Elysia marcusi (Ev. Marcus, 1972)

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 : 69-72

Elysia marcusi (Ev. Marcus, 1972)

( Figs. 6 Q, 39–41)

Bosellia marcusi Ev. Marcus 1972a: 293 –295, figs. 19–22 (Type locality: Grassy Key and Bear Cut Rocks, Florida, USA) — Ev. Marcus 1973: 819 –820, figs. 1, 7 (part), 11–12, 15; Ev. Marcus 1980: 76; Thompson 1977: 132 –133, figs. 22e, 29a–b; Clark 1994: 905 –906; Krug 2009: 361–365, figs. 1B, 4C, 5C, 6; Christa et al. 2014: fig. 1D

Elysia marcusi (Ev. Marcus, 1972) — Händeler et al. 2009: fig. 7; Redfern 2013: 282–283, fig. 785; Christa et al. 2014: fig. 3; Krug et al. 2015: 990–991, figs. 3B, 4.

Type material. Bosellia marcusi— Untraceable, not at USNM. A specimen lot labeled “type specimen” at the MZSP (76043) contains a piece of calcareous algae ( Siqueira Dornellas & Simone 2011).

Material examined. San Salvador Island, Bahamas, 10 July 2010, 2 specimens ( LACM 178647–48 ).

Additional material examined. Bahamas: San Salvador Island, Bahamas, 23 June 2007, 23 specimens (isolate Emar_07Ssal02-24) , 9 July 2010, 2 specimens (isolate Emar_10Ssal02-03), Goulding Point , New Providence , 13 July 2010, 21 specimens (isolate Emar_10NPr01-21); Discovery Bay, Jamaica , March 2006, 48 specimens (isolate Emar_06Jam01-48); Geiger Beach , Key West, Florida, USA , 12 June 2007, 58 specimens (isolate Emar_07Gei01-58).

Live animal. Body highly plastic. Resting slugs contract into perfectly round circles, thick in the center like gumdrops; crawling specimens elongate into a long, thin, typical slug form. Specimens observed ranged from 2–5 mm in length ( Fig. 39 A–C)

External anatomy. Overall coloration uniformly light to dark green ( Fig. 39 A), overlaid by white pigment patches if present ( Fig. 39 B–C). Body edge may be rimmed by whitish-blue spots or short, lateral lines perpendicular to edge ( Fig. 39 C). White patches scattered across dorsal surface in some specimens, absent in others. White spots may be concentrated in patch behind head, where anterolateral groove terminates ( Fig. 39 C– D). Front of head smooth-edged, no oral lobes. Widely spaced eyes. Rhinophores solid white, simple, flattened (not rolled); fully retractable into head. Some specimens with white bar extending between parapodia, posterior to eyes, like a solid eyebrow. Most Florida specimens with iridescent deep blue spots scattered across body. Specimens from San Salvador Island uniformly green. Specimens from Jamaica with scattered white spots across dorsum, denser along side of foot, sometimes with scattered blue dots.

Parapodia superficially absent, having secondarily fused to body; fusion line forming mid-dorsal line and antero-lateral furrow on right side behind head ( Fig. 39 A). Body surface uniformly smooth. Curved, anterolateral furrow extending up right side of body, starting from side of foot about one-quarter of total body length from front of head. Furrow narrowing and straightening to form mid-dorsal longitudinal line, running straight down midline of dorsum; terminating just anterior to tail, where vaginal opening appears in a white patch ( Fig. 39 A). White pigment patches clustering along midline in some specimens.

Internal anatomy. Radula with 15 teeth (LACM 178647), 7 teeth in ascending limb and 8 in descending limb ( Fig. 40 A). Radula with 18 teeth according to Ev. Marcus (1973). Leading tooth robust and elongate with 7–8 triangular denticles on cusp from mid-tooth to sharp tooth tip ( Fig. 40 B). Housing depression for interlocking teeth “V”-shaped and extending ½ of tooth length ( Fig. 40 A). Base of tooth ½ total tooth length. Ascus containing jumbled heap of discarded large teeth.

According to Ev. Marcus (1972a; 1973), pharynx small (230 µm long × 150 µm high, on 3–4 mm long animals). Esophagus narrowing into small stomach. Anus opening into anterior transverse groove. Heart and kidney reduced, not visible externally.

