December 22, 2001
From: Mary Jane Adams
Since Gymnodoris nigricolor has only been reported from the Ryukyu Archipelago in southern Japan, I was very surprised to find it in Morovo Lagoon, Solomon Islands. I had already made a dozen dives at this site before spotting the two slugs on the goby's fins. They seemed to be very securely attached to the fin rays. The slug on the
dorsal fin was tossed from side to side every time the goby flicked it's fin. The one on the pelvic fin was dragged across the sand whenever the goby changed position. When disturbed, the goby seemed to bolt into it's burrow as rapidly as any other.
Location: Wickum Island, New Georgia group, Solomon Islands. Depth: 13 meters, 16 November 2001
Goby: Amblyeleotris steinitzi
Shrimp: Alpheus djeddensis
firstname.lastname@example.orgAdams, M.J., 2001 (Dec 22) Full house - goby, shrimp & Gymnodoris. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5867
Dear Mary Jane,
This is indeed a wonderful Christmas present. Not only does it show that G. nigricolor may be widespread in the Indo-West Pacific, but your description and excellent close-up photos help us to understand the nature of the relationship between nudibranch and fish.
Although it has been suggested that they feed on the fin tissue, I could find no sign of damage on any of the fish I have examined. From your photos it seems that the the Gymnodoris, for some reason, firmly attach to the fish by grasping the fins with their buccal apparatus. In your photo you can clearly see the buccal bulb extended out enveloping one or more fin rays. My feeling is that rather than eating the fins, these Gymnodoris are using the fins as a relatively easy part of the body to attach to. This doesn't of course answe the question of what they eat.
I can offer no suggestion about why they have evolved such a relationship and guess we will only properly understand it when we determine just what the Gymnodoris feed on. It is a fascinating relationship.