# Aplysia

Aplysia (/əˈplɪʒ(i)ə/) is a genus of medium-sized to extremely large sea slugs, specifically sea hares, which are a kind of marine gastropod mollusk.
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, 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. 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 repair enzyme poly 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. Cohen-Aromon et al. 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.
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.

## 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. 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." 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.
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.

## 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.
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.
Studies of multiple matings in the California sea hare, Aplysia californica, have provided insights on how conflicts between the sexes are resolved.

## 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. The ability of the Aplysia species to hold toxins within their bodies without poisoning themselves 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 argus Rüppell & Leuckart, 1830
* Aplysia atromarginata Bergh, 1905
* Aplysia brasiliana Rang, 1828
* Aplysia californica (J.G. Cooper, 1863) California sea hare
** Distribution: Northeast Pacific
* Aplysia cedrocensis (Bartsch & Rehder, 1939)
** Distribution: Northeast Pacific
* Aplysia cervina (Dall & Simpson, 1901)
** Distribution: West Atlantic
* Aplysia cornigera Sowerby, 1869
** Distribution: Indian Ocean, West Pacific
* Aplysia cronullae Eales, 1960: synonym of Aplysia extraordinaria (Allan, 1932) (uncertain synonym)
** Distribution: Southwest Pacific
* Aplysia dactylomela (Rang, 1828) spotted sea hare
** Distribution: Cosmopolitan; tropical and temperate seas.
** Color: from pale gray to green to dark brown.
** Description: large black rings on the mantle; good swimmer
* Aplysia denisoni Smith, 1884: synonym of Aplysia extraordinaria (Allan, 1932) (possible senior synonym)
** Distribution: Indian Ocean, West Pacific
* Aplysia depilans (Gmelin, 1791)
** Distribution: Northeast Atlantic, Mediterranean.
** Description: thin, yellow inner shell
* Aplysia dura Eales, 1960
** Distribution: Southeast Atlantic, Southwest Pacific
* Aplysia elongata (Pease, 1860)
* Aplysia euchlora Adams in M.E.Gray, 1850: represented as Aplysia (Phycophila) euchlora (Gray, 1850) (alternate representation)
** Distribution: Northwest Pacific
* Aplysia extraordinaria (Allan, 1932) (possibly = Aplysia gigantea)
** Distribution: Western Australia, New Zealand.
** Length: more than 40 cm
* Aplysia fasciata (Poiret, 1798) ( Aplysia brasiliana Rang, 1828 is a junior synonym).
** Distribution: East Atlantic, Mediterranean, West Africa, Red Sea
** Length: 40 cm
** Color: dark brown to black.
** Description: sometimes has a red border to the parapodia and oral tentacles
* Aplysia ghanimii Golestani, Crocetta, Padula, Camacho, Langeneck, Poursanidis, Pola, Yokeş, Cervera, Jung, Gosliner, Araya, Hooker, Schrödl & Valdés, 2019
* Aplysia gigantea Sowerby, 1869
