Opisthobranch of the Week Data
Aplysia dactylomela is a commonly encountered animal during low spring tides in Okinawa, especially those spring tides occurring at night having a minus value. In researching my opisthobranch records I found that I've never collected a specimen, although I've taken quite a few photographs of the animals as found in various tidal pools. The reason for this is perhaps twofold: first, the animals are of large size, and therefore produce problems of preservation and storage, and second, they are so commonly found here that I haven't been too anxious to collect specimens. The common name, Sea Hare, has been doubtlessly coined due to their superficial resemblance to the prominent ears of rabbits; the "ears" being the slender paired chemosensory structures (rhinophores) found on the dorsal portion of the head. Unlike the nudibranchs which never possess shells as adults, members of the family (Aplysiidae) have a vestigial internal shell. In Aplysia dactylomela the shell is fairly large, heavily calcified, and buried below the parapodia of the mantle.
These large, slow moving animals are frequently found at night in shallow tide pools as well as being exposed to the air if there is not enough available sediments for them to burrow into. I frequently take students on both nighttime as well as daytime field excursions and sometimes students will initially discover, quite by accident during the day, Aplysia dactylomela when they inadvertently step on one of the animals buried in the substrate and discover the bright purple pigment which these animals discharge when disturbed. The pigment is possibly a defense mechanism used to dissuade predators from feeding attempts.
The animals, at least on Okinawa, are typically found in shallow protected intertidal pools or in protected lagoons. In both environments there is the presence of various sea grasses (Cymodocea spp., Halodule spp., Halophila ovalis, Thalassia hemprichii, Zostera japonica) and numerous thallophytes of several phyla. The eggs of Aplysia dactylomela are frequently seen intertidal and down to approximately three meters; they appear as a long tangled string of variably shaded green or brown and look suspiciously similar to certain types of pasta (capelli d'angelo, capellini, fedllini, spaghetti). For a good discussion in pair- and chain-mating in Aplysia spp. see See Bill Rudman's Slug Forum article in response to a query from Erwin Koehler.