West Coast Marine Molluscs
Katherine Veness's blog | Created 2 years ago
Western Australia (WA) has a unique and diverse marine fauna. You may have noticed, walking along our beaches, a wide variety of seashells. Who do these shells belong to? And how do they survive?
WAs coastal waters boast many species of mollusc. Representatives are present from a variety of classes and sub-classes, as listed on the World Register of Marine Species (WORMS database), including well-known and less-known groups:
- Bivalvia (clams, oysters, mussels)
- Cephalopoda (squid, cuttlefish, octopus)
- Gastropoda (snails + allies)
- Polyplacophora (chitons)
- Scaphopoda (tusk shells)
Many factors have contributed to high molluscan biodiversity in WA. The enormous coastline, which spans tropical and temperate environments, results in species from the Indo-West Pacific through to cold temperate southern waters, and everything in between.
The warming and cooling Leeuwin and Cape Currents and continental geography affect marine fauna. Geographical effects on the Indian Ocean have changed over time. Today, little sediment flows into the sea due to a lack of mountains, but in the past things were different. Tides, and substrates such as rocks, mangroves, sand, and mud, also affect marine habitats.
What do molluscs eat?
Molluscs feed on a variety of different foods, depending on the species. Common food sources are plankton, algae, other molluscs, sponges, other invertebrates, and fish.
The cone shell is a creature to watch out for. It kills its prey by injecting venom produced by salivary glands. One species of cone shell, Conus geographus, is venomous enough to kill its preferred prey, fish, in an instant.
But a fish isn’t the only thing this venom can dispatch – humans are also at risk! This is why you should never pick up a cone shell if you see one lying on the beach. Watch out for this species at the Abrolhos islands!
The cone shell’s venom is notable for another reason too – medicine. Cone shell venoms contain conotoxins. These help to suppress their prey by targeting ‘receptors and ion channels of excitable tissues’ (Layer and McIntosh 120).
These properties have caught the attention of medical experts, who believe conotoxins may be of use to treat a variety of debilitating medical conditions including ‘pain, convulsive disorders, stroke, neuromuscular block, and cardioprotection’ (Layer and McIntosh 119). Careful and extensive testing must be completed before conotoxins can be used as a therapeutic treatment. Stay tuned!
Mussels: Food or Foe?
The Western Australian coastline is a success story of food production of some of the world’s favourite seafoods, from fish and crayfish to mussels.
According to the Western Australian Fishing Industry Council inc. (WAFIC), the Cockburn Sound mussel fishery is a sustainability success story. Mussels are grown on ropes to minimise the impact on the seabed. Mussels naturally feed on algae, and so mussel farming in the Cockburn Sound helps to preserve seagrass beds, and thus the health of the marine environment (WAFIC).
Not all mussels are welcome in our waters. According to the Department of Fisheries, the Asian green mussel (Perna viridis) has the potential to become a major pest. It is a rapidly growing animal that can easily adapt to new environments. It also poses a potential threat to humans through the accumulation of toxins and heavy metals associated with shellfish poisoning.
Oysters – from pearl to plate
Oysters have played a major role in WAs history, both as a food and a source of the much sought after pearl.
Many consider oysters a delicacy. A plate of oysters offers a rich serving of omega-3 fatty acids, which are essential to human health but not produced by the body. A deficiency in fatty acids is thought to lead to cognitive and neurological impairment, so it’s important to consume them in some way!
Oysters became important to WAs fledgling economy in the 1800s. Once the discovery of pearl oysters became more widely known, people flocked to try their luck at harvesting pearl oysters off WAs north-west coast.
One such man who had already put down roots in the northwest was Charles Broadhurst, and he was quick to try and capitalise on the newly formed industry. Although the pearling industry became a mainstay of WAs economy for many years and built up a rich history, Broadhurst didn’t find success searching for shell, although he is considered a pearling pioneer for trialling the use of the ‘hard hat’ when diving for shell.
Molluscs – from mega to micro
‘Mega’ molluscs are highly sought after by shell collectors and conchologists all over the world. Volutes, cowries and cones are prized additions to shell collections and displays. Many of these species are endemic to Western Australia, making it a hotbed for shell enthusiasts!
One particularly impressive mollusc is Syrinx aruanus, or Australian Trumpet. Growing to a maximum length of 91 cm and weighing up to 18 kg, this sea snail is the largest gastropod in the world.
Compare this monster with the hyper-diverse group of micro molluscs, some of which grow no larger than a few millimetres or even less, and you can begin to appreciate the extreme diversity and variation within the mollusc family.
Solar powered molluscs
Dr. Lisa Kirkendale is the Western Australian Museum’s Curator of Molluscs. She has published a variety of papers on molluscs, and here explains the concept of photosymbiosis.
The giant clam is just one of a number of marine invertebrates, including corals, which have developed a symbiotic relationship with algae. Part of what gives the soft body of the giant clam its beautiful variegated colour is the alga that resides within its soft tissues (called mantle).
When sunlight reaches algal cells, the cells photosynthesise and create energy (carbohydrates). Some of these products are then transferred to the clam to supplement the host’s energy budget.
Giant clams, like most other photosymbiotic organisms, live with soft tissues exposed in often clear waters of shallow tropical reefs. However, other photosymbiotic clams do not expose soft tissues, but have developed a different strategy to capture light.
Corculum cardissa and many other species in the genus Fragum have shell transparencies or windows that permit light to penetrate through the shell to reach algal laden mantle protected within. Truly living solar panels!
Further Information and References
This article is based on a lecture given by Shirley Slack-Smith and further consultation and papers provided by Western Australian Museum Curator of Molluscs Dr. Lisa Kirkendale.
T. Layer, and J.M. McIntosh, 2006. Conotoxins: Therapeutic Potential and Application. Drugs, Vol 4: 119-142.