** Distribution: Indian Ocean, West Pacific
* Aplysia hooveri Golestani, Crocetta, Padula, Camacho, Langeneck, Poursanidis, Pola, Yokeş, Cervera, Jung, Gosliner, Araya, Hooker, Schrödl & Valdés, 2019
* Aplysia inca d'Orbigny, 1837
** Distribution: Southeast Pacific
* Aplysia japonica G. B. Sowerby II, 1869
* Aplysia juanina (Bergh, 1898)
* Aplysia juliana (Quoy & Gaimard, 1832) Walking sea hare
** Distribution: cosmopolitan, circumtropical in all warm seas
** Color: various, from uniform to pale brown
** Description: no purple gland, therefore no ink secretions; posterior end of the foot can act as a sucker
* Aplysia keraudreni Rang, 1828
** Distribution: South Pacific
** Length: 25 cm
** Color: dark brown
* Aplysia kurodai (Baba, 1937)
** Distribution: NW Pacific
** Length: 30 cm
** Color: dark brown to purplish black, dotted with white spots
* Aplysia lineolata A. Adams & Reeve, 1850: synonym of Aplysia oculifera A. Adams & Reeve, 1850
* Aplysia maculata Rang, 1828
** Distribution : Western Indian Ocean
* Aplysia morio (A. E. Verrill, 1901) Atlantic black sea hare, sooty sea hare
** Distribution: Northwest Atlantic
** Length: 40 cm
** Color: black to deep brown; no spots
* Aplysia nigra d'Orbigny, 1837
** Distribution: Southwest Atlantic, South Pacific
* Aplysia nigra brunnea Hutton, 1875
** Distribution: New Zealand
** Length: 10 cm
** Color: dark brown
* Aplysia nigrocincta von Martens, 1880
* Aplysia oculifera (Adams & Reeve, 1850) spotted sea hare
** Distribution: Indian Ocean; West Pacific; common along the north, east and south coast of South Africa
** Length: 15 cm
** Description: greenish brown, with small brown to black spots with white centres
** Habitat: shallow bays and estuaries
** Behaviour: hides by day; emerges at night to feed on seaweed
* Aplysia parva Pruvot-Fol, 1953
* Aplysia parvula (Guilding in Moerch, 1863) pygmy sea hare, dwarf sea hare
** Distribution: worldwide in warm to temperate seas
** Length: 6 cm
** Color: brown to green spots
* Aplysia peasei (Tryon, 1895) (taxon inquirendum)
* Aplysia perviridis (Pilsbry, 1895)
* Aplysia pilsbryi (Letson, 1898)
* Aplysia pulmonica Gould, 1852
* Aplysia punctata (Cuvier, 1803)
** Distribution: NE Atlantic
** Length: 20 cm
** Color: very variable
* Aplysia rehderi Eales, 1960
** Distribution: Northeast Pacific
* Aplysia reticulata Eales, 1960
** Distribution: Southwest Pacific
* Aplysia reticulopoda (Beeman, 1960) net-foot sea hare
** Distribution: Northeast Pacific
* Aplysia robertsi Pilsbry, 1895
** Distribution: Northeast Pacific
* Aplysia rudmani Bebbington, 1974
** Distribution: Indian Ocean
* Aplysia sagamiana (Baba, 1949)
** Distribution: East Australia, Japan; Northwest Pacific
* Aplysia sowerbyi Pilsbry, 1895
** Distribution: Southwest Pacific
* Aplysia sydneyensis (Sowerby, 1869)
** Distribution: Australia
** Length: 15 cm
** Description: not clearly defined
* Aplysia tanzanensis Bebbington, 1974
** Distribution: Indian Ocean
* Aplysia vaccaria (Winkler, 1955) California black sea hare (possibly ?= Aplysia cedrocensis)
** Distribution: Pacific Coast of California
** Length: very big – up to 75 cm
** Color: black
** Description: no purple ink; huge internal shell
* Aplysia venosa Hutton, 1875 (taxon inquirendum)
* Aplysia vistosa Pruvot-Fol, 1953

