Elysia evelinae Er. Marcus, 1957

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 : 66-68

Elysia evelinae Er. Marcus, 1957

( Figs. 6 M, 36–38)

Elysia evelinae Er. Marcus 1957: 410 –416, figs. 48–57 (Type locality: Ilhabela, island of São Sebastião, and Enseada de Guarujá, island of Santa Amaro, state of São Paulo, Brazil) — Ev. Marcus & Er. Marcus 1960: 153, fig. 35; Ev. Marcus & Er. Marcus 1967: 27, fig. 21; Jensen & Clark 1983: 4 –5; Clark 1994: 904 –905; Valdés et al. 2006: 64 –65; García et al. 2008: 72; Krug et al. 2015: 990 –991, figs. 3B, 4.

Type material. Elysia evelinae— possible type specimen, ex. Marcus collection (HMCZ 288304).

Material examined. Manzanillo, Limón, Costa Rica, 13 March 2001, 1 specimen ( MZUCR INB 0003312779 ).

Live animal. There is little information on the behavior of this species.

External anatomy. Animal small, short and wide. Color variable, from pale cream to brownish gray, with numerous red and opaque white spots covering head or entire body. Conspicuous black circle surrounding anal papilla, a distinctive characteristic of this species ( Fig. 36 , black arrows). Anal papilla situated on right side of body, at base of head. Rhinophores short, wide, pale cream in color. Posterior end of head pigmented with dark brown or black. Tail short, conical. Eyes conspicuous, black, each crossed by a longitudinal dark line running from the base of the rhinophores to the anterior end of the parapodia.

Parapodia short, thick. Pericardium large, rounded, not covered by parapodia. Paired posterior dorsal vessels emerging from end of short renopericardium and running to end of body, with short lateral side branches shooting off at intervals ( Fig. 37 ). One or two short anterior vessels emerging from renopericardial sac, branching not evident on preserved specimen.

Internal anatomy. Radula with 11 teeth (MZUCR INB0003312779), 5 teeth in ascending limb and 6 in descending limb ( Fig. 38 A). Leading tooth elongate and straight, with characteristic blunt cusp lacking denticles ( Fig. 38 B). See Jensen (1997) for additional views of the radular teeth. Base of tooth elongate.

Penis narrow and elongate, tapering into a conical apex devoid of armature. Deferent duct narrow and convoluted ( Fig. 6 M).

Reproduction and development. Larval development is lecithotrophic. Most veligers hatched prior to metamorphosis (‘ type 2’ development) but some larvae in many clutches underwent encapsulated metamorphosis (‘ type 3’) ( Clark & Jensen 1981; Jensen & Clark 1983). Egg diameter was given as 104 µm; there is no ECY, but cloudy albumen inside the egg capsules vanished once cleavage began ( Clark & Jensen 1981). Larvae hatched after 12 d at 20°C, at a shell length of ~210 µm. Larvae had eyespots, swam little and actively explored available substrata. Starved slugs produced clutches with reduced proportions of encapsulated metamorphosis, as noted for the lecithotrophic morph of Alderia willowi Krug, Ellingson, Burton & Valdés, 2007 (see Krug 2001).

Host ecology. The host was erroneously reported in the original description (Er. Marcus 1957). Elysia evelinae primarily feeds on chain-forming benthic diatoms in the genus Biddulphia , but also on epiphytic diatoms fouling algae such as Bryopsis and Caulerpa in shaded areas ( Jensen & Clark 1983). Jensen (1981a) described feeding; the pharyngeal bulb is extruded from the mouth, engulfing a diatom cell which is then pierced by the tip of the leading tooth; slugs emptied 5–10 cells per minute, selecting Biddulphia over other diatoms but not exhibiting preference for any particular cell size. Slugs preferred to feed on diatoms intermingled with other algal substrata. No other Elysia is known to feed suctorially on diatoms.

Phylogenetic relationships. Elysia evelinae was recovered within subclade 2, sister to the North Atlantic species E. viridis with 100% support in all phylogenetic analyses, despite their lack of apparent similarities in morphology or ecology ( Fig. 4 ).

Range. Brazil (Er. Marcus 1957; García et al. 2008), Costa Rica ( Espinosa & Ortea 2001), Florida ( Ev. Marcus & Er. Marcus 1960, 1967; Jensen & Clark 1983; Clark 1994).

Remarks. Externally, the most distinctive feature of E. evelinae is the anus, which opens slightly anterior to, and to the right of, the pericardial bulge. The anus opens at the center of a large dark spot that is visible even on preserved specimens. Among Atlantic elysiids, only E. canguzua has a similarly placed and prominent anus, but lacks the dark circum-anal band of E. evelinae .

No pharyngeal pouches (used to pump algal cytoplasm in other species) are present in E. evelinae , being replaced by a pair of what Er. Marcus (1957: fig. 66) termed “muscular pharyngeal diverticula … on both sides of the radular pouch.” The intestine is elongated in E. evelinae relative to body length, compared with other Elysia spp. (Er. Marcus 1957).

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.

FIGURE 6. Penial morphology of some species examined. A, Elysia ornata (LACM 178583). B – E, Elysia crispata (LACM 178640) (B); (LACM 178641) (C); (LACM 2004.5.1) (D); (isolate Ecri _ 10 LKS 03) (E). F, Elysia chlorotica (LACM 178597). G – H, Elysia patina (LACM 178650) (G); (LACM 178651) (H). I, Elysia flava (LACM 178626). J, Elysia subornata (LACM 178629). K, Elysia canguzua (LACM 178644). L, Elysia serca (CPIC 00027). M, Elysia evelinae (MZUCR INB 0003312779). N, Elysia velutinus (LACM 178642), O, Elysia papillosa (LACM 178607). P, Elysia cornigera (LACM 173227). Q, Elysia marcusi (LACM 178647), R, Elysia pratensis (CPIC 00068). S, Elysia zuleicae (LACM 178656). Abbreviations: bw, body wall; dd, deferent duct; pe, penis; s, stylet.

FIGURE 36. Elysia evelinae, external morphology of two specimens; actual sizes: 5 mm (A), 4 mm (B).

FIGURE 37. Elysia evelinae, dorsal vessel diagram reproduced from Ev. Marcus (1980).

FIGURE 38. Elysia evelinae, SEM of the radula (MZUCR INB 0003312779). A, Radula. B, Leading tooth.