Meeting of the Biodiversity and Ecology Journal Club
Tuesday April 18, 2006
4:00 pm - 5:00 pm
DBS Conference Room, Department of Biological Sciences, NUS
"Nudibranchs: successful but unsung globe-trotters"
Richard Willan
Research Scientist,
Museum and Art Gallery of the Northern Territories (MAGNT),
Darwin, Australia
Host: Dr. Tan Koh Siang
About the talk - Nudibranchs are highly visible components of marine ecosystems. However most people don't realise how many of them are not native to the countries where they presently occur and have been transported around the globe by humans. In fact the original distribution of some of them will never be known with certainty. This lecture considers why some nudibranchs and not others are amenable to transportation by shipping and why nudibranchs, unlike other molluscs, never achieve the status of "pests" in their new countries and so go unnoticed.
About the speaker - Richard Willan is Curator of Molluscs at the Northern Territories Museum in Darwin, Australia. He has wide-ranging systematic interests in nudibranchs and bivalves. He has studied molluscs for more than 40 years and joined the MAGNT in 1992. He is an experienced diver and has carried out field work in much of Australia, also in Vanuatu, Papua New Guinea, New Zealand, North America and Europe. He is a Past President of the Malacological Society of Australasia and Life Member of the Conchology Section of Auckland Museum. He has authored or co-authored four books, over 100 scientific papers, and numerous popular articles.
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My duck managed to catch a portion of this talk despite severe weather conditions that threatened to detain him amidst a centenary of campus celebrations. But happily, Siva was able to drop me off below the department. It was one of those days when I ventured outdoors without a watch, and worse, minus a pen and spare batteries. So I waddled breathlessly into the conference room while the talk was about a third through, and plonked my damp duck on one of the chairs close to the front row. The unfortunate lady beside me was disturbed by a blatant request for a writing utensil, which she finally produced after rummaging through her bags. The pen was florescent pink, just like my duck!
By the time I arrived, Dr. Willan was already wrapping his introduction to those colourful sea slugs better known as nudibranches. Basically, these are shell-less molluscs in the suborder Nudibranchia, order Opisthobranchia, which utilise camouflage, or more commonly nasty stinging cells and unpleasant flavours to ward off hungry seafood-seeking ducks. I am sure there are lots of other interesting facts about these creatures, but what I can recall at the moment is that Dr. Willan believes those species that are able to retract their gills (nudibranch means 'naked gills') fully into their mantle (the cryptobranch dorids) are monophyletic, while those who do not do so (the phanerobranch dorids) are paraphyletic. Or was it the other way around?
Dr. Willan also showed some pretty pictures of a thing he calls the Spanish dancer. It looks like something that Catherine Zeta-Jones wore to the ball in The Mask of Zorro and I gathered that apart from its ability to dance a mean samba, this undulating wave of scarlet is significant for some other reason that I just can't remember. Something to do with its morphological symmetry, I believe. Nudibranches are also hemaphrodites, meaning they have both ducks and .....
My cold, wet and hungry duck also learnt that there is virtually nothing edible in the sea that nudibranches do not eat. Because nudibranches are the hot-rods of the molluscan world (excepting those flyboys the cephalopods), cruising at speeds that far exceed a limpet's pace, they run on heavy fuel. So while smaller relatives may thrive on mere algae, most nudibranches subscribe to the Atkins diet of protein-rich meals such as sponges (including tough, calciferous species), hydroids, bryozoans and other cnidarians such as anemones and jellyfish.
One pelagic species, Glaucus, cruises the high seas gobbling up Portugese men 'o war colonial jellyfish. And instead of being digested, the cnidarian's stinging nematocysts are separated in the nudibranch's body and reextruded to serve as protective organs. Some nudibranches specialise in feeding on other species of nudibranches, utilising the gory procedure of chewing a hole in the side of the prey animal and sucking out the contents to leave behind a sorry sack of skin. Others dedicate themselves to devouring the caviar of fellow nudibranches.
Dr. Willan also mentioned that recently, a new species of colourful nudibranch with pretty black and yellow spots was discovered in Singapore waters. However, this slimey finding should by no means result in any unnecessary disruption of budgetary meetings, especially since marine biology and its obtuse branches such as the study of fouling species, should never impose its concerns on greater affairs of state. Many other species of nudibranch plague local waters, such as Tritonia bollandi and a selection can be seen here, here and here.
Moving on, Dr. Willan also happily revealed that he was the key villain responsible for the deaths of countless marine life from big barramundi to bite-size bivalves at a marina in the coastal city of Darwin, Australia. Before Ria could throttle him, he explained that the mitigating factor was a small shellfish called the black-striped false mussel (Mytilopsis sallei). This creature belongs to the same family as Dreissena polymorpha, aka the zebra mussel that has wrecked havoc on the ecology and economy of the Great Lakes region in North America.
