|Aplysia californica, a typical sea hare displaying inking behavior|
P. Fischer, 1883
The clade Aplysiomorpha, commonly known as sea hares (Aplysia species and related genera), are medium-sized to very large Opisthobranchia with a soft internal shell made of protein. These are marine gastropod molluscs in the superfamilies Aplysioidea and Akeroidea.
The common name "sea hare" is direct translation from Latin lepus marinus, as the animal was called so already in Roman times. The name derives from their rounded shape and from the two long rhinophores that project upwards from their heads and that somewhat resemble the ears of a hare.
Many older textbooks and websites refer to this suborder as Anaspidea. The original author Paul Henri Fischer described the taxon Anaspidea at unspecified rank above family. In 1925 Johannes Thiele established the taxon Anaspidea as a suborder.
However, since the taxon Anaspidea was not based on an existing genus, this name is no longer available according to the rules of the ICZN. Anaspidea has been replaced in the new Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005) by the clade Aplysiomorpha.
The scientific name for the order in which they used to be classified, the Anaspidea, is derived from the Greek for "without a shield" and refers to the lack of the characteristic head shield found in the cephalaspidean opisthobranchs. Many anaspideans have only a thin, internal and much-reduced shell with a small mantle cavity; some have no shell at all. All species have a radula and gizzard plates.
Jörger et al. (2010) have moved this taxon (named as Anaspidea) to Euopisthobranchia.
Sea hares are mostly rather large, bulky creatures. The biggest species, Aplysia vaccaria, can reach a length of 75 centimetres (30 in) and a weight of 14 kilograms (31 lb) and is arguably the largest gastropod species.
Sea hares have soft bodies with an internal shell, and like all opistobranch mollusks they are hermaphroditic. Unlike many other gastropods, they are more or less bilaterally symmetrical in their external appearance. The foot has lateral projections, or "parapodia".
Sea hares are herbivorous, and are typically found on seaweed in shallow water. It seems to be the case that some young sea hares are capable of burrowing in soft sediment leaving only their rhinophores and mantle opening showing. Sea hares have an extremely good sense of smell. They can follow even the faintest scent using their rhinophores, which are extremely sensitive chemoreceptors.
Their color corresponds with the color of the seaweed they eat: red sea hares have been feeding on red seaweed. This camouflages them from predators. When disturbed, a sea hare can release ink from its ink glands, providing a potent deterrent to predators. This release acts as a smoke screen, while at the same time, adversely affecting the smell sensors of their predators. In a small environment, this ink could be toxic to the inhabitants. The color of the ink is white, purple or reddish, depending on the color of the pigments in their seaweed food source. Their skin contains a similar toxin that renders sea hares largely inedible to many predators.
Sea hares are eaten. Called Chinese: 酱爆海兔; literally: "sauce-fried sea hare" (pinyin=jiàng bào hǎi tù) in China, sea hares are typically quick fried in a sauce. In Hawaii sea hares or Kualakai were typically cooked in an imu wrapped in ti leaves. Aplysia californica is a typical example of a sea hare, and is noteworthy for its usefulness in studies of neurobiology because of the unusually large size of the axons that are a part of its nerve cells. See Aplysia gill and siphon withdrawal reflex.
Sea hares are often used as a method of eradicating nuisance algae and cyanobacteria ("red slime algae") in reef aquariums. The hares usually do an excellent job, but when they have eaten all of the algae and cyanobacteria they often shrink from starvation and eventually starve to death. Many reef keeping clubs have started programs where groups of hobbyists "share" a single sea hare among a large group.
- Jörger K. M., Stöger I., Kano Y., Fukuda H., Knebelsberger T. & Schrödl M. (2010). "On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia". BMC Evolutionary Biology 10: 323. doi:10.1186/1471-2148-10-323.
- Fischer, P. (1883). Manuel de conchyliologie et de paléontologie conchyliologique fasc. 6. Paris: Savy. pp. 513–608.
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 376. ISBN 0-03-056747-5.
- Packard, A. (1972). "Cephalopods and Fish: the Limits of Convergence". Biological Reviews 47 (2): 241–307. doi:10.1111/j.1469-185X.1972.tb00975.x.
- For details of locomotion in the Aplysiomorpha, see Bebbington; Hughes (1973). "Locomotion in Aplysia (Gastropoda, Opisthobranchia)". Journal of Molluscan Studies 40 (5): 399–405.
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