Penis small and oval to spherical in shape (LACM 178647–48), with a pointed stylet ( Fig. 6 Q, Fig. 40 C). Deferent duct narrow.

Reproduction and development. Ev. Marcus (1973) described the reproductive system as triaulic, with anterior male and female apertures and a posterior vaginal pore leading to bursa for receipt of allosperm. Follicles (n = 5–6) of ovotestes large (250 µm across). Penis transparent, saclike, round and flat at the end with a curved, pointed stylet. Female opening in dorsal furrow on right side, posterior to male aperture containing penial sheath. Bursa copulatrix inside posterior end of body, connected to outside by vaginal pore; duct running forward along midline of body to region where sperm duct and oviduct separate ( Ev. Marcus 1973: fig. 15). The specimens here examined also had a round penis with a hollow apical stylet ( Figs. 6 Q, 40C), confirming Ev. Marcus’ (1973) description.

During mating, some pairs lined up with the head of one aligned with the tail of the other, and reciprocally inseminated for periods of>1 hour. Occasional group mating was observed, with three slugs forming a circle, each inseminating the slug before it. Inseminations occurred either into the posterior furrow, at or near the location of the terminal vaginal pore leading to the internal bursa copulatrix ( Fig. 39 D), or into the antero-lateral furrow. Slugs settled into their round, flat resting morphology immediately after mating.

Larval development is described in Krug (2009) as “ Bosellia ” marcusi . Development is lecithotrophic. Mean clutch size was 26.0 ± 14.3 SD, including clutches from Florida (n = 4), Jamaica (n = 9) and San Salvador, Bahamas (n = 3). Grand mean diameter of uncleaved ova was 103.9 µm ± 2.8 SD (n = 8 clutches; range of mean egg diameters from different clutches was 99.5 ± 3.3 to 108.5 ± 4.4). White or yellow eggs are laid in a typical spiral, one egg per capsule, usually with ECY of the same color attached to the outside of the egg capsules ( Fig. 39 E–H). One clutch from San Salvador had both white and yellow ova, and white and yellow ECY ( Fig. 39 G). Of 13 clutches observed sufficiently early in development, ECY was present as one rounded blob per capsule (6 clutches; Fig. 39 E); two small streaks per capsule (one clutch); continuous threads of almost translucent pale yellow (two clutches; Fig. 39 F); one irregular blob or streak for every third capsule (one clutch; Fig. 39 G); or was absent (all three clutches from Florida; Fig. 39 H). Capsules were deposited with an opaque material, presumably albumen, surrounding the uncleaved ova; capsular fluid cleared within an hour.

Clutches held at ~25°C developed large eyespots after about 10 d, and in some clutches the encapsulated larvae developed a brick-red color shortly before hatching. Intra-capsular metamorphosis was only recorded in one of 11 clutches (Krug 2009). Larvae hatched after 14 d (± 0.7 SD, n = 2), releasing swimming veligers that swam actively for 4–7 d before either undergoing spontaneous metamorphosis or dying in the absence of an algal cue (Krug 2009). Mean larval shell length was 190.3 µm (± 13.9 SD, n = 5 clutches, grand mean; range = 175.5 ± 7.63 to 210.3 ± 13.3), the smallest lecithotrophic larvae reported for any elysiid, and the second-smallest out of 22 sacoglossans for which data exist; only the poecilogonous Alderia willowi has smaller lecithotrophic larvae (Krug 2007).

Host ecology. In Florida and Jamaica, specimens were collected from clumps of Halimeda opuntia , generally in shaded and protected habitats, in water as shallow as a few cm. Slugs maintained in aquaria fed readily on H. opuntia and rarely crawled off their host, on which they are highly cryptic. In Bahamas sites, specimens were collected from clumps of H. goreaui that grew hanging down in highly shaded pockets of dead coral (San Salvador) or under rock ledges (New Providence). Specimens were not collected from H. incrassata , H. tuna or H. monile .

Phylogenetic relationships. Our phylogenetic analyses indicated E. marcusi is a derived species of Elysia ( Fig. 4 ) consistent with prior molecular analyses ( Händeler et al. 2009; Christa et al. 2014; Krug et al. 2015). Its misclassification as a Bosellia resulted from the fusion of parapodia over the dorsum, and convergence on a flattened boselliid shape due to their shared niche (adhering tightly to flat pieces of Halimeda ). Elysia marcusi was not closely related to any of the seven other sampled Elysia spp. that feed on Halimeda , and was one of the few species for which no sister taxon was identified with significant support ( Fig. 4 ). The relatively long branch of E. marcusi could result from a failure to sample related species, and/or from the high degree of adaptive evolution that this taxon has undergone, reflected in both the highly derived morphology of this taxon and a large number of fixed mutations.

