Here’s another strange one: A nudibranch that attaches to and eats the fins of shrimp gobies! Shrimp gobies are cool fishes – teaming up with nearsighted burrow-building shrimp, the vigilant gobies act as sentinels, watching for danger, while the shrimp build communal burrows. Life is good out on the sand flat, right? Well yes, until the little nudibranch, Gymnodoris nigricolor, enters the scene. In the photo, the nudibranch is the little black job hanging onto the fin of the goby.
We were diving in Lembeh back in 2000 when Mary Jane Adams told us about this relationship of the nudibranch on the goby and asked if we had ever seen it in Lembeh. She had recently seen it in the Solomon Islands, which was a range extension, since at that time, it had only been reported from Okinawa. We didn’t even know such a relationship existed, but our guide said he had seen it once at a site called Critter Hunt, so off we all went in search of it.
No luck in Lembeh but in 2005, I found it in Fiji (which I think was the first time it had been recorded there) – but of course at the end of a dive and 65′ deep. Ned got a shot but the goby spooked and darted into its hole, taking the nudibranch with it. We weren’t sure if the nudi would still be hanging on when the goby re-emerged but our allowed dive time was over so we couldn’t stay around to find out. I marked the spot with a very distinctive rock and we planned a second dive so a couple of others could photograph it. A comedy of errors ensued – mainly because I didn’t realize there were two large bommies at the site and the tender driver had dropped us on the other one. After a little head scratching, I finally saw the second bommie, found my marker rock and the goby with the attached nudibranch! I got a few seconds of video then Burt Jones showed up and got a couple of shots before the goby freaked again and dived into its hole – we marveled at the strength of the grip the nudibranch had on the goby fin.
In 2009, on a sandy slope in the Florida Islands in the Solomons, I saw another! I was scouting for Ned and by the time he got some photos, the goby decided I was a threat and dived into its hole. The nudibranch hung on during what was a very fast and certainly violent by nudibranch standards, dive by the goby into the burrow.
Bob Bolland’s Okinawa Slug Site has more photos and lists references, including the paper, The First Association of an Adult Mollusk (Nudibranchia: Doridae) and a Fish (Perciformes: Gobiidae) and the 2000, paper that confirms the nudibranch does feed on the goby fins – I think before that, the nature of the relationship was speculation. And the paper has a close-up of the hooked radula of the nudibranch’s mouth, which helps explain why the nudi is able to hang on the way it does. I’ve loaded a little bit of video on our BlennyWatcher YouTube channel so you can see how the nudi hangs on when the goby bolts, or you can click below to watch.