Phylum Echinodermata: sea stars, sand dollars, urchins, and cucumbers


Echinoderms are the largest marine-only phylum, and its ~7,000 species are found at every ocean depth from rock pools to the deep abyss (Dubois, 2014). Sea stars, sea urchins, sand dollars, and sea cucumbers comprise this phylum and let me tell you why they’re so darn cool.


Echinoderms all possess radial symmetry, even though this can be quite hard to see in sea cucumbers, trust me, it’s there. Usually, the oral surface is on the underside of the animal, and the anus is located on top. The calcareous echinoderm endoskeleton is composed of ossicles which can be in the form of plates, spines, or lumps (Evamy & Shearman, 1965). The ossicles form a sponge-like structure called the stereom and are supported by a tough epidermis (Bottjer, Davidson, Peterson, & Cameron, 2006). Each ossicle develops from the deposition of a single cell which divides into more cells that deposit calcium carbonate in the original orientation. Ossicles are connected by collagen and may connect to muscles, allowing for flexibility and manoeuvrability. Spines are modified ossicles used for protection, locomotion, burrowing, gathering food and can also contain venom.


Pedicellariae are small, pincer-like structures used for protection against anything that may settle or graze on an animal’s body (Campbell, 2020; Coppard, Kroh, & Smith, 2012). Some pedicellariae are involved in capturing food, and some may be venomous. Each pedicellaria has its own muscles and sensory receptors, and therefore each pedicellaria is capable of reacting to a stimulus. There are four types of pedicellaria in urchins and two types for sea stars; one animal may have multiple types.

Pedicellaria of a crown-of-thorns sea star (Acanthaster planci).
By Philippe Bourjon – The uploader on Wikimedia Commons received this from the author/copyright holder., CC BY 3.0,
The incredibly dangerous flower urchin (Toxopneustes pileolus) with its long, venomous pedicellariae.
By Vincent C. Chen – Own work, CC BY-SA 3.0,

Water-vascular system

Before we dive into this topic, take a look at this diagram, and feel free to refer to it as we dive into the incredibly unique water-vascular system.

Basic anatomy of an echinoderm. By CNX OpenStax, CC BY 4.0,

The water vascular system is an elaborate closed system of canals used to help locomote the organism via its tube feet (Blake & Guensburg, 1988; Prusch & Whoriskey, 1976). The madreporite is a small sieve filter that creates an external link to the water-vascular system and is usually located to one side of the aboral surface and connects to a stone canal that extends vertically until it meets the ring canal. Radial canals emerge from the ring canal to the rays of the body in sea stars and around the body in sea urchins. From the radial canal stem lateral canals, which connect to ampullae and tube feet. Ampullae are bulb-like swellings that serve as reservoirs for water and fill the tube foot with water as they contract.

Tube feet have suction cup endings allowing for strong attachment to substrata and provide a general direction of movement. Pressure is exerted on the end of the sucker, and mucous functions as an adhesive. When the tissue inside the sucker contracts, it forms a cup that is secured to the substrate. In the case of sea stars living on soft substrates, tube feet are pointy so they can penetrate sediment and bury themselves.

Tube feet of the helmet urchin (Colobocentrotus atratus).
By Sébastien Vasquez – The uploader on Wikimedia Commons received this from the author/copyright holder., CC BY-SA 4.0,


The feeding strategies differ significantly between species of Echinodermata. Some are filter feeders, most urchins are grazers, and most sea stars are carnivorous predators.

This predatory strategy of sea stars is perhaps the most interesting of the echinoderms (Melarange, Potton, Thorndyke, & Elphick, 1999; Semmens et al., 2013; Wulff, 1995). The oesophagus connects to a stomach with two sections called the cardiac stomach and the pyloric stomach. In evolutionarily advanced sea stars, the cardiac stomach can be everted out of the mouth and engulf and digest food. They can use their tube feet to create suction on bivalve shells and open them, where they will evert a section of their cardiac stomach inside the shell to release enzymes to digest the animal inside. The stomach retracts back inside and the partially digested prey can be passed to the pyloric stomach.

A starfish (Circeaster pullus) everting its cardiac stomach to feed on coral.
By NOAA – Flickr, CC BY-SA 2.0,

Regular vs Irregular Echinoids

Regular echinoids (e.g., sea urchin) have no front or back end, and the oral end is underneath and the aboral end is on top. From above, they are circular and radially symmetrical because regular echinoids roam the seafloor in search of food and need to move in any direction. This means they are exposed to predators and have evolved elaborate spines for defence and locomotion. Spines vary between species: needle-like, club-like, poisonous, or thorny. Regular echinoids are usually scavengers with a diet of plant matter, animal detritus, and other inverts, and can use their tube feet to grasp food. They have powerful, complex jaws called Aristotle’s Lanterns, which extend through the mouth to collect food and leave a distinctive star-shaped grazing trace.

Irregular echinoids (e.g., sand dollars) lead different lifestyles from the regulars. They burrow in the seafloor and extract nutrients from sediment and have one plane of symmetry – the oral end is at the front of the animal to collect food, and the aboral end is at the rear to leave waste behind. The spines have lost their defensive role and have become reduced and hair-like to help burrow, move through sediment, gather food, and generate currents in the burrow. Many have lost their jaws as they are unnecessary to their mode of life. The tube feet are modified into flanges for respiration and gathering food.


