These benthic herbivorous creatures can become rather large compared with most other mollusks. They graze in tidal and subtidal zones of tropical waters, mostly in the Indo-Pacific Ocean (23 species); but they can also be found in the Atlantic Ocean (12 species), with a few species occurring in the Mediterranean.
Aplysia species, when threatened, frequently release clouds of ink, it is believed in order to blind the attacker (though they are in fact considered edible by relatively few species).
Following the lead of Eric R. Kandel,[1] the genus has been studied as a model organism by neurobiologists, because its gill and siphon withdrawal reflex, as studied in Aplysia californica, is mediated by electrical synapses, which allow several neurons to fire synchronously. This quick neural response is necessary for a speedy reaction to danger by the animal. Aplysia has only about 20,000 neurons, making it a favorite subject for investigation by neuroscientists.[2] Also, the 'tongue' on the underside is controlled by only two neurons, which allowed complete mapping of the innervation network to be carried out.
Long-term memory
In neurons that mediate several forms of long-term memory in Aplysia, the DNA repairenzymepoly ADP ribose polymerase 1 (PARP-1) is activated. In virtually all eukaryotic cells tested, the addition of polyADP-ribosyl groups to proteins (polyADP-ribosylation) occurs as a response to DNA damage. Thus the finding of activation of PARP-1 during learning and its requirement for long-term memory was surprising.[3] Cohen-Aromon et al.[3] suggested that fast and transient decondensation of chromatin structure by polyADP-ribosylation enables the transcription needed to form long-term memory without strand-breaks in DNA. Subsequent to these findings in Aplysia, further research was done with mice and it was found that polyADP-ribosylation is also required for long-term memory formation in mammals.[4]
In 2018, scientists from the University of California, Los Angeles, have shown that the behavioral modifications characteristic of a form of nonassociative long-term memory in Aplysia can be transferred by RNA.[5]
Operant conditioning
Operant conditioning is considered a form of associative learning. Because operant conditioning involves intricate interaction between an action and a stimulus (in this case food) it is closely associated with the acquisition of compulsive behavior.[6] The Aplysia species serve as an ideal model system for the physical studying of food-reward learning, due to "the neuronal components of parts of its ganglionic nervous system that are responsible for the generation of feeding movements."[7] As a result, Aplysia has been used in associative learning studies to derive certain aspects of feeding and operant conditioning in the context of compulsive behavior.[8]
In Aplysia, the primary reflex studied by scientists while studying operant conditioning is the gill and siphon withdrawal reflex. The gill and siphon withdrawal reflex allows the Aplysia to pull back its siphon and gill for protection. The links between the synapses during the gill and siphon withdrawal reflex are directly correlated with many behavioral traits in the Aplysia such as its habits, reflexes, and conditioning. Scientists have studied the conditioning of the Aplysia to identify correlations with conditioning in mammals, mainly regarding behavioral responses such as addiction. Through experiments on the conditioning of the Aplysia, links have been discovered with the synaptic plasticity for reward functions involved in the trait of addiction within mammals. Synaptic plasticity is the idea that the synapses will become stronger or weaker depending on how much those specific synapses are used. Conditioning of these synapses can lead them to become stronger or weaker by causing the neurons to fire or not fire when influenced by a stimulus. The conditioning of behavioral traits is based on the idea of a reward function. A reward function is when a stimulus is conditioned to fire according to a certain stimulus. The neurons will adapt to that stimulus, and fire those neurons more easily, even if the stimulus has a negative effect on the subject (in this case the Aplysia). In mammals, the reward function is mainly controlled by ventral tegmental area (VTA) dopamine neurons. During conditioning (in mammals), the VTA dopamine neurons have an increased effect on the stimuli being conditioned, and a decreased effect on the stimuli not being conditioned. This induces the synapses to form an expectation for reward for the stimuli being conditioned. The properties of the synapses displayed in the tests on conditioning involving the Aplysia (which has dopamine neurons but not a ventral tegmental area) are proposed to be directly comparable to behavioral responses such as addiction in mammals.[9]
Reproduction
Aplysia are simultaneous hermaphrodites, meaning each adult individual sea hare possesses both male and female reproductive structures that may be mature at the same time.[10]
Aplysia have the ability to store and digest allosperm (sperm from a partner) and often mate with multiple partners. A potent sex pheromone, the water-borne protein attractin, is employed in promoting and maintaining mating in Aplysia. Attractin interacts with three other Aplysia protein pheromones (enticin, temptin or seductin) in a binary fashion to stimulate mate attraction.[11]
Studies of multiple matings in the California sea hare, Aplysia californica, have provided insights on how conflicts between the sexes are resolved.[12]
Self-defense
Aplysia species was once thought to use ink to escape from predators, much like the octopus. Instead, recent research has made it clear that these sea slugs are able to produce and secrete multiple compounds within their ink, including the chemodeterrant Aplysioviolin and toxic substances such as ammonia for self-defense.[13] The ability of the Aplysia species to hold toxins within their bodies without poisoning itself is a result of the unique way that the toxin is stored within the slug. Different molecules essential to the creation of the toxin are accumulated in separate parts of the body of the slug, rendering them benign, as only the mixing of all the molecules can result in a toxic chemical cloud. When the sea hare feels threatened it immediately begins the process of defending itself by mixing the distinct molecules in an additional part of the body used specifically for that purpose. At which point, enzymes within the sea slug begin the process of making the substance toxic, and the mixture is ejected out at the predator in self-defense.
Species
Species within the genus Aplysia are as follows.
This list follows the studies of Medina et al. who established a phylogenetic hypothesis for the genus Aplysia through study of the partial mitochondrial DNA (mtDNA) sequence data of ribosomal genes (rDNA).
Aplysia benedicti Eliot, 1899: synonym of Aplysia dactylomela Rang, 1828
Aplysia bourailli Risbec, 1951: synonym of Aplysia dactylomela Rang, 1828
Aplysia brasiliana (Rang, 1828) mottled sea hare, sooty sea hare (junior synonym of Aplysia fasciata; different geographical populations of the same species): synonym of Aplysia fasciata Poiret, 1789
Howson, C.M.; Picton, B.E. (Ed.) (1997). The species directory of the marine fauna and flora of the British Isles and surrounding seas. Ulster Museum Publication, 276. The Ulster Museum: Belfast, UK. ISBN0-948150-06-8. vi, 508 (+ cd-rom) pp
Gofas, S.; Le Renard, J.; Bouchet, P. (2001). Mollusca, in: Costello, M.J. et al. (Ed.) (2001). European register of marine species: a check-list of the marine species in Europe and a bibliography of guides to their identification. Collection Patrimoines Naturels, 50: pp. 180–213
