Elysia Risso, 1818

Krug, Patrick J., Vendetti, Jann E. & Valdés, Ángel, 2016, Molecular and morphological systematics of Elysia Risso, 1818 (Heterobranchia: Sacoglossa) from the Caribbean region, Zootaxa 4148 (1), pp. 1-137 : 16-18

Elysia Risso, 1818

Actaeon Oken 1815: 305 [non Acteon Montfort, 1810 ] (Type species Aplysia viridis Montagu, 1804 [= Elysia viridis ]), rejected under plenary powers (ICZN 1956: Opinion 417).

Elysia Risso 1818: 375 –376 (Type species: Notarchus timidus Risso, 1818 [= Elysia timida ], by monotypy).

Aplysiopterus Delle Chiaje 1830: 31 (Type species: Aplysiopterus neapolitanus Delle Chiaje, 1830 [= Elysia viridis ], by monotypy).

Rhyzobranchus Cantraine 1835: 384 (Type species: Elysia viridis ( Montagu, 1804) , by monotypy)

Rhycobranchus Herrmannsen 1846 –47 [1846]: 17, error for Rhyzobranchus .

Thallepus Swainson 1840: 250 , 359 (Type species: Thallepus ornatu Swainson, 1840 [= Elysia ornata ], by monotypy).

Tridachia Deshayes 1857: 142 (Type species: Elysia schrammi Mörch, 1863 [= Elysia crispata ], by subsequent monotypy).

Hydropsyche Kelaart 1858: 107 (Type species: Elysia grandifolia Kelaart, 1858 , by monotypy).

Elysiella Verrill 1872: 283 –284 (Type species: Placobranchus catulus Gould, 1870 [= Elysia catula ], by monotypy).

Elysiella Bergh 1872: 201 , pl. 9, fig. 3, pl. 24, figs. 20–25 [non Elysiella Verrill, 1872 ] (Type species: Elysiella pusilla Bergh, 1871 [= Elysia pusilla ], by monotypy).

Pterogasteron Pease 1860: 35 –36 (Type species: Pterogasteron ornatum Pease, 1860 [= Elysia ornata ], here designated).

Thridachia P. Fischer 1880 –87 [1883]: 545, unjustified emendation for Tridachia .

Elysiobranchus Pruvot-Fol, 1930: 230 (Type species: Elysiobranchus mercieri Pruvot-Fol, 1930 [= Elysia mercieri ], by monotypy).

Tridachiella MacFarland 1924: 405 (Type species: Tridachia diomedea Bergh, 1894 [= Elysia diomedea ], by original designation).

Elysiopterus Pruvot-Fol 1946: 39 (Type species: Elysiopterus verrilli Pruvot-Fol, 1946 [= Elysia verrilli ], here designated).

Pattyclaya Ev. Marcus 1982: 17 (Type species: Elysia arena Carlson & Hoff, 1978 , by original designation).

Checholysia Ortea, Caballer, Moro & Espinosa 2005: 512 (Type species: Elysia patina Ev. Marcus, 1980 , by original designation) n. syn.

Diagnosis. Species of Elysia have a differentiated head bearing slender, dorsal rhinophores. Eyes located behind the rhinophores. Parapodia vary in size from narrow folds, barely covering the dorsal body surface, to wide extensions of the body. Body surface typically smooth, sometimes covered with papillae, which can be ramified. Dorsal vessels normally extensively branched, sometimes anastomosing distally. Body color usually green of different shades, but some species may be dark or light. In some species the parapodial margins may have brightly colored bands or spots, and spots of varying sizes and color may be distributed over the body. Pharynx lacking a pharyngeal pouch. Longitudinal ascus-muscle long and attached to the ventral surface of the pharynx throughout its length. Radular teeth blade-shaped, denticulate or smooth. Reproductive system triaulic, but a separate vaginal opening may be absent. There may be one, two or many ampullae. Penis is usually unarmed but in some species has a hollow apical stylet.

Remarks. The genus Elysia has a long and complex taxonomic and nomenclatural history. Several genus names are currently considered synonyms of Elysia for different reasons. Montagu (1804) described the species Aplysia viridis Montagu, 1804 (under the incorrect spelling “Laplysia”) from Devonshire, England . Oken (1815) reexamined the original description of this species and considered it different from the true Aplysia Gmelin, 1791 , thus erecting the new genus Acteon Oken, 1815 (under the incorrect spelling “ Actaeon ”). However, Oken’s name Acteon is preoccupied by Acteon Montfort, 1810 , and subsequently Oken’s (1815) publication was rejected for nomenclatural purposes by the ICZN (1956: Opinion 417). Risso (1818) named a new species from Nice , France as Notarchus timidus Risso, 1818 , based on manuscript notes from 1812 in which he refers to the species as Elysia timida . Because Risso (1818) cites the species as the binomen Elysia timida , this work constitutes the original description of the genus Elysia . Another synonymous genus name based on Mediterranean-Atlantic species is Aplysiopterus , originally introduced by Delle Chiaje (1830) for the new species Aplysiopterus neapolitanus Delle Chiaje, 1830 , which was later found to be a synonym of Elysia viridis (see Iredale & O’Donoghue 1923; Bouchet 1984). Five years later, Cantraine (1835) indicated that, in personal correspondence during 1827, he had created the name Rhyzobranchus for Elysia viridis , but now recognized that Risso’s name, Elysia , had priority.