Class Echinoidea

Class Echinoidea, aka echinoids, is composed of sea urchins and sand dollars.

Top – West Indian sea egg (Tripneustes ventricosus). Bottom – reef urchin (Echinometra viridis).
By Nhobgood, Nick Hobgood – Own work, CC BY-SA 3.0,
Lateral view of Aristotle’s Lantern of a sea urchin.
By Philippe Bourjon – The uploader on Wikimedia Commons received this from the author/copyright holder, CC BY-SA 4.0,
Sand dollar (Mellita species) burying itself in the sand.
By John Tracy from Snellville, GA, USA – End of the line, CC BY 2.0,

Class Holothuroidea

Sea cucumbers have leathery skin and an elongated body that is radially symmetrical along its longitudinal axis. They have no oral or aboral surface but instead, stand on one of their sides. Extraordinarily, they can loosen or tighten the collagen that forms their body wall and can essentially liquefy their body to squeeze through small gaps.

Brown sea cucumber (Actinopyga echinites) displaying its feeding tentacles and tube feet.
By François Michonneau – d2008-Kosrae-0084.jpg, CC BY 3.0,
A giant sea cucumber (Thelenota ananas).
By Leonard Low from Australia – Flickr, CC BY 2.0,

Class Asteroidea

There are around 1,500 species of sea star that make up the class Asteroidea.

Necklace sea star (Fromia monilis).
By Nhobgood Nick Hobgood – Own work, CC BY-SA 3.0,
Crown-of-thorns sea star (Acanthaster planci) is one of the largest sea stars, and it gets its name from the venomous, thorny ossicles covering its surface.
By jon hanson on flickr. –, CC BY-SA 2.0,


Blake, D. B., & Guensburg, T. E. (1988). The water vascular system and functional morphology of Paleozoic asteroids. Lethaia, 21(3), 189–206.

Bottjer, D. J., Davidson, E. H., Peterson, K. J., & Cameron, R. A. (2006). Paleogenomics of echinoderms. Science, 314(5801), 956–960.

Campbell, A. C. (2020). Form and function of pedicellariae. In Echinoderm studies (pp. 139-167). CRC Press.

Coppard, S. E., Kroh, A., & Smith, A. B. (2012). The evolution of pedicellariae in echinoids: an arms race against pests and parasites. Acta Zoologica, 93(2), 125–148.

Dubois, P. (2014). The skeleton of postmetamorphic echinoderms in a changing world. The Biological Bulletin, 226(3), 223–236.

Evamy, B. D., & Shearman, D. J. (1965). The development of overgrowths from echinoderm fragments. Sedimentology, 5(3), 211–233.

Melarange, R., Potton, D. J., Thorndyke, M. C., & Elphick, M. R. (1999). SALMFamide neuropeptides cause relaxation and eversion of the cardiac stomach in starfish. Proceedings of the Royal Society of London. Series B: Biological Sciences, 266(1430), 1785–1789.

Prusch, R. D., & Whoriskey, F. (1976). Maintenance of fluid volume in the starfish water vascular system. Nature, 262(5569), 577–578.

Semmens, D. C., Dane, R. E., Pancholi, M. R., Slade, S. E., Scrivens, J. H., & Elphick, M. R. (2013). Discovery of a novel neurophysin-associated neuropeptide that triggers cardiac stomach contraction and retraction in starfish. Journal of Experimental Biology, 216(21), 4047–4053.

Wulff, L. (1995). Sponge-feeding by the Caribbean starfish Oreaster reticulatus. Marine Biology, 123(2), 313–325.


Phylum Arthropoda: the largest phylum in the animal kingdom


Arthropoda: you may remember them from such fears as arachnophobia and your recent nightmare, “Help! I’m Locked in a Coffin of Cockroaches!” But, no fear, I won’t be burdening you with any terrestrial garbage because, as you know, it’s all underwater from here.


To qualify for Phylum Arthropoda, you must be one of over 10 million species that lack a backbone, have an exoskeleton, segmentation, bilateral symmetry, a coelom, and paired, jointed appendages. Their segments are grouped into body divisions called tagmata, where segments and limbs have specialised functions; the three tagmata are the head, thorax, and abdomen, although some species have a combined head and thorax called a cephalothorax.


Arthropod exoskeletons are a cuticle that is secreted by the epidermis and is composed of two layers which aid in support and protection (Chen, Lin, McKittrick, & Meyers, 2008). The thin, waxy outer layer is called the epicuticle and is used in waterproofing. The thick, inner layer is called the procuticle and is the central structural part composing the majority of the exoskeleton. The exoskeleton is attached to the soft body by muscles and the animal uses those muscles to flex their joints (although some use hydraulic pressure to extend them).


The exoskeleton is not flexible and, therefore, restricts arthropod growth. In order to grow, arthropods moult and shed the old exoskeleton in an almost continuous cycle until they reach their full size. First, the epidermis secretes a moulting enzyme that separates the old cuticle from the body. While the old cuticle is detaching, the epidermis secretes a new layer that will form part of the procuticle. After this is complete, the animal will take on seawater to split the old cuticle along predetermined weaknesses, and the animal will crawl out of its old exoskeleton. The new cuticle is exceptionally soft, and the animal is highly vulnerable as it continues to pump itself up with seawater to stretch the soft cuticle out. The cuticle will harden, and the animal can relax and eat its old exoskeleton to get back some nutrients (this gives me big Goldmember vibes iykyk).

