Tag: mollusc

The dead ugly Dolabella auricularia

nudibranch

The sea slugs, or nudibranchs, include some beautifully frilled and coloured characters, Dolabella, the Sea Hare, is not one of them. Even when alive and looking at its best it has the mottled lumpy look of decay, they are well camouflaged for a life  hidden in amongst the weed. It will make itself known if you accidentally stand on it and it releases a tell-tale reddish purple  “ink”. When they wash up on the beach, after a storm perhaps, the dead Dolabella shrink and dry and their inner ear-shaped shell will split the body from within.

The beached carcass reveals the basic anatomical plan.  At the anterior end there is a pair of tentacular feelers at the mouth and a pair of club-like chemosensory rhinophores set further back. No eyes as such. Sea slugs are hermaphrodites, with a long stalked penis housed inside the head and a female opening to the rear. The penis is seen here as a lump on the right hand side of the head, it connects by an exterior groove to the inhaling siphon, seen here with shell protruding. When mating the sperm travel up the groove to the penis. Water taken into the enclosed mantle cavity is expelled through another siphon in the middle of the basal disc. The gut also empties into this mantle cavity and excreta and ink are all expelled through the posterior siphon.

Dolabella can grow up to 400 mm in length and they make a living as browsing herbivores feeding mainly on macroscopic algae. Like all molluscs the sea slug has highly modified mouthparts, developed here as a radula, which resembles a tiny chainsaw.  The diet of plant material is based on a preference for softer tissues, so the tougher calcareous seagrass blades will be stripped clean of softer filamentous algal growth. These sea slugs may be grooming the seagrass beds and restoring photosynthetic function in  polluted waters where excess nitrogen has resulted in abundant algal growth.

If cleaning the seagrass is not enough to earn our admiration, it seems this species of sea slug can also cure cancer. From its varied diet the animal stores various anti-mitotic chemicals which are used to suppress tumour growth in cancer patients.  And of course the Dolabella and its long filamentous egg mass are edible, apparently semi-cooked with vinegar. Hopefully they escape the notice of local gourmands.

For anyone interested in going down the rabbit hole of sea-slug taxonomy there is forum group seaslugforum.net. This is possibly the best organised forum group I have EVER seen. Photos, description, Q&A, lists of scientific references. Just for a laugh check out the extensive species list, our little seaslug Dolabella auricularia is placed partway down the list under FAMILY Aplysiidae, which is under SUPERFAMILY APLYSIOIDEA, which is under ORDER ANASPIDEA – Alternatively, rather than searching blindly, use the find function on your computer – command+F for macs, or control+F for PC. http://www.seaslugforum.net/specieslist.htm

Sunset on the Fan Shell?

Here lies the remains of the last Fan shell on Deeban Spit.

Possibly.

Over the last two years the number of Fan shells poking out of the sea grass on Deeban Spit at low tide has plummeted. Where once there was a field of a thousand gaping shells, there are now almost none, and those that do remain are empty.

I miss them.

Empty shells on oyster beds

Fan shells are known locally by the more dramatic, but incorrect name of Razor clams. Yet another animal menace just waiting to extract human blood. They are popular in recipes of an Asian flavour, put “razor clam” in your search engine and cooking pops up first, but you will notice that true Razor clams look like old-school cut-throat razors, while our Fan shells look like fans, or perhaps the ear of a donkey. They are all edible regardless of name.

The big shells out on Deeban Spit  are  Pinna bicolour, bivalve molluscs in the genus Pinna, latin for ear. Pinna bicolour is a widely distributed species named for the bright iridescence of the inner shell. It is restricted to the sandy seagrass beds where the larvae are able to settle and feed. Research done in the warm waters of Malasia and the cooler waters of South Australia shows that they grow quickly and reach 20cm in length in the first year. By the second year they reach 26 cm and they are full size in the third year at 35 cm. Lucky Fan shells may go on to live well over a decade, the old ones gather epibiota, just like the hull of a boat.  Eventually they get tired, weighed down and stop moving. Where the seagrass beds remain there is a good chance the Fan Shells will return.

live fan shell survivor
empty fields empty fields

The carnivorous Moon Shell egg mass

Often claimed to be shark poo, these gelatinous croissants made of mucus and a sprinkle of sand are in fact masses of eggs laid by the Moon Shell, a carnivorous snail that feeds on other shelled molluscs. On the sand flats the molluscs include the oysters seen in the photo and the whelks that lie around casually waiting for the returning tide.

If you are a keen observer of sea shells on the sand you will have noticed that some types of empty shells have a tiny neat hole usually drilled at the apex, in the ideal place for threading a necklace. The molluscs that produce these shells have a pair of shells that come together to form a shelter for the soft innards, they are known as bi-valves. The mollusc that made the hole is another type entirely, a gastropod – like the garden snail. The class Gastropoda contains a vast total of named species, second only to the insects in overall number. Moon shell gastropods are able to travel along quite efficiently by extending a big muscular foot. These active mollusc are then able to hunt the less agile bi-valves and anything else that they come across. Moon shells can be carnivorous and they are also known to feed on soldier crabs.

Molluscs have a body plan that  incorporates hardened elements, usually a shell, in the case of an octopus it is a horny beak, while gastropods have a shell and a radula, a tongue structure that looks like a miniature chainsaw composed of many tiny teeth on a band that will easily saw through plant material or drill holes into shells. The moon shell will grab hold of the prey with its foot and locate the precise spot to start drilling with the aid of an acid secretion, the muscle that clamps the two shells together is disabled and the shells can then be prised open and the contents removed.

The moon shell that is commonly found on the Deeban Spit is Polinices (Conuber) sordidus

There is an illegal market for shells collected in Port Hacking with specimens selling for $10 each.