Cullen Bay Marina is a posh seaside neighbourhood flanked by expensive villas, shining yachts and topless Sheilas. Due to Darwin's phenomenally high tides (up to 8 metres), a lock is used to maintain the water level in the marina. This physical barrier between the open sea and the marina created an artificial estuary habitat of sorts, providing a home for brackish water animals such as archer fish and barramundi. In 1999, a team led by Dr. Willan found that boats and other hard substrates in the marina were heavily infested by M. sallei. The species is not native to Australian waters and it was feared that the mussels would spread along the country's coast, disrupting native ecosystems as well as causing severe fouling problems on seawater intake pipes, marine vessels and harbour facilities.
The discovery was declared a national disaster (and a state cabinet budgetary meeting disrupted in the process) and the marina and its fleet quarantined. As M. sallei has a high tolerance for various salinity levels, freshwater flooding was not a solution. Dr. Willan thus opted for the "Duke Nukem" approach: adding six metric tons of copper sulphate to eradicate all the mussels (and other life) in the marina. Though drastic, the measures were effective and after the treatment period, marine life in the marina enjoyed a recovery. The authorities also put in place strict shipping rules to keep vessels with fouled hulls (particularly those from Indonesian waters) away from Australian waters, as such ships are suspected to be the primary source of the infestation.
Another innocuous-looking invader in Australasian waters is the Asian Bag Mussel (Musculista senhousia ). Native to the north-western Pacific, ballast water from ships is believed to be the dispersing force that brought vast beds of this bivalve (up to 472 animals per 0.1 square metre) to the southern seas, displacing native benthics and seagrasses. I note with some pleasure that diving ducks are known to feed on this pest.
In Tasmania, a prolific filter-feeding gastropod from New Zealand called the Screw Shell (Maoricolpus roseus) has firmly established itself, thanks to the lack of native predators equipped to tackle its hard shell. Dr. Willan believes the snail was inadvertently introduced earlier last century when oysters from New Zealand waters were imported to replenish dwindling Australian stocks.
I know the above has little to do with nudibranches, but in the final leg of his talk, Dr. Willan returned to his original trail of slime and discussed the observation that many species of nudibranches have taken advantage of shipping routes to become globe-trotters. For instance, the South African nudibranch Polycera capensis has reached Australia, probably via bryozoan colonies on ship bottoms. Dr. Willan noted that 19 mollusc species are known to have been introduced to Australia. Fourteen are nudibranches, while the rest are mainly mussels. However, there has been no observed negative effect on local ecosystems by the naturalised nudibranches. In other words, the nudibranches are invasives, but not pests, unlike many other molluscs. But why not?
To this duck, one possible explanation could be that the troublesome invasive molluscs tend to be generalists: filter-feeders and detritius munchers with high rates of reproduction as befits species who are grazers and mowers by habit. In habitats far from their native waters, the absence of customary predators and parasites removes a population ceiling that would otherwise keep populations in check, allowing them to outcompete local species that occupy the same niches.
Dr. Willan also suggests that food specificity is a limiting factor for nudibranches. Being predators of emphemeral prey, nudibranches probably need to cover much ground in between the discovery of suitable feeding locales. As top predators that also invest much energy into specialised biochemical defenses, their reproduction rates (and larval survival) may also be lower, and time to maturity longer, compared to molluscs that adopt a 'survival by sheer volume' strategy. Thus, the constraint of their ecological roles and selective diet may well contribute to prevent nudibranches from running rampant in new habitats.
Sorry, but that's all I can recall from the talk. By the end of it, my duck was in a state of suspended animation and was probably in such a bad condition that Joe pretended not to recognise me before she ran out of the room shrieking. :P I also failed to notice any tall, Beng-like ang moh or snake-eating E.R. doctors who might have helped to examine my duck before it expired in a mouldy heap at the back seat of a taxi that took far too long to arrive.
Additional references:
www.marine.csiro.au/CRIMP/reports/CRIMPTechReport22.pdf
www.marine.csiro.au/crimp/Reports/Infosht7_Musc0700S3.pdf
good summary. thank you
i enjoyed it :)
it's good to know mussels are not the only stowaways and that there are benign and comparatively "positive" illegal immigrants too :P
Posted by: monkey | 18 April 2006 at 11:11 PM
that's already quite alot that you covered. very interesting read.
so now i know about other bad mussels besides the one on my dinner plate :P darwin used to have great abundance of prawns! hmm ... that was 10years ago? hope they are not impacted as i do hope to go for prawn fest in the near future hehehe ...
Posted by: slurp! | 19 April 2006 at 11:37 PM