### Species brought into synonymy

* Aplysia aequorea Heilprin, 1888: synonym of Aplysia dactylomela Rang, 1828
* Aplysia albopunctata Deshayes, 1853: synonym of Aplysia punctata (Cuvier, 1803)
* Aplysia angasi G.B. Sowerby II, 1869: synonym of Aplysia dactylomela Rang, 1828
* Aplysia annulifera Thiele, 1930: synonym of Aplysia dactylomela Rang, 1828
* Aplysia ascifera Rang, 1828: synonym of Dolabrifera dolabrifera (Rang, 1828)
* 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
* Aplysia cirrhifera Quoy & Gaimard, 1832: synonym of Barnardaclesia cirrhifera (Quoy & Gaimard, 1832)
* Aplysia concava Sowerby, 1869: synonym of Aplysia parvula Mørch, 1863
* Aplysia depressa Cantraine, 1835: synonym of Phyllaplysia depressa (Cantraine, 1835)
* Aplysia dolabrifera Rang, 1828: synonym of Dolabrifera dolabrifera (Rang, 1828)
* Aplysia donca (Ev. Marcus & Er. Marcus, 1960): synonym of Aplysia morio (A. E. Verrill, 1901)
* Aplysia eusiphonata Bergh, 1908: synonym of Aplysia maculata Rang, 1828
* Aplysia fimbriata Adams & Reeve, 1850: synonym of Aplysia dactylomela Rang, 1828
* Aplysia gargantua Bergh, 1908: synonym of Aplysia maculata Rang, 1828
* Aplysia geographica (Adams & Reeve, 1850): synonym of Syphonota geographica (A. Adams & Reeve, 1850)
* Aplysia gilchristi Bergh, 1908: synonym of Aplysia maculata Rang, 1828
* Aplysia gracilis Eales, 1960: synonym of Aplysia fasciata Poiret, 1789
* Aplysia griffithsiana Leach, 1852 synonym of Aplysia punctata (Cuvier, 1803)
* Aplysia guttata Sars M., 1840 synonym of Aplysia punctata (Cuvier, 1803)
* Aplysia hamiltoni Kirk, 1882: synonym of Aplysia juliana Quoy & Gaimard, 1832
* Aplysia hybrida Sowerby, 1806: synonym of Aplysia punctata (Cuvier, 1803)
* Aplysia longicauda Quoy & Gaimard, 1825: synonym of Stylocheilus longicauda (Quoy & Gaimard, 1825)
* Aplysia megaptera Verrill, 1900: synonym of Aplysia dactylomela Rang, 1828
* Aplysia nettiae Winkler, 1959: synonym of Aplysia californica J. G. Cooper, 1863
* Aplysia norfolkensis Sowerby, 1869: synonym of Aplysia parvula Mørch, 1863
* Aplysia oahouensis Souleyet, 1852: synonym of Dolabrifera dolabrifera (Rang, 1828)
* Aplysia ocellata d'Orbigny, 1839: synonym of Aplysia dactylomela Rang, 1828
* Aplysia odorata Risbec, 1928: synonym of Aplysia dactylomela Rang, 1828
* Aplysia operta Burne, 1906: synonym of Aplysia dactylomela Rang, 1828
* Aplysia petalifera Rang, 1828: synonym of Petalifera petalifera (Rang, 1828)
* Aplysia poikilia Bergh, 1908: synonym of Aplysia maculata Rang, 1828
* Aplysia protea Rang, 1828: synonym of Aplysia dactylomela Rang, 1828
* Aplysia pulmonica Gould, 1852: synonym of Aplysia argus Rüppell & Leuckart, 1830
* Aplysia radiata Ehrenberg, 1831: synonym of Aplysia dactylomela Rang, 1828
* Aplysia rosea Rathke, 1799: synonym of Aplysia punctata (Cuvier, 1803)
* Aplysia schrammi Deshayes, 1857: synonym of Aplysia dactylomela Rang, 1828
* Aplysia scutellata Ehrenberg, 1831: synonym of Aplysia dactylomela Rang, 1828
* Aplysia sibogae Bergh, 1905: synonym of Aplysia juliana Quoy & Gaimard, 1832
* Aplysia striata Quoy & Gaimard, 1832: synonym of Stylocheilus longicauda (Quoy & Gaimard, 1825)
* Aplysia tigrina Rang, 1828: synonym of Aplysia dactylomela Rang, 1828
* Aplysia tigrinella Gray, 1850: synonym of Aplysia maculata Rang, 1828
* Aplysia tigrinella Gray, 1850: synonym of Aplysia maculata Rang, 1828
* Aplysia velifer Bergh, 1905: synonym of Aplysia dactylomela Rang, 1828
* Aplysia willcoxi (Hellprin, 1886): synonym of Aplysia fasciata Poiret, 1789
* Aplysia winneba Eales, 1957: synonym of Aplysia fasciata Poiret, 1789