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Of Tiger Shrimp, Flatworms and Ascidians

Sometimes a good memory helps.

Like most divers I have always been interested in nudibranchs & flatworms and, like many divers at times, it became, for a while, a bit of an obsession. In the 1980's I was lucky to be able to volunteer to assist Dr Bill Rudman, who was the head of the Australian Museum's malacology department, with collections and preparation for field collecting trips around my home Coffs Harbour, Australia, which includes the unique Solitary Islands - so observing nudibranchs was a large part of every dive in those years.

At this time my diving mates used to joke - "don't dive with Derek, he spends the whole dive on one rock looking at things the size of a grain of rice". They were right, I had my own little project and was tracking the growth and colour change of a species of aeolid nudibranch for 2 years. The same wall of rock, basically every 2 weeks, weather permitting - starting with juveniles about 3-5mm long. For our group of divers at that time, showing nudibranch slideshows at our BBQs, to non-divers, was a bit like continually showing wedding photographs to people who were not there.

So in July 2011, I was diving "Bolo I" and saw a reddish brown flat worm moving quickly across a small outcrop of reef that was covered with hydroids, sponges, ascidians etc. I had my Nikons V with me and, though the lens was not optimum, I decided to try to take a photo (below). While I was taking the photograph, the flatworm stopped at a small purple / blue ascidian and very quickly consumed it. I was surprised to see the flatworm eat the ascidian so quickly - it seemed to be gone in minutes..

Flatworm (Prostheceraeus sp.) eating a solitary ascidian.

12 months later in July 2012, diving on "Bolo II" - this time equipped with a digital camera in a housing (Nikon D300 with 60mm lens in a Subal ND-30 housing with Inon Z240 strobes). We came across what looked like a slightly battered Spiny Tiger Shrimp (Phyllognathia ceratophthalmus) on a purple blue ascidian. I set up for a photograph and spent some significant time just looking at what it was up to through the viewfinder.

To my surprise, out of the side of the viewfinder's frame came the same species of flatworm I had seen eat the ascidian twelve months before. So I sat down to watch what would happen through the camera.

Would the flatworm try to eat the ascidian the Spiny Tiger Shrimp was sitting on and, if it did, what would be the reaction of the shrimp.

SpinyTiger Shrimp (Phyllognathia ceratophthalmus) and intruding flatworm (Prostheceraeus sp.) on the ascidian.

The flatworm went straight for the ascidian, moving straight under the shrimp as if it was not there. Initially the shrimp moved off a slightly but then came back to disturb the flatworm which then also moved off a bit.


Eventually the shrimp gave the flatworm what looked like a bit of a nip and the flatworm moved on.

The Spiny Tiger Shrimp again took up its earlier position on the ascidian.

This dive lasted 98 minutes in 8m of water. Most of the time I spent in one place watching the action on a single ascidian. The print out of my dive computer is a classic. The ascidian survived another day.

Often when photographing underwater, particularly macro, you are really capturing animals in their habitat, feeding and living their normal lives.

Many flatworms feed on ascidians, so when you see one moving across the reef, it is worth taking a little time to see where it is going and what it is doing.

On the last trip to Nth Sulawesi, in July this year (2017), I again photographed a flatworm this time a different species and feeding on a different species of ascidian. You can almost see that the ascidian is not happy (if this is possible) when you look at its puckered appearance compared to those below.

Racing Stripe Flatworm (Pseudoceros bifurcus) feeding on a social truncate

Ascidians have been found to contain chemicals that are thought to provide them with a form of toxic chemical defence. Research has shown that those specific flatworms and nudibranchs that have adapted to feed on these ascidians, have also absorbed these chemicals and potentially transferred then into their own bodies as part of their own chemical defence system. This is similar to what many chromodorid nudibranchs do with the toxins from sponges and also what aeolid nudibranchs do with the stinging cells (nematocysts) from hydroids. In some cases the presence of a chemical defence means that camouflage can be dispensed with in favour of bright warning colours.

The nudibranch Crested Nembrotha (Nembrotha cristata) also feeds on ascidians and potentially utilises chemicals absorbed from the ascidians in a similar chemical defence system as the flatworm above may do. Neither could be said to be cryptic or depend on camouflage for defence.

On the same dive that I photographed the flatworm feeding I was able to photograph and observe this Crested Nembrotha nudibranch appearing to feed on a hydroid. It may be that it was just positioned in that specific area at the time - but it raises the question as to whether they also periodically feed on hydroids to enhance their inbuilt defences as do some other species of nudibranch? It is something for me to watch for on future dives to see if I can again observe their immediate proximity to these particular hydroids and if the evidence of them actually feeding on the hydroid can be definitively observed.

Nembrotha cristata possibly feeding on hydroids.

When you get older, you really appreciate the magnification that an SLR camera lens provides.

I hope that this short background story behind the photographs has been of interest. Seeing something is one thing, but being able to take the time to watch and observe it can often be even more rewarding. Sometimes small observations over time can build up a story.



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