Two additional genus names were introduced for species with convoluted parapodial margins. Deshayes (1857) described the genus Tridachia based on a species to be named after Schramm, but did not name the species. Mörch (1863) introduced for the first time the binominal name Tridachia schrammi in reference to Deshayes’ (1857) description, thus becoming the type species by subsequent monotypy. MacFarland (1924) described Tridachiella as different from Tridachia because the parapodia did not unite in front as in Tridachia . Because both Tridachia and Tridachiella are nested within Elysia in phylogenetic analyses based on both morphological ( Gosliner 1995) and molecular ( Händeler et al. 2009) data, these three names are considered synonyms.

Three additional genus names were introduced for members of the Elysia ornata species complex. Swainson (1840) described the genus Thallepus for Thallepus ornatus Swainson, 1840 but Verrill (1901) considered it to be a synonym of Elysia . Kelaart (1858) described the species Elysia grandiflora Kelaart, 1858 from Sri Lanka. At the end of the description he suggests to use the new genus name Hydropsyche for this species if it is later found that it does not belong to any know genus. Pease (1860) described the genus Pterogasteron for two species collected in the Hawaiian Islands , Pterogasteron ornatum Pease, 1860 and Pterogasteron bellum Pease, 1860 . No type species was indicated. Pease’s illustrations were published in Bergh (1881: pl. G, fig. 18–19), who transferred Pterogasteron ornatum Pease, 1860 to the genus Elysia , making it a homonym of Elysia ornata Swainson, 1840 , of which it is also a synonym ( Jensen 1992). Krug et al. (2013) showed that although there may be undescribed Indo- Pacific species, all members of the Elysia ornata species complex form a clade; thus, the genera Thallepus , Hydropsyche , and Pterogasteron are synonyms of Elysia .

In 1872 two authors independently introduced the same genus name for two different species of Elysia . Bergh (1872) described the genus Elysiella for Elysiella pusilla Bergh, 1871 and distinguished it from Elysia because of the short tentacles and the carinated side of the head. The name Elysiella pusilla was first introduced in the caption of plate 9 for Bergh’s (1872) paper, which was published in 1871, a year before the actual text. Verrill (1872) introduced the genus Elysiella for Placobranchus catulus Gould, 1870 as different from Elysia and Placobranchus because the posterior end of the parapodia are fused together. According to Wheat (1918) Elysiella Verrill, 1872 was published earlier and therefore has priority, thus Elysiella Bergh, 1872 is unavailable. Jensen & Wells (1990) and Jensen (1992; 1997b) considered Elysiella a valid genus based on a broad, demarcated foot, short parapodia and rhinophores, an elongated renopericardial extension radiating dorsal vessels, and radular teeth with triangular, unidenticulate cusps. Jensen (1997b) described an additional species from Australia, Elysiella stylifera Jensen, 1997 . However, Elysia is paraphyletic with respect to Elysiella in both morphological ( Gosliner 1995; Jensen 1997a) and molecular ( Händeler et al. 2009; Krug et al. 2015) phylogenetic analyses. Thus, Elysiella is considered a synonym of Elysia .

Similarly, the genus Pattyclaya Marcus, 1982 was erected for the species P. arena Carlson & Hoff, 1978 , which has dorsal lamellae running perpendicular to the main body axis; a second species, P. brycei Jensen & Wells, 1990 , was subsequently described in this genus. However, Pattyclaya nested within Elysia in all morphological phylogenetic analyses ( Gosliner 1995; Jensen 1997a), and is considered a synonym of Elysia pending molecular confirmation.

Several additional genus-level names have been introduced more recently because of the presence of unique anatomical traits in some species. For example, Pruvot-Fol (1930) described the new genus Elysiobranchus for Elysiobranchus mercieri Pruvot-Fol, 1930 , which has long and ramified tubercles. Later, Pruvot-Fol (1946) considered Elysiobranchus as a subgenus of Elysia , and Carlson & Hoff (1978) re-described E. mercieri treating Elysiobranchus as a synonym of Elysia . Pruvot-Fol (1946) introduced the subgenus Elysiopterus for Verrill’s (1901) misidentification of Elysia crispata (as “ Elysia crispa ”), which she named Elysiopterus verrilli Pruvot-Fol, 1946 . Also, Pruvot-Fol (1946) included Placobranchus expansa O’Donoghue, 1924 in this new subgenus. As discussed below, Elysia velutinus Pruvot-Fol, 1947 (= Elysiopterus verrilli Pruvot-Fol, 1946 ) is a senior synonym of Elysia tuca Ev. Marcus & Er. Marcus, 1967 which is nested within other Caribbean Elysia , and therefore there is no phylogenetic basis for the maintenance of Elysiopterus. Finally Ortea et al. (2005) introduced the genus Checholysia Ortea, Caballer Moro & Espinosa, 2005 for species with a penial stylet, with Elysia patina Marcus, 1980 as the type species. Because Elysia velutinus Pruvot-Fol, 1947 has a penial stylet, Elysiopterus is the oldest available genus-level name for such a group of species. However, according to the phylogenetic analysis here presented, the penial stylet in Elysia evolved multiple times, so there is no phylogenetic support for either Elysiopterus or Checholysia ( Fig. 4 ).

Figures

FIGURE 4. Phylogenetic hypothesis for family Plakobranchidae rooted on genus Bosellia (not shown) to illustrate relationships within the genera Elysia, Plakobranchus and Thuridilla. Topology and branch lengths are from Maximum Likelihood analysis of concatenated DNA sequences (2,807 bp total) representing portions of two mitochondrial (COI, 16 S) and two nuclear (H 3, 28 S) genes. Significant support values are given as ML bootstrap percentages (below branch), or BI posterior probabilities (above branch); asterisk = 1.0 or 100 % support. Species discussed in this study are bolded. Triangle denotes presence of a penial stylet.