March 2013, Halmahera, Indonesia ~ Oh no! Right before my eyes, my beloved benthic ctenophores, so delicate and colorful, have metamorphosed from gentle plankton netters to smothering killers of fishes and crabs! Drifting over a black rubble slope off Makian, our guide Yann Alfian points out a ctenophore-covered starfish. During our October trip around Batanta aboard the Dewi Nusantara, Yann asked me why I was spending so much time looking at these things on the starfish. I explained that these colorful invertebrates that look like flatworms are related to the comb jellies that we see floating by in the water column. In the past year, I have found benthic ctenophores on at least four different species of starfish and on several corals. Each one has two feeding tentacles that they cast out to catch passing plankton. I think the delicate tentacles drifting in the current are beautiful and dreamy, so I shoot video whenever I see them (see my previous post and video, “Ctenophores Galore“). “I don’t show them to the guests because I don’t think they find them interesting,” is Yann’s understatement.
Here on Makian, the starfish is a different color from others I’ve recorded, so I settle to video the ctenophores casting their feeding tentacles into the current. I hear Yann tapping and look up to see his hand signal, “Ghost pipefish” and signal back that I’ll be there in a minute. To my astonishment, in the few seconds that I looked away, a ctenophore has trapped a tiny fish, which is now struggling for its life. I’ve been obsessed with ctenophores for the past year but always thought they ate microscopic zooplankton, so I am shocked as I watch one entangle and slowly engulf the fish – shocked but excited, of course, to witness something so unexpected. This prompts me to go back and examine photos and video from our trip to Bali last year and sure enough, some clearly show lumps – fish perhaps?
Over the next two days, I seize every chance to visit the rubble slope, which is just below a dry riverbed, so the organic matter deposited after rain makes the site rich with crabs, shrimp and many small fishes. The starfish, about a half dozen, travel rapidly around the bottom in bursts that last about 15 minutes and during those times, I see the ctenophores snag two crabs, another fish and yank a poor tube worm right out of its tube. After giving this some thought, it makes sense, since the benthic ctenophores’ pelagic cousins are dominant carnivores in the zooplankton food chain. Many other divers in our group, forced into ctenophore-awareness by my obsessive chatter, also witness the action and Ned gets a great series of photos of yet another fish in the grips of the ctenophores.
At lunch, we discuss the many ways one could meet one’s end in the sea and decide a smothering death by ctenophore would be one of the worst – we are happy these creatures are small. Back home, we asked Dr Gerry Allen to help identify the fish: possibly Limnichthys nitidus, a sand burrowing fish that lives in dense populations in shallow, sandy bottoms and dart quickly when disturbed. Dr. George Matsumoto, a ctenophore expert, has kindly provided reference material that is serving to fuel the ctenophore flame, so stay tuned for more ctenophore excitement (video is in the works).