Aplysia vaccaria

This sea hare species lives in the northeast Pacific Ocean off California, United States and Baja California, Mexico, including the Gulf of California.[3]

The black sea hare can grow to be very large: the longest recorded specimen measured 99 cm (39 in) when crawling (and thus fully extended), and weighed nearly 14 kg (31 lb).[3]

Unlike Aplysia californica, the body of this species is relatively firm, and the parapodia are joined behind the siphon.[3]

Unlike many other members of the same family and genus, this species is incapable of producing ink.

All Aplysia species are herbivorous. This species eats brown seaweeds and kelp, which give the animal its typically very dark coloration.

Aplysia vaccaria have a life cycle of one year.[4] They grow very quickly, at an average rate of 4.9 grams per day.[5] This high rate of growth makes sense given that Aplysia vaccaria is the largest known species of sea slug.[6] Aplysia vaccaria is a herbivore which feeds on brown algae, which is notable given that its close relative Aplysia californica feeds almost exclusively on red algae.[7] This difference in food sources likely minimizes competition between the two species, which share much of the same habitat. Aplysia vaccaria is found in the intertidal and upper-subtidal zone, meaning that it can survive exposure to air while the tide is out. Aplysia vaccaria is rarely preyed upon by other organisms, which has made it the subject of various experiments testing for a secreted bio-toxin that discourages predation.[7] The large size of Aplysia vaccaria also contributes to its lack of predators. It’s simply too big for many predators to attack or consume.

Aplysia vaccaria is a simultaneous hermaphrodite.[4] Simultaneous hermaphrodites are organisms that have both male and female sex organs that are able to contribute to sexual reproduction at the same time, which is distinct from non-simultaneous hermaphrodites which only have one sex organ present or functional at any one point in time. Being simultaneously hermaphroditic is advantageous because any two individuals are able to reproduce with one another, which facilitates Aplysia vaccaria finding a mate. However, despite having both sex organs functional during reproduction, only one Aplysia vaccaria acts as a sperm donor during the mating encounter.[4] Which individual acts as the sperm donor is dependent upon the relative sizes of the two individuals. The smaller individual most often acts as the sperm donor, while the larger individual acts as the sperm recipient.[4] Because the reproductive role of Aplysia vaccaria is dependent on the relative sizes between a mating pair, one individual can act as both a sperm donor and a sperm recipient during a mating season. It has been suggested that larger individuals are preferentially selected for mating, and they act as the sperm recipient to avoid the competition present between smaller individuals to be sperm donors. This mechanism of size-assortative mating is common among gastropods. There is circumstantial evidence to suggest cross-fertilisation between Aplysia vaccaria and smaller teroir slugs in stagnant rock pools, though this is based on anecdotal evidence and has never been independently verified.

The scarcity of natural predators for sea hares has long suggested to scientists that some kind of toxin must be produced to discourage predation.[4] Unlike most members of its family, Aplysia vaccaria is incapable of producing ink. The absence of this defense mechanism while maintaining a very low level of predation further suggests that Aplysia vaccaria is protected by a secreted toxin effective enough that the production of ink for defense purposes was no longer evolutionarily beneficial. Sea hares form their toxins from compounds they receive from their food source.[8] This is significant because the type of algae the sea hare eats will determine the toxin it produces.[7] Aplysia vaccaria primarily feeds on brown algae, meanings its toxins are derived from this source and are distinct from sea hares such as Aplysia californica which feeds on red algae. Food source also determines the color of the sea hare, which explains why Aplysia vaccaria appears black or dark brown while Aplysia californica appears red.[7] In order to discourage predation by fish, brown algae produces the toxin acetoxycrenulide.[7] During digestion, Aplysia vaccaria extracts this compound and allows it to accumulate in its tissue. Because acetoxycrenulide is highly toxic to fish, its presence in Aplysia vaccaria is sufficient to discourage predation.[8]

• Bebbington A. (1977) Aplysiid species with notes on the Pacific Ocean Aplysiomorpha (Gastropoda, Opisthobranchia). Transactions of the Zoological Society of London 34: 87-147.
• Turgeon, D.; Quinn, J.F.; Bogan, A.E.; Coan, E.V.; Hochberg, F.G.; Lyons, W.G.; Mikkelsen, P.M.; Neves, R.J.; Roper, C.F.E.; Rosenberg, G.; Roth, B.; ScheltemaaghsfaeeedtuquRQA6FSDVYASCTWRDQuvand scientific names of aquatic invertebrates from the United States and Canada: mollusks. 2nd ed. American Fisheries Society Special Publication, 26. American Fisheries Society: Bethesda, MD (USA). ISBN 1-888569-01-8. IX, 526 + cd-rom pp. page(s): 122