The Mighty Molluscs of the Tropics!
There is nothing quite like a mollusc. It is the largest of all marine phylum, and consists of a diverse array of invertebrates, characterised by their soft bodies, a radula (excluding the bivalve molluscs) and the presence of an internal or external shell. It encompasses all beings great and small, ranging from the well-known gastropods (snails and slugs), to the magnificent cephalopods (octopus, squid, and cuttlefish).
This phylum contains some of the most extraordinary individuals found within our oceans, exhibiting some bizarre and fascinating adaptations to survive throughout all marine environments. The most weird and wonderful of these can be found living amongst tropical coral reefs, an environment where temperatures must be above 18°C, salinities in between 32-42ppt and where competition and predation is high.
The Flamingo-Tongue Snail
The Flamingo-Tongue Snail, Cyphoma gibbosum, is a mysterious species of gastropod mollusc, found abundantly throughout the Caribbean. Whilst most molluscs have a preferred diet of algae, the Flamingo-Tongue chooses to feast almost exclusively on gorgonian corals. This is a taxonomic group including sea fans,
that produce chemical and structural defences that successfully deter other species from preying upon it. This includes the production of calcium carbonate, in the form of sclerites, for structural defence, nematocysts, and the production of secondary metabolites, most commonly terpenes, that consist mainly of nitrogenous compounds. However, Cyphoma gibbosum is not most predators. This small but mighty snail contains a type of detoxification enzyme, known as Glutathione S-transferases, that allows the snail to sequester the secondary metabolites into its own body for its own personal gain. These chemicals are recycled, rendering the snail itself toxic, and therefore decreases its susceptibility to predation. It makes this explicitly apparent to predators by completely enveloping its entire shell with its vibrant mantle, described scientifically to be of an aposematic colouration, in a bid to ward off potential predators. As most species are not able to consume sea fans, competition between species is rare, allowing this snail to flourish in the competitive ecosystem that is the coral reef. Unfortunately, recent numbers have been in decline as a result of divers collecting them due to their beautiful colours, not realising that these colours are produced by the live body and when the snail dies only the plain, cream shell is left behind.
The Blue-Ringed Octopus
Another species that uses poison is a very different mollusc: The Blue-Ringed Octopus, Hapalochlaena lunulate. This species is found in the Indian and Pacific oceans, commonly inhabiting coral reef environments, and is
widely known one of the most poisonous animals on the planet. Their salivary glands contain a symbiotic bacterium that synthesises a poisonous neurotoxin, known as terodotoxin, that is “1000 times more powerful than cyanide.” A single octopus contains enough to wipe out over 25 grown men in the space of just a few minutes. When hunting or threatened, it delivers an almost undetectable bite, injecting the venom into its prey or attacker, causing immediate paralysis and vascular depression, and eventual death. The Blue-Ringed Octopus is also a master of camouflage, containing specialised cells known as chromatophores that allow them to blend easily into their environment out of view of their predators. These defence mechanisms have allowed it to become a highly successful and conspicuous predator in the coral reef ecosystem.
The Blue Sea Slug
Our third and final mollusc is The Blue Sea Slug, Glaucus atlanticus, commonly found in the Pacific, Atlantic and Indian Oceans. Like the Flamingo-Tongue Snail, it is small but mighty, reaching a maximum size of only 3cm. This tropical and temperate beauty hides some unique and mind-blowing adaptations. This nudibranch is characterised by a flattened body, consisting of a dark blue underbelly and a silvery-white topside, providing it with a serious advantage considering its unusual lifestyle. The Blue Dragon lives a life floating at the surface of the ocean, able to do so by swallowing and storing air into their gastric cavity, allowing them to float almost effortlessly. The gentle movement of their protrouding horn-like structures, known as cerata (see photo below), enable them to change direction when searching for prey or for a mate. Their blue bellies provide camouflage from marine predators whilst their silvery topside provides protection against aerial predators, a prime example of countershading, indicating that these small creatures are preyed upon by animals with good vision.
Contradictory to its size, this species is an insatiable predator with unusual prey choices, namely hydrozoan species, including Physalia utriculus: The Portugese Man-O-War. Like the Flamingo-Tongue Snail, the Blue Dragon can avoid fatality from consuming these toxins by means of a protective layer of specialized cells, lining their skin, and the inside of their mouths, and by secreting large amounts of mucous to avoid being stung. The vast majority of nematocysts consumed are not digested, and are instead distributed throughout their cerata in a small sack found at their tips called a cnidosac. This allows it to deliver a nasty venomous sting when it encounters prey or predators and is not to be messed with!
Molluscs are a highly underappreciated diverse phylum, exhibiting extraordinary adaptations to survive and thrive in a tropical marine environment. These three species demonstrate only some of the unusual and wonderful ways in which they are able to protect themselves from predators, and must not go unnoticed!