Subphylum Crustacea

Crustaceans are what I like to call the insects of the ocean, and incudes isopods, copepods, barnacles, shrimp, krill, crabs, lobsters… the list goes on.


The head region contains two pairs of sensory antennae, mandibles for crushing food, and first and second maxillae to sort and deliver food to the mandibles. The thoracic regions appendages are called thoracopods and may be specialised into maxillipeds which are specialised for feeding, and pereiopods, specialised for walking and swimming. The abdominal region contains pleopods which can be specialised for swimming, jumping, respiration, egg brooding, or copulation. The final pleopods may modify into a tail called a uropod. The abdomen terminates at the telson, which usually sits above the uropod and contains excretory organs. The number and diversity of appendages vary from amongst crustacean species.

File:Anatomy of a shrimp 3.jpg - Wikimedia Commons
The appendages of a shrimp: A: antennae. R: rostrum. C: carapace. Mx: maxilliped. U: uropod. T: telson. P: pereiopod. Pl: pleopod. 1–9: abdominal segments.

Crustaceans usually have biramous appendages that branch into two, where each branch consists of a series of segments attached end-to-end. The branching takes place on the second article. The external branch of the appendages is known as the exopodite, while the internal branch is known as the endopodite. Crustacean appendages have adapted to function in sensing their environment, defending against predators, swimming, walking, grasping, transferring sperm, generating water movement, and gas exchange. Some crustaceans have uniramous appendages thought to result from evolutionary loss of the second branch.

The difference between biramous and uniramous appendages within the phylum Arthropoda.


Class Cirripedia

The most famous cirripeds are the acorn and gooseneck barnacles, and they live attached to hard substrates (Doyle, Mather, Bennett, & Bussell, 1996). They have a hard carapace made from calcareous plates that enclose the soft body parts. Their thoracic appendages are called cirri, which are biramous. The endopodites and exopodites are covered with setae to filter food particles from the water; they can also respire through the cirri. Most barnacles are hermaphroditic, and the penis extends into neighbouring barnacles to deposit spermatophores (Charnov, 2018). The larvae are planktonic and moult until they find a suitable substrate in which they settle on their “back”, the carapace, which adheres permanently to the substrate.

Gooseneck barnacles (order Pedunculata) growing in a tidal cave.
Northern acorn barnacles (Semibalanus balanoides).

Order Amphipoda and Order Isopoda

Amphipods are the most annoying crustacean. They’re the ones that bite you at the beach, aka sandflies. Isopods are similar in some ways but are lice. Let me break them both down for you.

Amphipods are scavengers and consume smaller invertebrates and plant matter; that’s why you often find them around driftwood or decaying seaweed at the beach. They are frequently consumed therefore making them an integral part of coastal food webs. Their bodies are laterally compressed (flattened from side to side) with no carapace and have the three main arthropod tagmata. They have strong uropods which aid them in jumping all over your lovely picnic.

A freshwater amphipod species (Gammarus roeseli).

Isopods have a range of feeding strategies from scavengers to carnivores and parasites to filter feeders. Their bodies are dorsoventrally flattened (flattened top to bottom, creating a wide, flat profile), and they lack a prominent carapace; it’s more of a helmet, if anything. Like amphipods, they contain all three main body parts and have a pleotelson where the last abdominal segment is fused with the telson.

A carnivorous isopod called the speckled sea louse (Eurydice pulchra).
A giant, marine isopod (Bathynomus giganteus).

Order Decapoda

Decapods, meaning “ten-footed”, are your supermarket crustaceans, e.g., crabs, lobsters, prawns, and shrimps, although I’m sure you’ll agree they look a lot better in the ocean! One of their thoracic appendages may be specialised into large pincers called chelae (think lobster claws), used to crush shells, tear up food, and pass pieces to the maxillipeds. The maxillipeds are the first three pairs of thoracic appendages and are modified for feeding. The abdominal appendages function to carry eggs, brood young, or transfer spermatophores. They usually have a uropod and telson that serve as a strong tail. Although, some decapods, e.g., crabs, have short abdomens, which are typically folded under the thorax. In males, this fold is triangular, and in females, it is broader so it can hold the eggs. Their carapace extends low enough to cover their gills.

Many decapod species can demonstrate the ability to autotomise, whereby they can regenerate an appendage after it has been dropped (Juanes & Smith, 1995; Shinji, Miyanishi, Gotoh, & Kaneko, 2016). They usually drop their limbs when threatened by a predator as a deterrent; the predator will be distracted by the limb, and the decapod can escape. A blot clot will prevent bleeding, and regeneration of the new limb will start immediately and can usually be seen after the successive moult. Growing a new appendage is extremely energy taxing, so dropping it in the first place is usually a last resort.

European lobster (Homarus gammarus).
Purple rock crab (Leptograpsus variegatus).
Mantis shrimp (Odontodactylus scyllarus).


Charnov, E. L. (2018). Sexuality and hermaphroditism in barnacles: a natural selection approach. In Barnacle biology (pp. 89–103). Routledge.

Chen, P. Y., Lin, A. Y. M., McKittrick, J., & Meyers, M. A. (2008). Structure and mechanical properties of crab exoskeletons. Acta biomaterialia, 4(3), 587–596.

Doyle, P., Mather, A. E., Bennett, M. R., & Bussell, M. A. (1996). Miocene barnacle assemblages from southern Spain and their palaeoenvironmental significance. Lethaia, 29(3), 267–274.

