December 29, 1998
From: Cory Pittman
NOMENCLATURAL NOTE: Most references to Stylocheilus longicauda (Quoy & Gaimard, 1825) on this page and in the references listed are incorrect and really refer to Stylocheilus striata (Quoy & Gaimard, 1832). See my message and the separate page where the correct identity of these species is discussed. (Added 5 August 1999)
Dear Jussi & Dr. Rudman,
My apology for the late response to this thread. I'm just catching up on things I set aside during my last trip. :)
I've seen drifting individuals of several species over the years (including Gymnodoris okinawa, Philinopsis pilsbryi, Doriopsilla coronata and Cyerce sp.). With one exception, however, I haven't seen any of them with sufficient frequency to be sure that they represent anything more than dislodged animals. The exception is the sea hare, Stylocheilus longicauda, which I've seen drifting many times and, on occasion, in very large numbers.
For background, S. longicauda may well be the most common Opisthobranch in shallow subtidal habitats on Maui [Hawaiian Islands]. Hundreds of animals can often be seen on a single snorkeling excursion and it often exceeds 50% of the opisthobranchs picked up in random samples. Like most pecies, however, it may have a quite patchy distribution.
The most spectacular examples I've observed occurred just west of Hekili Point, Maui in 1994. That section of reef has a broad, shallow platform. Inshore depths are about 1 m and the crest (100-200 m offshore) is only slightly below the zero tide datum. At low tide, it blocks the surf almost completely so the near-shore area is extremely calm. All of the following observations were made at low tide and well inshore of the crest.
Every few days starting on Aug. 1 I'd been working the area at night without recording S. longicauda. On Aug. 20, I recorded 4. The night of Aug. 21, however, I found many hundreds of animals on the bottom and hundreds more drifting in the water column. Drifting animals typically hung head-downward and motionless with the body in a slight arc. Each animal had a mucus string extending from the tip of the foot for several times the length of its body (the mucus was clearly highlighted by my dive light). I next worked the area on Aug. 29 (after a few days of moderately high surf) and found no S. longicauda. Five sessions between Sept. 14 and Sept. 21 yielded only a few animals (all on the 19th). On Sept. 26, I again found large numbers both on the bottom and drifting. I also found many on the bottom and moderate numbers drifting on Sept. 30. After that, I recorded "many S. longicauda on the bottom" on Oct. 5, "several" on Oct. 7 and none during my last visit of the year on Oct. 9. This suggests that large populations not only use this method for dispersal but can be carried both into and out of an area very rapidly.
Most of the drifting animals seen in 1994 were relatively large and I saw many copulating pairs on the bottom. This brings to mind another S. longicauda behavior that I've seen only once (although John Hoover and others have reported it, before): very dense mating aggregations involving thousands of animals that form "windrows" on the bottom. My impression is that these numbers are well in excess of what could be supported by the readily available food supply. That makes me wonder if such aggregations might result when waves or currents concentrate drifting populations into small areas. Perhaps this could be one function for the behavior?
Another observation that might be of interest occurred on May 30, 1992 (this time in the trough just east of Hekili Point). This is an area with somewhat restricted circulation and depths of 1-2 m. If I recall correctly (I'm relying on memory for the details of this one), it was at high tide late in the afternoon during a period of unusually low surf and warm weather. I'd gone snorkeling earlier and was returning to shore when I encountered many hundreds of S. longicauda drifting in the water column just off the beach. I can't be sure (I hadn't noted the resident population in the trough prior to the event) but, due to the timing and conditions, I suspect that they had recently released from the bottom. This is the only time I've seen large numbers drifting during the day. Also, although I didn't have a thermometer, it was my impression that the water in the trough was unusually warm. That's not enough for any conclusions, of course, but it might provide some support for the idea that they sometimes release in response to stress.
I hope this isn't too late to be of use.
firstname.lastname@example.orgPittman, C., 1998 (Dec 29) Re: adult dispersion /drifting Stylocheilus. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/424
There's no time limit on answering queries. Answers and observations are always welcome. I have added a Stylocheilus longicauda page for future reference. I wonder if your observations represent post spawning exhaustion preceeding death? I have never seen Stylocheilus in such aggregations but they remind be of Bursatella leachii in New Zealand which I often found washing up in huge numbers. I presumed that these washed up animals were dying (of old age?) after spawning or were caught in particularly hot weather in mid-summer.
If your Stylocheilus were similarly a great swarm of dying animals, how did they aggregate in the first place? Many aplysiids have been reported in great breeding aggregations. This has led to discussions about migrations to breeding grounds and other behavioural suggestions. Although some species of Aplysia do move inshore as they grow, it seems that most "aggregations" are the result of large numbers of similarly aged animals growing very rapidly to maturity. Their close proximity leads inevitably to much breeding activity and in some cases a huge death toll afterwards. Whether death always occurs so rapidly and to all the population at the same time we don't know. Perhaps as you suggest external forces, like particularly hot weather, or water, can some years cause massive mortality over a very short period - one or two days - rather than a more natural death rate spread over some weeks or months.
Best wishes for the New Year,