Elysia evelinae Er. Marcus 1957

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).