June 27, 2002
From: Edwin Cruz-Rivera
Concerning the Aplysia inking message.
P.M. Johnson and I did a number of field experiments with Stylocheilus striatus and Aplysia parvula and found no differences in the susceptibility of sea hares with or without ink. Manually "de-inked" sea hares (no hooks, Bill) were either consumed (Stylocheilus) or rejected (Aplysia) with equal frequency as conspecifics with a full complement of ink. Still, we think ink has a defensive purpose, just not in the conventional way that one usually thinks. It might serve to distract the predator rather than simply repell it. P.M. presented our results recently in the meeting of the AChemS but I don't want to spill the beans too much here. The work will be submitted to publication soon.
As for anemones and lobsters as sea hare predators, I have seen P.M.'s videos of Metridium gobbling Aplysia and I have no doubt in my mind that they can do it in the field. The fact is one hardly observes predation in the field for most organisms. This does not mean it does not occur. Out of hundreds of day and night dives, I have only seen one episode of predation on sea hares: a portunid crab (Thalamita sp.) feeding on a Dolabella. However, if I bring crabs and sea hares to the lab, these crabs will eat sea hares even in the presence of alternative foods. If one considers how voracious, generalized and abundant some of these larger crustaceans are, it is not hard to envision crabs and lobsters as sea hare nemeses.
firstname.lastname@example.orgCruz-Rivera, E., 2002 (Jun 27) Re: Inking in Aplysia. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/7373
Thanks for the input. I look forward to reading of your research results when they are published. I couldn't agree more with your comments about the difficulty of getting information on predation. It's why I started up the Predation Records page on the Forum. While I do not dispute that in certain places, large sea anemones and large crustaceans may eat sea hares, in many parts of the world there are not large sea anemones in areas sea hares are common, and from my experience - not backed by any rigorous sampling - large sea hare populations can be found in places where large crabs abound.
The other point that needs to be considered when considering the function of the purple gland is its phylogenetic history. The purple gland is an 'ancient' organ which Sea Hares have inherited from primitive shelled ancestors. They are represented today by the family Akeridae. Although its present function may have changed, the purple gland evolved in shelled ancestors. It is not therefore a special organ which has evolved to meet the defensive requirements of an effectively shell-less slug. It may be useful then to look at this gland in Akera to get some clues to its function. Certainly the gland does not protect Akera from being eaten by a variety of bottom-feeding fish. An interesting observation was made by Thompson & Seward (1989) when studying populations of Akera bullata in England. They noted that normal populations produced purple ink but populations of a dwarf form [A. bullata nana] did not. It would be interesting to know if this form lacked a purple gland or perhaps fed on algae without the chemical constituents required to produce purple secretions.
This is a fascinating topic. If you would like to keep us up to date on developments, it would be much appreciated.
• Thompson, T.E. & Seaward, D.R. (1989) Ecology and taxonomic status of the aplysiomorph Akera bullata in the British Isles. Journal of Molluscan Studies, 55: 489-496.
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