Juanes, F., & Smith, L. D. (1995). The ecological consequences of limb damage and loss in decapod crustaceans: a review and prospectus. Journal of Experimental Marine Biology and Ecology, 193(1-2), 197–223.

Shinji, J., Miyanishi, H., Gotoh, H., & Kaneko, T. (2016). Appendage regeneration after autotomy is mediated by Baboon in the crayfish Procambarus fallax f. virginalis Martin, Dorn, Kawai, Heiden and Scholtz, 2010 (Decapoda: Astacoidea: Cambaridae). Journal of Crustacean Biology, 36(5), 649–657.

Lophophorata: Phylum Phoronida, Brachiopoda, and Bryozoa


Lophophorata is a clade composed of three Phyla: Phoronida, Brachiopoda, and Bryozoa. They are grouped like this due to one uniting structure, the lophophore (Jang & Hwang, 2009; Temereva & Kuzmina, 2017).


Let’s begin with the pièce de resistance: the lophophore. It is a feeding structure, similar to a bunch of ciliated tentacles, that surround the mouth; therefore, lophophorates are suspension feeders. The cilia create strong water circulation, allowing for gas exchange, the exportation of gametes, and food particle delivery. The lophophore is thought to be the result of convergent evolution (Halanych, 1996). The gut of a lophophorate is U-shaped, so the direction of water flow prevents the mixing of food and waste products.

Phylum Phoronida

Phoronids are also known as horseshoe worms and build chitinous tubes to protect and support their soft bodies (Abele, Gilmour, & Gilchrist, 1983). Phoronids can retract and extend their lophophore, and cilia manipulate food into their mouth. Phoronids actively assess the flow of the water current and can reorient themselves as water flow changes to maximise their food-capturing ability. Their diet includes zooplankton, detritus, and invertebrate larvae.

Phoronis australis.

Phylum Brachiopoda

Please don’t be a pleb and confused them with Bivalvia. Brachiopoda has upper and lower valves, as opposed to left and right valves of bivalves. Anyway, they don’t even share the same ancestry.

The lophophore is connected to the lower brachial valve and is supported by cartilage and, sometimes, a brachidium (calcareous support attached to the brachial valve). Most brachiopods attach themselves to hard substrates by a stalk called the pedicle, a connective tissue that is part of the upper pedicle valve.

Inarticulate brachiopods (Lingula anatine).

Class Articulata

This class is defined by its “tooth-and-socket” hinge arrangement and a simple muscle strategy that opens and closes the hinges.

Class Inarticulata

This class has untoothed hinges and a more complex muscular strategy for aligning the valves.

Phylum Bryozoa

Bryozoans resemble phoronids, except bryozoans are microscopic, typically about 0.5mm in length. All genera, except one, form colonies that resemble moss. Each individual is called a zooid and has a hard casing called a cystid and a polypide that holds the organs in place. Within the colony, individual zooids may share resources through internal connections, and some zooids may specialise in a function. Vibracula zooids have a long bristle thought to function as defence or vibrate to keep the colony from becoming covered with sediment.

A freshwater bryozoan species.


Abele, L. G., Gilmour, T., & Gilchrist, S. (1983). Size and shape in the phylum Phoronida. Journal of Zoology, 200(3), 317–323.

Halanych, K. M. (1996). Convergence in the feeding apparatuses of lophophorates and pterobranch hemichordates revealed by 18S rDNA: an interpretation. The Biological Bulletin, 190(1), 1–5.

Jang, K. H., & Hwang, U. W. (2009). Complete mitochondrial genome of Bugula neritina (Bryozoa, Gymnolaemata, Cheilostomata): phylogenetic position of Bryozoa and phylogeny of lophophorates within the Lophotrochozoa. Bmc Genomics, 10(1), 1–18.

Temereva, E. N., & Kuzmina, T. V. (2017). The first data on the innervation of the lophophore in the rhynchonelliform brachiopod Hemithiris psittacea: what is the ground pattern of the lophophore in lophophorates?. BMC evolutionary biology, 17(1), 1–19.

Phylum Mollusca: send nudes (nudibranchs)


There’s a mollusc, see? And he walks, well he doesn’t walk up, he swims up. Well, actually, the mollusc isn’t moving, he’s in one place and then the sea cucumber… I mixed up. There was a mollusc and a sea cucumber. None of them were walking. Normally, they don’t talk, but in a joke, everyone talks. So, the sea mollusc says to the cucumber, “with fronds like these, who needs anemones!”

Mollusca forms the second largest invertebrate phylum and has a diverse evolutionary history and wide range of feeding and life-history strategies that have led to its success on land, freshwater, and the ocean (Rosenberg, 2014). Molluscs can be herbivores, carnivores, scavengers, and filter feeders, and they are responsible for the consumption of large amounts of organic matter. They themselves serve as food for a range of predators, and thus, are essential links in the food chain.


All species of Mollusca have bilateral symmetry, lack of segmentation, a structured nervous system, and a mantle. When discussing mollusc morphology, a generalised overall structure is usually referred to, though many species are an exception to this structure.

