Introduction
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.
Morphology
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:
- 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.
- Middle layer – aka prismatic is made of columnar calcite.
- Inner layer – is often nacreous (think iridescent and pearl-like) and laid down in thin sheets by the epithelial parts of the mantle.
Torsion
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.
Feeding
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.
Systems
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.
Classifications
Neogastropoda
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.
Bivalvia
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.
Nudibranchia
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.
Cephalopoda
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.
References
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.
The Ocean: Explained 