Is Aeolidiopsis a good genus?

July 24, 2002
From: Bill Rudman

To accompany the pages I have just posted on Aeolidiopsis ransoni and Aeolidiopsis harrietae it is necessary to discuss what genus these species should be placed in, particularly since Terry Gosliner, described a third species from South Africa, which he placed in the genus Baeolidia, (B. palythoae).

Placing these species in a genus is somewhat problematic. The radula and jaw plates undoubtedly place them in the Aeolidiidae. Pruvot-Fol erected the genus Aeolidiopsis for A. ransoni, citing the general body form, the single row of cerata, the dorsal position of the anus, and absence of cnidosacs, as features distinguishing it from other genera of the Aeolidiidae. Of these characters, the body form as described by her with the foot folded up the sides of the body, was an artefact of preservation, and the absence of cnidosacs, was an error on her part. Despite this, the position of the anus and the way the cerata are arranged, suggest that this genus is justified.

However are the other two Palythoa feeders closely related to A. ransoni or are their similarities a result of their same lifestyle? Neither A. harrietae nor A. palythoae have a dorsal anus, and unlike A. ransoni they both have papillate rhinophores. It is difficult to determine which morphological characters possessed by these Palythoa feeders are related to their development of symbiosis with zooxanthellae, and which characters can be considered of phylogenetic importance. For instance, the relatively small number of cerata arranged in a few sloping rows is unusual in the family. The similarity may suggest close phylogenetic affinity, but it could also suggest convergence of two unrelated species which have evolved similar symbiosis. The papillate rhinophores in A. harrietae are reminiscent of 'Berghia' and the ceratal arrangement could be derived from the Berghia-Spurilla group by the loss of the posterior limbs of the anterior arches. The radula is also similar to that of Berghia.

All three species differ from most genera in having a simple rounded anterior end to the foot, and their jaw plates differ from other genera in having a great elaboration of the anterior flange. This may be functionally related to their tough food, so again it is difficult to know whether this common structure is functional convergence or phylogenetic. Miller (2001), in his recent review of the family considers Aeolidiopsis to be a valid genus for A. ransoni but considers the other two species, rather than being placed in Baeolidia, as proposed by Gosliner (1985), should be placed in a new genus. I think we have to remember the lesson presented to us by the soft-coral feeding genus Phyllodesmium when comparing anatomical similarities and differences. Within the genus we can find most of the external anatomical features which are used elsewhere in the Aeolidioidea to delineate families and genera. In fact until relatively recently most species, of what we now place in the genus Phyllodesmium, were placed in their own monotypic genera, often in monotypic families (Rudman, 1981, 1982). I might be shown to be quite wrong, but I feel that all three Palythoa - feeders are closely related, and so feel it is most appropriate to leave them all the in the genus Aeolidiopsis.

References:
• Gosliner, T.M. (1985) The aeolid nudibranch family Aeolidiidae (Gastropoda: Opisthobranchia) from tropical southern Africa. Annals of the South African Museum, 95(6): 233-267.
• Miller, M.C. (2001) Aeolid nudibranchs (gastropoda: Opisthobranchia) of the family Aeolidiidae from New Zealand waters. Journal of Natural History, 35: 629-662.
• Rudman, W.B. (1981) The anatomy and biology of alcyonarian feeding aeolid opisthobranch molluscs and their development of symbiosis with zooxanthellae. Zoological Journal of the Linnean Society 72: 219-262.
• Rudman, W.B. (1982) The taxonomy and biology of further aeolidacean and arminacean nudibranch molluscs with symbiotic zooxanthellae. Zoological Journal of the Linnean Society, 74: 147-196.
• Rudman, W.B. (1991) Further studies on the taxonomy and biology of the octocoral-feeding genus Phyllodesmium Ehrenberg, 1831 (Nudibranchia: Aeolidoidea). Journal of Molluscan Studies, 57(2): 167-203.

Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Jul 24) Is Aeolidiopsis a good genus?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/7614

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