Image shows a gastropod shell, a type of mollusc, against a black background

What are they?

Molluscs are a diverse group of soft bodied, typically shelled organisms, including everything from the more familiar garden snails, oysters, and mussels, to the more peculiar octopuses and sea hares. At more than 200,000 species, molluscs not only make up a large proportion of animal life within our seas (some 23%), but also constitute a large portion of all known eukaryote lifeforms -Plants, Animals, Fungi & microscopic plants, and animals called Protists.

Molluscs are invertebrates- meaning that they lack a spine, however, many do have a shell. The shell that we associate with many molluscs, particularly garden snails, is secreted by a layer on their back known as a mantle, which also functions for respirative and excretory purposes – in some classes this shell is internalised and can act as a buoyancy control. One of two key ingredients in mollusc shells- calcite, is also a principal ingredient in the formation of marble!

They are also considered the most “intelligent” invertebrate. For example, so rich in neurons (or “brain cells”) are octopuses that each arm is essentially a “brain” of its own, capable of responding to stimuli independent of the actual brain!

Ecological importance

In the marine ecosystem, the role of molluscs varies widely. Some of the smaller and/or younger species are considered “low” on the food chain and can be found feeding on algae, phytoplankton, and bacteria (as such, we call them Primary Consumers). Other, larger species, such as the Giant Pacific Octopus, fulfil the role of top (or Apex) predator, and have even been known to feed on mudsharks. In some cases, marine molluscs are filter feeders, meaning that they strain through the water to feed on smaller, suspended organisms. Bivalves, such as clams and scallops, specialise in this mode of feeding. Most molluscs are herbivores, and typically feed on anything from microalgae to kelp, doing so with use of their radula, or “rasping tongue”, a serrated muscle which scrapes away at the food surface. Besides their huge diversity and the myriad of species throughout the marine ecosystem, molluscs provide an important and highly nutritious food source for many marine organisms – so much so that certain species have earned the label “potato chips of the sea” by local fishermen. Without them, many marine ecosystems would collapse. Predators of molluscs can include pufferfish, crabs, seals, and whales. Shelled molluscs, such as mussels, require specialist feeding habits from predators, including “slurpers” like walruses, and the more extreme behaviour adopted by starfish -insertion of its stomach through an agape mollusc shell, before digestive enzymes effectively make soup of the mollusc inside!

Mollusc shells do not only provide protection to the mollusc itself – many other organisms, including hermit crabs, seek refuge from predators within hollowed out shell interiors. Not only this, but some creatures have been known to “hitch a ride” astride a moving mollusc on the seabed, facilitating migrations which may have been too dangerous alone. The structure created by a community of oysters on the rocky shore or sea bed, termed an “oyster reef”, provides a habitat to millions of barnacles and anemones, a nursery to juvenile fish, sustains the diet of wading birds throughout the winter and buffers against coastal erosion. Consequently, oysters are termed “ecosystem engineers” due both the creative outlets and balance they provide marine habitats.

Bivalves, the class of hinged molluscs, are known for their ability to remove pollutants from the sea water. Additionally, the more sedentary molluscs are effective at monitoring water pollution, as they accumulate harmful substances, including heavy metals such as mercury and hydrocarbon products including petroleum, in their gills. Industrial borne chemicals including phosphates and ammonia are filtered by the oyster and converted into less harmful and biodegradable products and in some cases, even incorporated into the bivalve shell. So effective are they at their job (an adult oyster can filter up to 227L of seawater a day – more than the average bathtub capacity), that some have even called them “the kidneys of the sea”. These capabilities have seen initiatives sparked worldwide, to employ oysters as a method of removing contaminants from the seas, commendably so across the U.S., Australia and on the Hong Kong coast.

Molluscs are liable to suffer the effects of climate change and industry. Excess CO2 trapped within the ocean acidifies the seawater and hinders molluscan growth, particularly shell development – calcite, the main factor in shell formation, is readily corroded and dissolved in acidic seawater. This is not only detrimental to the organism itself, but also the whole ecosystem.

Life characteristics

The lifespan of an adult mollusc varies widely: a garden snail could live on average 2-3 years, while the oldest mollusc recorded, the black clam, was found to have lived more than 500 years. Planktonic molluscs can live for as little as a few hours to up to a year – shelled organisms are much more durable and better protected in the water column.

Many molluscs are meroplanktonic, meaning that the plankton lifestage is just a steppingstone on the way to adulthood for them, prior to settling in more familiar habitats such as the rocky shore, in the case of the periwinkle, for example. Molluscs can be hermaphrodites, meaning that it’s possible that just one parent is needed for fertilisation, but in most cases, male and female molluscs release eggs (often 100,000’s) and sperm into the water column where fertilisation occurs. Planktonic offspring are known as trochophore or veliger larvae.

Trocophore larvae are translucent, shell-less and acorn shaped. They range in size from 50 to 200 µ, equivalent to the size of a single strand of hair up to a single pixel on your tv screen! Veliger larvae are pigmented, shelled, and generally have a lobed, fringed profile. They can be a “next step up” from the trochophore larvae or else independent. They range in size from 70 to 400µ, similar to the size of a brain cell up to the size of a grain of salt!

There are holoplanktonic molluscs – those which spend their whole lives in the plankton. The shelled Heteropods, also known as sea elephants, due to their long “trunk”, have developed “fins” which bob around in the water. Similarly, the Pteropods, or sea butterflies, have what are essentially a modified pair of “wings” which wave through the water. Cephalopods, the group including squids and octopus, emerge from eggs as miniature versions of their parents before spending time in the plankton, in what is known as direct development.

By Niamh McCaffrey