By Steve Gillespie
Have you ever heard someone say “Look at that slug!”?
As divers, we hear it all the time, mostly through the sound of our bubbles and often after a dive over a few beers. In truth, we do get to see lots of slugs – sea slugs, that is. There are lots of them out in the ocean and they come in all different shapes, sizes and colours.
They’re not like your common “garden variety” of slug. We have pretty ones, ugly ones, big ones, little ones, some with stripes and dots and multi-coloured. Even some that look like tiny, little red rubber gloves. Others are see-through, while some just look like gelatinous blobs. These creatures are all related to each other in some way, although I bet they don’t always admit it.
All nudibranchs are sea slugs but not all sea slugs are nudibranchs. Confused? Join the club.
One of the best places to see beautiful sea slugs is Port Stephens and Nelson Bay but our “home patch” around Shellharbour has some outstanding nudibranchs as well. They can be found in shallow water, including local rock pools as well as down to relatively deep areas along the coast only accessible to scuba divers.
You are what you eat
Nudibranchs are carnivorous. Their prey includes sponges, coral, anemones, hydroids, barnacles, fish eggs, sea slugs and other nudibranchs. They are picky eaters with individual species or families of nudibranchs eating only one kind of prey/food and they get their bright colours from the food they eat.
Like land slugs and snails, they emit slime as they move. They can emit chemicals into this slime to communicate with other nudibranchs, either to inform them of danger from predators or to tell them where they are so that they can mate. That adds another dimension to the term “snail trail”, doesn’t it?
'Nudis' have few predators and are only at risk from other nudibranchs, turtles, some crabs and humans. Over years of evolution they have discarded their protective shell and so rely on other forms of protection and defence to deter would-be predators. Some even retain the foul-tasting poisons of their prey and secrete them as a defence against predators.
Mating, but with a difference
Nudibranchs are simultaneous hermaphrodites, meaning that they possess both male and female sex organs. That would certainly make their sex lives interesting! However, unlike many other hermaphroditic animals, nudibranchs do not self-fertilise. A pair of mating nudibranchs usually fertilises each other’s eggs, which are then laid in coiled jelly-like masses to await hatching.
If you want to enjoy the company of a nudibranch, or even to spot their ribbons of eggs, all you need to do is get out there and have a look. You can start by looking into some nice shallow rock pools and, for the more adventurous, don the scuba gear and go and have a look. Don’t forget the underwater camera. Hope you find lots of them.
Steve Gillespie has been scuba diving for 54 years and is an alumni member of the Professional Association of Diving Instructors (PADI). He lives in the Illawarra with his wife, Jayne, and believes that diving is good for the soul.