Phylum Arthropoda: the largest phylum in the animal kingdom

Introduction

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.

Morphology

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.

Exoskeleton

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).

Moulting

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.

Appendages

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.

Classifications

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).

References

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.