Their head contains the mouth and feeding structures and a ganglion of nervous systems. The visceral mass is the metabolic region containing the stomach, heart, intestines, gonads, etc. Covering the visceral mass is the mantle which secretes the shell, and a fold on the mantle known as the mantle cavity contains excretory and respiratory organs. The shell thickens with age as it is secreted by the mantle and consists of three layers:

  1. Outer layer – aka periostracum is composed of a durable organic material and may develop as a thin smooth coating, into hairs or into flexible spine-like outgrowths.
  2. Middle layer – aka prismatic is made of columnar calcite.
  3. Inner layer – is often nacreous (think iridescent and pearl-like) and laid down in thin sheets by the epithelial parts of the mantle.


Torsion is evident in gastropods (snails) during larval development. Two torsion events of 90° result in 180° rotation of the mantle cavity and the organs it contains to an anterior position above the head (Page, 2006). A possible disadvantage of torsion is that the anus excreting above the head could cause fouling of the mouth and sensory organs. However, the success of the class Gastropoda suggests this may not be an issue. Possible advantages of torsion are that it allows the animal to retract its vulnerable head into the shell efficiently and in marine species, the anterior positioning may prevent sediment from entering the mantle cavity.


The radula is unique to molluscs and is found in every class except bivalves (Steneck & Watling, 1982). It is a chitinous ribbon studded with small, hard teeth, used for scraping or cutting food before it enters the oesophagus. The radula protrudes from the floor of the buccal cavity, where the odontophore underlies the radula membrane and controls its protrusion and return. As the radula retracts into the buccal cavity, the teeth rasp food particles from the substrate and food is deposited into the pharynx. As teeth wear, new teeth are continuously being secreted, shaped, and added to the cuticle ribbon inside the radula sac.

Food particles pass from the pharynx to the oesophagus and then to the stomach, where digestion occurs intracellularly and extracellularly within folds of the stomach called diverticula. The stomach may have several functions: sorting, grinding, and digesting food particles.


The circulatory system is usually open, and blood flows through the haemocoel cavity. The respiratory pigment is called hemocyanin, which is pale blue when oxygenated and clear when deoxygenated. This blue is from the copper contained in the oxygen-binding molecules, as opposed to the red blood of mammal’s iron found in haemoglobin.

The nervous system is typically a mass of nerve cell bodies that associate with the sensory organs. The sensory organs often include eyes, statocysts (the sensory organ that orients animal to gravity, located in the foot), osphradia (sensory epithelium which act as chemoreceptors), and tentacles.



Neogastropoda includes sea snails and is primarily carnivorous. They have a proboscis that extends out and can drill through shells of bivalves or is used to suck up nutrients from its prey (much like a butterfly feeding on nectar). Some species have a siphon to draw water into the mantle cavity to oxygenate the gill.

Mud whelks (Nassarius jacksoniania) eating a dead fish.


Bivalvia includes your classic shellfish, e.g., clams, mussels, oysters, scallops, and cockles. They lack a radula, an odontophore, and a head. The name bivalve means “two shells”, which is exactly what they have, and these shells are connected by a hinge and are left and right, as opposed to top and bottom as with Brachiopoda (don’t worry, we’ll get there). The lack of head is made up for with their foot. It is usually well-developed and excellent for digging and ploughing through sediment. Some bivalves, e.g., mussels, have a byssus thread used to attach to hard substrates, and I guarantee you will notice it the next time you eat a mussel.

Little black mussel (Xenostrobus pulex).
Small giant clam (Tridacna maxima).


Nudibranchia contains the crazy, colourful nudibranchs: a group of around 3,000 species of soft-bodied molluscs. But how can they be molluscs if they don’t have a shell? They shed their shell in the larval stage (Thompson, 1959). Along with a naked body, they also lack a mantle cavity, meaning that the nudibranchs probably like the term mollusc to be used loosely! To add to their peculiar nature, they are all carnivorous mostly feeding with a radula, and some store nematocysts from their Cniadrian prey and use them as a defence mechanism (Frick, 2003). They are hermaphroditic, but cannot fertilise themselves, and mate after a courting dance takes place.

Phyllidia babai.
Mexichromis macropus.
Dendronotus albus.


And if you thought nudibranchs didn’t fit into the Mollusca mould, then don’t even bother reading about cephalopods. With their name meaning “head-feet”, Cephalopoda contains over 800 living species of octopus, squid, cuttlefish, and nautilus. They have bilateral symmetry and, as their name suggests, a prominent head atop a set of arms or tentacles that have evolved from the molluscan foot. And before you ask, “arms” are the suction cup ones, and “tentacles” only have suction cups at the end – some species have one or the other or both.

When you think of squid, you don’t immediately associate them with their bivalve or gastropod relatives mainly because, well, the latter two have a shell and a squid does not, right? Well, the answer is tricky, and, like with every marine invertebrate, I will forgive you for thinking something is something that it is not. Nautiluses have an external shell that is visible to the naked eye, still the case of cuttlefish, octopuses, and squid is slightly more complex. Some cephalopods have a vestigial shell, some have organic, internal, calcium carbonate structures, and some may have just evolved to lose their shell entirely (Baratte, Andouche, & Bonnaud, 2007; Furuhashi, Schwarzinger, Miksik, Smrz, & Beran, 2009; Warnke & Keupp, 2005).

Cephalopods are often regarded as extremely intelligent, with complex nervous systems and the ability to use tools and problem solve (Budelmann, 1995; Finn, Tregenza, & Norman, 2009; Richter, Hochner, & Kuba, 2016; Schnell, Amodio, Boeckle, & Clayton, 2021).