Range. Bahamas ( Redfern 2013; present study), Costa Rica ( Espinosa & Ortea 2001), Cuba (Espinosa et al. 2005), Florida, USA ( Ev. Marcus 1972a; Clark 1994; present study), Jamaica ( Thompson 1977; present study).

Remarks. Ev. Marcus (1972a, 1973) placed this species in Bosellia presumably due to the absence of apparent parapodia, despite the absence of dorsal vessels. Preceding molecular phylogenetic ( Händeler et al. 2009; Christa et al. 2014; Krug et al. 2015) and developmental studies (Krug 2009) noted that E. marcusi was a derived Elysia species, but no formal taxonomic transfer has been published previously. Anatomical study indicates the parapodia secondarily fused to the body, forming the curved antero-lateral furrow and the mid-dorsal line. Thompson (1977) speculated about this, noting the “dorsal longitudinal line resembled the line of fusion of lateral parapodial lobes found in some aplysiomorphs”. The fusion of parapodia would have covered the ancestral dorsal vessel network and renopericardial complex, becoming hidden from external observation. Ev. Marcus (1972a, 1973) did not question her assignment of this species to Bosellia , despite remarking on the numerous features that distinguished E. marcusi (slender, curved radular teeth; ascus storing discarded teeth; no evident dorsal vessels, pericardium or pharyngeal crop; triaulic reproductive system with few, large follicles in the ovotestes; penial stylet) from B. mimetica . Ev. Marcus (1973) described B. corinnae from Florida, which in most respects was more similar to E. marcusi than to B. mimetica , but had the exposed dorsal vessels of Bosellia yet highly distinct radular teeth. Given the absence of any subsequent studies, the identity and generic placement of B. corinnae remains unclear; it could be a derived Elysia that has lost parapodia, like E. serca , or a second Atlantic Bosellia sp.

Together with its small size, the secondary fusion of parapodia over the dorsum allows E. marcusi to flatten its body onto blades of Halimeda opuntia or H. goreaui in protected microhabitats. Several elysiids have a flattened body shape and reduced parapodia ( E. pusilla , E. stylifera , E. serca ) analogous to the fused parapodia in E. marcusi , likely reflecting convergent evolution. Selection may favor a flattened morphology for greater adherence to hosts presenting flat surfaces such as seagrass ( E. serca ) and some Halimeda spp. ( E. pusilla , E. stylifera , E. marcusi , Bosellia mimetica ). Homoplasy in body form resulted in the erroneous generic placement of E. marcusi , highlighting the need to consider ecological information when making taxonomic diagnoses.

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 39. Elysia marcusi, external morphology and egg masses. A, Crawling specimen from Jamaica showing vaginal pore (vp) and antero-lateral furrow (af). B, Crawling specimen from Florida with white spots. C, Resting specimen from Jamaica, showing rounded “ gumdrop ” posture adopted by slugs on algae. Size = 4 mm (A), 5 mm (B), 4 mm (C). D, Copulating specimens from Jamaica. Translucent penis of slug (left) is visibly being inserted into vaginal pore of larger slug (right); note antero-lateral furrows on both slugs. Field of view = 5 mm. E – H, Egg masses showing variation in ECY. Clutches deposited by Jamaica specimens had either regularly spaced blobs of whitish ECY, one attached to the outside of each capsule (E; width of egg mass = 1.2 mm), or else a thread of yellow ECY running along the inside edge of the string of egg capsules (F; width of egg mass = 1.6 mm). An egg mass from San Salvador (G) containing both white and yellow ova at the four-cell stage had ECY deposited as one white blob or streak per capsule for the first turn of the egg spiral, but thereafter as one yellow blob or streak every third capsule. Width of egg mass = 1.5 mm. An egg mass from Florida lacked ECY (H; width of egg mass = 1.0 mm).

FIGURE 40. Elysia marcusi, SEM of the radula and penis (LACM 178648). A, Partly disarticulated but complete radula. B, Tooth showing denticles. C, Penial stylet.