[3] The black sea hare can grow to be very large; the longest recorded specimen measured 99 cm (39 in) when crawling (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.

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

A. vaccaria is found in the intertidal and upper-subtidal zone, meaning that it can survive exposure to air while the tide is out.

It is rarely preyed upon by other organisms, which has made it the subject of various experiments testing for a secreted biotoxin that discourages predation.

Aplysia vaccaria is a simultaneous hermaphrodite;[4] it has both male and female sex organs that are able to contribute to sexual reproduction at the same time, which is distinct from nonsimultaneous (or sequential) hermaphrodites, which only have one sex organ present or functional at any one time.

Despite having both sex organs functional during reproduction, only one Aplysia vaccaria acts as a sperm donor during the mating encounter,[4] which is determined by the relative sizes of the two individuals.