The genus Hapalochlaena contains four extremely venomous octopus species, more commonly known as the blue-ringed octopuses. They are tiny, reaching maximum sizes of 20cm, but deadly, with one animal containing enough tetrodotoxin to kill 26 adult humans with a painless bite that can paralyse within minutes.

The greater blue-ringed octopus (Hapalochlaena lunulata.).
The Palau nautilus (Nautilus belauensis).
The common cuttlefish (Sepia officinalis) – note its “W” shaped pupil thought to be useful in improving horizontal vision (Mäthger, Hanlon, Håkansson, & Nilsson, 2013).
File:Euprymna scolopes - image.pbio.v12.i02.g001.png - Wikimedia Commons
The Hawaiian bobtail squid (Euprymna scolopes) reaches a max of 3cm.


Baratte, S., Andouche, A., & Bonnaud, L. (2007). Engrailed in cephalopods: a key gene related to the emergence of morphological novelties. Development genes and evolution, 217(5), 353–362.

Budelmann, B. U. (1995). The cephalopod nervous system: what evolution has made of the molluscan design. In The nervous systems of invertebrates: An evolutionary and comparative approach (pp. 115–138). Birkhäuser Basel.

Finn, J. K., Tregenza, T., & Norman, M. D. (2009). Defensive tool use in a coconut-carrying octopus. Current biology, 19(23), R1069–R1070.

Frick, K. (2003). Predator suites and flabellinid nudibranch nematocyst complements in the Gulf of Maine. DIVING FOR SCIENCE… 2003, 37.

Furuhashi, T., Schwarzinger, C., Miksik, I., Smrz, M., & Beran, A. (2009). Molluscan shell evolution with review of shell calcification hypothesis. Comparative biochemistry and physiology Part B: Biochemistry and molecular biology, 154(3), 351–371.

Mäthger, L. M., Hanlon, R. T., Håkansson, J., & Nilsson, D. E. (2013). The W-shaped pupil in cuttlefish (Sepia officinalis): functions for improving horizontal vision. Vision research, 83, 19-24.Page, L. R. (2006). Modern insights on gastropod development: reevaluation of the evolution of a novel body plan. Integrative and Comparative Biology, 46(2), 134–143.

Richter, J. N., Hochner, B., & Kuba, M. J. (2016). Pull or push? Octopuses solve a puzzle problem. PloS one, 11(3), e0152048.

Rosenberg, G. (2014). A new critical estimate of named species-level diversity of the recent Mollusca. American Malacological Bulletin, 32(2), 308–322.

Schnell, A. K., Amodio, P., Boeckle, M., & Clayton, N. S. (2021). How intelligent is a cephalopod? Lessons from comparative cognition. Biological Reviews, 96(1), 162–178.

Steneck, R. S., & Watling, L. (1982). Feeding capabilities and limitation of herbivorous molluscs: a functional group approach. Marine Biology, 68(3), 299–319.

Thompson, T. E. (1959). Feeding in nudibranch larvae. Journal of the Marine Biological Association of the United Kingdom, 38(2), 239–248.

Warnke, K., & Keupp, H. (2005). Spirula—a window to the embryonic development of ammonoids? Morphological and molecular indications for a palaeontological hypothesis. Facies, 51(1), 60–65.

Phylum Ctenophora: is it a bird? Is it a plane? Is it a jellyfish?


There are less than 200 known species of ctenophores, all of which are found exclusively in marine habitats. Ctenophores, more commonly known as the comb jellies, resemble cnidarian medusa (I will forgive you for confusing them with jellyfish), but ctenophores have a few specific features that make them unique.


Opposed to jellyfish, who have radial symmetry, ctenophores have bilateral symmetry (Pang & Martindale, 2008). They don’t use jet propulsion like our Scyphozoan friends, but rather are the largest animals to swim with the help of cilia, with adults range from a few millimetres to 1.5 metres (Tamm, 2015). The cilia are packed in the thousands into ctenes, or comb plates, which are organised into comb rows around the body (Tamm, 2014).

Ctenophores are known to be bioluminescent, where the photoproteins, located in canals under the comb rows, are activated (Haddock & Case, 1999; Pang & Martindale, 2008). This is not to be confused with the rainbow effect of the comb rows produced as their cilia beat and scatter light (Welch, Vigneron, Lousse, & Parker, 2006).

A brief annotation of the anatomy of a ctenophore.


Ctenophores have sticky cells in the epidermis of their tentacles called colloblasts which capture food. The tentacles expand when ready to capture food and the branches on the tentacles separate. The stickiness of the colloblasts allows the organism to “fire” a fibre with adhesive granules to capture food (e.g., copepods).

Colloblasts are linked to a nervous system that triggers the retraction of the tentacles towards the mouth when prey is captured. Prey is then wiped inside the mouth, swallowed, and liquified into a slurry by enzymes and muscular contractions in the pharynx. Cilia beat and distribute the slurry through the canal system where digestion occurs intra and extracellularly. Some waste is released through the anal pores but usually they regurgitate waste from the mouth.

Colloblasts are unique to ctenophores and are found in the epidermis of the tentacles and, similar to nematocysts of Cnidaria, the colloblasts are discharged from the tentacles and capture prey (Franc, 1978). However, colloblasts are not venomous but rather they are adhesive and stick to their prey. The rare ctenophore, Haeckelia rubra, has rid itself of colloblasts completely and instead collects nematocysts from their cnidarian prey (Mills & Miller, 1984).

