Slugs, Snails, And The Things They Eat

July 21, 2014

So many nudibranchs, where to find them? What can we learn about these spineless creatures while we are observing them in the wild? One of the easiest things we can observe while out on the reef photographing these sometimes splendidly colourful seaslugs is the stuff that they eat. It also works the other way around! If you’re looking for a particular type of seaslug, it might be useful to find out what they eat, then look for them in parts of the reef where you are likely to find their food.

The Slender ceratosoma (above & top) is a sponge-feeding nudibranch. While in this photo it might look like it’s feeding on the gorgonian, closer inspection reveals that it’s feeding on sponge fibres at the base of the gorgonian colony. Many species of Ceratosoma store the yucky chemicals from the sponges they feed on in that long recurved dorsal ‘horn’ that acts as a defensive lure attracting potential predators to the part of the animal which contains most of the distasteful chemicals stored from their food (1). Clever slugs.

They may look kind of benign, but Allied cowries or Ovulids, are carnivorous sea snails that feed on coral polyps, that is, the individual animal within the coral colony. Ovulids live on, and eat, soft corals and sea fans, and they are usually regarded as parasites, similar to head lice, that live on the surface of its host. Ewe. Unfortunately for the coral colonies they cannot move themselves to escape, nor have arms to scratch them off their skin. Even if they could, ovulids don’t make it easy, anchoring themselves onto their host with a long and narrow foot. Ovulids are so extremely specialised in their specific food source, that each species actually has adapted mouths that are unique to their host! (2) So even if they wanted to, they can’t be vegetarian.

The little yellow thing sticking off of the hydroid is a special sea slug we call a Doto. There are many species of Doto, and a few difference varieties can be found at Pulau Hantu. You are most likely to find them on hydroids, because that’s what they feed on. Different type of hydoids would likely have a different species of Doto on them, so it also helps to know your hydroids if you’re looking for a Doto on our reef!

If you’ve ever encountered a hydroid, you’ll know that they are nasty creatures. Their sting causes a reaction similar to an electric shock for most humans. Yet these little Dotos that appear so fragile, seem unfazed as they feed upon the surface of their hydroid hosts. Unlike the Ceratosoma (top) and Pteraeolidia ianthina (below) Dotos don’t store the stinging cells (nematocysts) from its food source in its cerata (that is, the frilly or horn-like bits sticking out from all over its body) (3). If you take a closer look at this hydroid, you’d notice that its polyps are yellow. What a coincidence, so is the Doto. Does the Doto choose to feed on a yellow hydroid to get better camouflage? Dotos are actually yellow because of the hydroids they feel on. How weird is that?  It’s like turning yellow if you eat a mango. Thank goodness we’re not translucent. Dotos are. And a part of their gut extends into their cerata, that’s why we see them as yellow, because of all that wonderful yellow hydroid that’s mushing about in its gut. (4)

Pteraeolidia ianthina is an aeolid, like the Doto. Unlike the Doto, it does store those nematocysts in its cerata. So packed with stingers are its cerata that even humans can get stung. See those pretty, purple tips? Jammed packed with stingers. Pteraeolidia feeds on hydroids. So when they feed on the hydroids, not only do they ingest the nematocysts, they also ingest the the micro algae (Symbiodinium) that was living in the tissue of the hydroid. This symbiodinium continues to photosynthesise within the sea slug,  and the sugars produced through photosynthesis also feeds the seaslug (5)! I imagine this gives the seaslug a huge feeding advantage because it can still receive sugar as a food source even when its not feeding. Young Pteraeolidia have fewer cerata than more mature individuals. This may make identification confusing. They are better identified by the two dark purple bands on their antennae.

Pretty red sea snail feeding on a hydroid. What do you make of its bold self advertising on its mono-coloured host?

Glossodoris cinta is a beautiful nudibranch. What do you think it feeds on?

Glossodoris atromarginata is a carnivore, eating corals or sponges (6)

Michelle Ooi, who joined the Hantu Blog for this dive, also shares her experience on her blog. Check out her perspective here: http://underwaterasian.blogspot.sg/2014/07/diving-pulau-hantu.html

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