Written by Dr. Carsten Lüter, Museum für Naturkunde, Berlin.
The deep sea is full of weird looking creatures which through the course of evolution have adapted to the inhospitable conditions of their environment such as eternal darkness, low temperatures, high pressure and the erratic availability of food. We biologists onboard the SONNE are particularly interested in collecting and describing this deep-sea biodiversity to answer questions of species distribution in deep waters and their potential dispersal barriers. But we have one problem, which is best described by citing the famous protagonist of the US-movie Forrest Gump: “My mom always said life is like a box of chocolate…you’ll never know what you get!” This is exactly what happens if you send collecting gear 4000m down to the floor of the Indian Ocean. You have absolutely no idea what will show up on board once the sample has been lifted from its dark and cold surroundings. And sometimes even as a trained marine biologist you are completely lost when it comes to identifying the creature you just collected from the deep. This happened to us a few days ago, when the dredge brought a single stone, a few skeletal needles of a glass sponge and a pinkish, knobbly unknown creature back on deck.
This animal puzzled us for the whole day, speculations were going all across the animal kingdom from cnidarians to molluscs to sea cucumbers and back, but nothing was really conclusive. Since the animal was about 10 cm in diameter, it was quite unusual that we were even unable to tell the orientation of it. Where are front and back? Where is the head? Does it have one at all? Or is it probably just a part of an even larger creature with the main body still being down in the deep? We had no clue. But the essence of all our speculations was that it is most likely a sea cucumber…we couldn’t be more wrong! I sent a message with photos of the animal to a former PhD-student of mine from Indonesia, who is a specialist in sea cucumber taxonomy and she immediately came back to me with “I have never seen something like it” – so this seemed to be a dead end. The next morning one of us suggested that we should start all over again and said “what if this is a cnidarian after all?” This obviously triggered the right choice of keywords for an internet search and guess what: It is a cnidarian! Thanks to a paper published in 2016 by Chinese colleagues describing a new species of an aberrant deep-sea anemone (Li & Xu, 2016), we could clearly identify our mysterious animal as a fully contracted specimen of the genera Phelliactis or Paraphelliactis. The two genera can only be distinguished based on fine anatomical details, but have also been synonymized in the past. These are typical deep-sea creatures which are wide-spread and obviously common in all oceans although rarely reported from deep-sea samples. They live on the vast sediment plains, but need a piece of hard substrate to anchor their disc-like foot. This substrate can be little rocks, but also skeletal needles of glass sponges like the ones we found in the same dredge. Their tentacles are short but the tentacle basis is a half-folded large oral disc, giving the anemone the appearance of a Venus fly trap.
This is an interesting find, since the only reported Phelliactis species from the Indian Ocean have been found off Somalia and off Sumatra during the German Deep-Sea Expedition with the “Valdivia” in the late 19th century. Probably we have found a new species here, but this remains to be confirmed until we’re back home. Comes in handy that the original or type material of the two already described species from the Indian Ocean are stored in Berlin’s Museum für Naturkunde. Every sample we collect from the deep can contain this kind of weird and unusual creatures and, therefore, every time the dredge or any other collecting gear comes back onboard, we biologists get very excited. It’s like being on safari and desperately hoping for an exquisite sight of a spectacular animal around the next corner. That’s the real fun part of these deep-sea cruises.
Li Y, Xu K (2016) Paraphelliactis tangi n. sp. and Phelliactis yapensis n. sp., two new deep-sea species of Hormathiidae (Cnidaria: Anthozoa: Actiniaria) from a seamount in the tropical Western Pacific. Zootaxa 4072 (3): 358-372.