Nervous system and navigation

Ctenophores have a structure, know as the statocyst, that aids in their navigation and orientation through gravitational sensitivity (Pang & Martindale, 2008; Tamm, 2015). If a ctenophore is pulled off balance, its statocyst will direct beating of specific comb rows in order to right itself. For movement over long distances, ctenophores mainly rely on ocean currents (Pang & Martindale, 2008). Instead of a nervous system, ctenophores have a complex nerve net that works closely with the statocyst and ctenes (Pang & Martindale, 2008; Tamm, 2014; Tamm, 2015).


Floating somewhat aimlessly around in the ocean as a relatively tiny and hard-to-see individual means the likelihood of you meeting someone you can have babies with is quite low. To combat this, almost all ctenophores are self-fertile hermaphrodites (Pang & Martindale, 2008).


Class Tentaculata

Tentaculata have, you guessed it, tentacles. Commonly, they have long, feathery tentacles which are equipped with colloblasts.

Mertensia ovum, a species of ctenophore from Class Tentaculata (note the two, prominent tentacles trailing behind.

Class Nuda

If Class Tentaculata have tentacles, then Class Nuda must have no tentacles. Organisms from Nuda are known as beroids (from the monophyletic order Beroida), and they feed by using their large mouths to engulf prey. Alternatively, some species spread their lips over prey whilst a sword-like structure chops the prey up (Tamm & Tamm, 1991). Beroids actively hunt their prey, which is usually soft-bodied organisms such as ctenophores – yes, they eat their own kind.

A species of beroid ctenophore from the Class Nuda (note its lack of tentacles).


Franc, J. M. (1978). Organization and function of ctenophore colloblasts: an ultrastructural study. The Biological Bulletin, 155(3), 527–541.

Haddock, S. H., & Case, J. F. (1999). Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: ctenophores, medusae and siphonophores. Marine Biology, 133(3), 571–582.

Mills, C. E., & Miller, R. L. (1984). Ingestion of a medusa (Aegina citrea) by the nematocyst-containing ctenophore Haeckelia rubra (formerly Euchlora rubra): phylogenetic implications. Marine Biology, 78(2), 215–221.

Pang, K., & Martindale, M. Q. (2008). Ctenophores. Current Biology, 18(24), R1119–R1120.

Tamm, S. L. (2014). Cilia and the life of ctenophores. Invertebrate Biology, 133(1), 1–46.

Tamm, S. L. (2015). Functional consequences of the asymmetric architecture of the ctenophore statocyst. The Biological Bulletin, 229(2), 173–184.

Tamm, S. L., & Tamm, S. (1991). Reversible epithelial adhesion closes the mouth of Beroe, a carnivorous marine jelly. The Biological Bulletin, 181(3), 463–473.

Welch, V., Vigneron, J. P., Lousse, V., & Parker, A. (2006). Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora). Physical Review E, 73(4), 041916.

Phylum Cnidaria: jellyfish, corals, and anemnem… amenome… we give up.


Phylum Cnidaria contains all your favourite ocean stingers from jellyfish to corals to anenomes, and we can’t forget our favourite Hydrozoan, the Portuguese man o’ war.



The body wall is composed of three layers:

  1. Epidermis tissue – outer layer.
  2. Mesoglea – “jelly in the middle” composed of mucopolysaccharides & collagen; is not true tissue but provides support, buoyancy, and locomotion.
  3. Gastrodermis tissue – inner layer which lines the gastrovascular cavity.

A distinguishing feature of cnidaria is their simple gastrovascular cavity, present in only one other primitive phylum. It is a two-way system, where food enters through an opening that serves as a mouth and an anus and is extracellularly digested within the gastrovascular cavity, then waste exits back through the same hole (Shostak, 2001).

Medusa & polyp form

Cnidaria can exist in two forms: medusa or polyp (Piraino, Boero, Aeschbach, & Schmid, 1996; Seipel & Schmid, 2004). Some cnidarians only exhibit a polyp or medusa form, or have one predominantly over the other. Some may pass through both forms throughout their life histories.

Polyp form:

  • Sessile
  • Cylindrical tubes point upwards
  • The oral end is on top
  • The aboral end usually attached to the substrate
  • Tentacles point upward
  • E.g., sea anemone
  • Some polyps form colonies (e.g., coral)

Medusa form:

  • Swims
  • Is the inverse of the polyp
  • The oral end is below a bell-shaped body
  • Th aboral end is to top of the umbrella-like structure (velum)
  • Tentacles point downward
  • E.g., jellyfish
Difference between polyp and medusa. ( Copyright 2001 by Sinauer Associates Inc.

Nematocyte and nematocyst

Yikes. Let’s get this clear:

NematoCYTE is the CELL.

NematoCYST is the ORGANELLE.

The nematocyte is a specialised, ectodermal stinging cell involved in defence and prey capture. It contains the nematocyst (Beckmann & Özbek, 2012; David et al., 2008; Östman, 2000).

The nematocyst is an organelle inside the nematocyte consisting of an ejectable thread that causes a sting and injects toxins into predators or prey. The nematocyst capsule is firm and made from a type of collagen, and it holds a coiled thread that can be barbed, smooth, or hold toxins. A chemical or physical stimulant stimulates the ejection of this thread, and once stimulated, will uncoil and extrude out, penetrating or wrapping around the prey. Once paralysed, captured, or killed (who knew sea anemones are so gruesome??), the tentacles will move the prey to the oral cavity to be digested by the gastrovascular cavity.

