The sea hare belongs to the family Aplysiidae of marine sea slugs that appear very similar to nudibranchs, but do have small softened shell that is mostly covered by the epidermis. The only part of the shell that is sometimes exposed is the mantle cavity which covers the gills and female reproductive organs. Like most gastropods, sea hares are hermaphrodites, with the male organ near the head. Although the mass of eggs are bound to substrate or seagrass in a line or bundle, the larvae are planktonic and could explain how this family of sea slug has spread to all the tropical or lower altitude temperate ocean regions(2). Some species are known to have large flaps near the mantle cavity that can actually be used for swimming. One of the species found in Hawai‘i, Notarchus indicus is capable of jet propulsion to escape predators(1).
During our trip we saw at least 2 species of Aplysiidae, definitely the Dolabella auricularia, identified by the wedge shape posterior end of the slug, and possibly the Aplysia parvula as sometimes the A. parvula is diurnal, although there are 4 other species that have a similar flap shape(1). When Curtis Lee first pointed out a black D. auricularia I had thought it was just a large algae, as it had a very irregular surface and was stationary. When it was picked up I was very surprised, and startled when I accidentally agitated it into squirting violet ink all over my hand. And if any of you want to call me silly for thinking there could be a black algae, Aplysiidae apparently get their color and toxicity if any from the algae and grasses they eat(2). Given that there was also a couple of green D. auricularia that were seen, it is unclear what algae they were eating, as most in the tide pools were white tinged with green and purple tips.
None of the 13 species known to live in the Hawaiian islands are endemic, found on other atolls in the tropical zones. However, the significant difference in size of one species in Hawai‘i to the much larger sized individuals in other populations is suggesting diversification of that species(2). Though most species in Hawai‘i are small, in other parts of the world, some sea hares can mass up to 2 kg.
Kwok, S.F., 2009 (Jan 9) Unknown from Camden Haven, New South Wales. [Message in] Sea Slug Forum. Australian Museum, Sydney.
CP: 40 mm; mottled: Hekili Point, Maui; March 21, 2005.
(1) Pittman, Corey & Fiene, Pauline. Sea Slugs of Hawai‘i. http://seaslugsofhawaii.com/. Website. Febuary 17, 2012.
(2) Rudman, W.B., 2000. Sea Slug Forum. Australian Museum, Sydney. http://www.seaslugforum.net/. Website. Febuary 17, 2012.