File:Nematocyst discharge.png - Wikimedia Commons
Showing the discharge of a barbed nematocyst from a nematocyte.

There are three types of nematocysts:

  1. Penetrant – barbed thread with open tip; when discharged, it pierces the skin/exoskeleton and injects venom to paralyse or kill.
  2. Glutenant – smooth or bristled thread with an open tip that is sticky and has toxins.
  3. Volvent – smooth, lasso-like thread with closed tip entangles prey.


Cnidaria are capable of both sexual and asexual reproduction (Shostak, 2001). Sexual reproduction involves gametes, usually produced in separate individuals, and are fertilised in the gut, ovary, or water after being released by the mouth, tentacles, or breaks in the epidermal layer. Female gametes may produce a substance that attracts male gametes. Asexual reproduction usually happens in warmer months, where a bud develops via evagination from the adult body wall and contains an extension of the gastrovascular cavity. Once fully developed, it detaches from the parent.


Coral have a unique symbiotic relationship with zooxanthellae which gives the coral a range of different colours (Shostak, 2001). Coral produce carbon dioxide (CO2) and ammonium (NH4+) as a by-product of cellular respiration, and zooxanthellae use the CO2 and NH4+ to conduct photosynthesis which, in turn, supplies the coral with sugars, lipids, and oxygen.

Coral bleaching is a phenomenon where the coral consumes or expels their symbiotic inhabitants to ensure short-term survival when exposed to stressful conditions such as rising water temperature, leading to a white “bleached” appearance (Hoegh-Guldberg, 1999; Lesser, 2011; Nir, Gruber, Shemesh, Glasser, & Tchernov, 2014). The coral continues to live after bleaching, but under a prolonged, stressful environment, they will die from starvation.


Class Anthozoa

This group includes sea anemones, stony corals, and soft corals.

Apple anemone (Stomphia didemon).
Rare, long-lived, deep-sea Hawaiian gold coral (Kulamanamana haumeaae).

Class Scyphozoa

These are the true jellyfish.

Chrysaora melanaster, commonly known as the northern sea nettle or brown jellyfish, is a species of jellyfish native to the northern Pacific Ocean and adjacent parts of the Arctic Ocean.
A species of jellyfish from the genus Cephea, or the cauliflower jellyfish.

Class Cubozoan

Box jellyfish are distinguishable by their cube-shaped medusae. Some species have potent venom that can be extremely painful and potentially fatal to humans.

Chironex fleckeri, or the sea wasp, is thought to be the most lethal jellyfish in the world and is responsible for sixty-four deaths in Australia from 1884-2021 (Fenner & Williamson, 1996). C. fleckeri is said to contain enough venom to kill sixty adult humans, and stings are typically excruciatingly painful and, if left untreated, can kill within two to five minutes.

Chironex fleckeri.

Malo kingi, or the common kingslayer, is a species of Irukandji jellyfish named after one of its victims, Robert King (Gershwin, 2007). The Irukandji jellyfish are any of several box jellies that cause Irukandji syndrome after stinging their victims; Irukandji syndrome is characterised by severe pain, vomiting, and rapid rise in blood pressure. M. kingi are very small and inconspicuous in the water, making it hard for victims to see them.

Malo kingi.

Class Hydrozoa

To end on a fun note, let’s finish with Hydrozoa. They are a group of very small, predatory individuals that can live in solitude or within a colony. The colonial species can be large and sometimes the specialised individuals cannot survive outside of their colony. So, the next time you hear someone calling a Portuguese man o’ war (Physalia physalis) a jellyfish, feel free to roll your eyes and let them know they are actually a Hydrozoan.

Portuguese man o’ war (Physalia physalis) washed up on a beach.


Beckmann, A., & Özbek, S. (2012). The nematocyst: a molecular map of the cnidarian stinging organelle. International Journal of Developmental Biology, 56(6–7–8), 577–582.

David, C. N., Özbek, S., Adamczyk, P., Meier, S., Pauly, B., Chapman, J., … & Holstein, T. W. (2008). Evolution of complex structures: minicollagens shape the cnidarian nematocyst. Trends in genetics, 24(9), 431–438.

Difference between polyp and medusa. From Difference Copyright 2001 by Sinauer Associates Inc.

Hoegh-Guldberg, O. (1999). Climate change, coral bleaching and the future of the world’s coral reefs. Marine and freshwater research, 50(8), 839–866.

Lesser, M. P. (2011). Coral bleaching: causes and mechanisms. In Coral reefs: an ecosystem in transition (pp. 405–419). Springer, Dordrecht.

Nir, O., Gruber, D. F., Shemesh, E., Glasser, E., & Tchernov, D. (2014). Seasonal mesophotic coral bleaching of Stylophora pistillata in the Northern Red Sea. PLoS One, 9(1), e84968.

Östman, C. (2000). A guideline to nematocyst nomenclature and classification, and some notes on the systematic value of nematocysts. Scientia Marina, 64(S1), 31–46.

Piraino, S., Boero, F., Aeschbach, B., & Schmid, V. (1996). Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa). The Biological Bulletin, 190(3), 302–312.

Seipel, K., & Schmid, V. (2004). Mesodermal anatomies in cnidarian polyps and medusae. International Journal of Developmental Biology, 50(7), 589–599.

Shostak, S. (2001). Cnidaria (Coelenterates). e LS.