Emplectonema Stimpson, 1857

Mendes, Cecili B., Delaney, Paul, Turbeville, James M., Hiebert, Terra & Maslakova, Svetlana, 2021, Redescription of Emplectonema viride - a ubiquitous intertidal hoplonemertean found along the West Coast of North America, ZooKeys 1031, pp. 1-17 : 1

Genus Emplectonema Stimpson, 1857

Type species.

Emplectonema viride Stimpson, 1857: 163; Griffin 1898: 207.

Emplectonema gracile Coe 1901: 23, fig. 3; Coe 1904: 23, fig. 3; Coe 1905: 207-208, pl. 1, figs 14, 14a, 15, 15a, tex fig. 32; Coe 1940: 252, 278-280, pl. 30, fig. 40; Corrêa 1964: 517-518, 534-536; Kozloff 1999: 98, 100; Roe et al. 2007: 229, 232 pl. 89I.

Material examined.

Seven adults from Charleston Marina, OR (43°20.63'N, 124°19.38'W); 27 Nov. 2019; collected from wooden pilings among acorn barnacles, Balanus glandula (Table 1, Suppl. material 1). One specimen from OIMB Boathouse dock, OR (43°20.96'N, 124°19.80'W); 10 Oct. 2019; collected from concrete pilings among acorn barnacles, Balanus glandula (Table 1, Suppl. material 1). Eggs measured from two specimens collected from the jetty at the north end of Bastendorff Beach, OR (43°21.13'N, 124°20.66'W) on 29 Jan. 2020; and sperm from one male collected at Charleston Marina on 31 Jan. 2020. Voucher material is deposited at the Smithsonian Institution’s National Museum of Natural History, Washington, DC: USNM 1638666- USNM 1638673. Each specimen consists of a morphological voucher (anterior end fixed in formaldehyde, post-fixed in Bouin’s solution and stored in 70% ethanol) and a tissue sample for DNA extraction (pieces of posterior or midbody in 95% ethanol) .

Comparative material.

Three females and four non-sexed adults of E. gracile from Pawleys Island, SC (33°24.63'N, 79°7.88'W); 29 Nov. 2019; among scorched mussels, Brachidontes exustus on granite rocks; GenBank and NMNH accession numbers in Table 1 .

Description.

Based on specimens from Oregon, body long and thread-like, 35-103 mm long, 0.6-1.0 mm wide. However, Griffin (1898) found specimens nearly 1 m long. Dark green dorsally, cream-colored or pale yellow ventrally (Fig. 3A ). Head round, slightly wider than adjacent body when moving freely, with whitish-yellow or cream-colored margins matching the color of the ventral side (Fig. 3B ). A pair of small cerebral organ furrows (anterior cephalic furrows), each shaped as a small arch, is located ventrally, anterior to cerebral ganglia (Fig. 3C ). Head furrow (posterior cephalic furrow) is shaped as a dorsal posteriorly directed “V” located behind the cerebral ganglia, and only just barely noticeable in some individuals, and not detectable in many individuals. Rhynchostomopore is a small antero-ventral opening. Numerous small ocelli arranged in two groups on each side of head. Each anterior group has 8-10 eyes arranged in a narrow row along the anterior margin of the head. Each posterior group has 10-12 eyes in a dispersed cluster in front of the brain (Fig. 3B ). Cerebral ganglia are pinkish and show through the body wall, especially in lighter-colored individuals. Cerebral organs are not easily distinguishable in life, but with slight compression. Posterior tip of body tapered.

Rhynchocoel is short, approximately 1/3 of body length. Central stylet slightly curved, 170-326 μm long (n = 7), smooth. Basis is slender, 480-815 μm long (n = 7), its distal end abruptly widening into a truncated bulb (Fig. 3C ). Basis length/width ratio 11.5-16.0. Basis/stylet length ratio 2.0-2.8. Two accessory stylet pouches, each with 9-13 accessory stylets (Fig. 3C ). Proboscis bulb elongated. Lateral intestinal diverticula beginning at posterior of rhynchocoel, present until posterior tip of the body. Separate sexes. Gonads serially arranged between intestinal diverticula. Testes of mature males are visible through the body wall as whitish sacs. Ovaries of mature females are visible through the body wall, and the oocytes are orange to light pink, with distinct germinal vesicles. Spermatozoa with elongated head 16-20 μm. Oocytes are 110-140 μm in diameter and surrounded by a tight chorion and a jelly coat (Fig. 4 ).

Reproduction and larval development.

Reproductive individuals of E. viride were collected in Charleston, OR, in September 2009, October 2019, January 2020, and June 2020. When ripe, males and females free-spawn gametes into the water, with no known reliable spawning cue. Swimming larvae hatch from the egg chorion after ~30 h and begin feeding on small planktonic crustaceans after developing a functional proboscis and stylet (~4 d). Planktonic period lasts several months (Mendes unpublished observations). Wild-caught larvae of E. viride were found in the plankton samples taken with 50-153 μm net at the Charleston Marina, OR, in October 2013, March 2019, February 2020, and June 2020. Emplectonema viride larvae are easily recognized by their distinctive green color (Fig. 5 ).

Distribution.

Northeastern Pacific Ocean from Alaska to California. Type locality is San Francisco Bay, California, USA.

Morphological comparison with Emplectonema gracile .

As has been pointed out by Griffin (1898), specimens of E. viride have a darker dorsal surface, with a distinctly lighter colored ventral side and head margins, compared to those of E. gracile (Fig. 3 ). We confirm this finding and can also add that E. viride has smaller eggs: 110-140 μm (n = 9), compared to 181-198 μm eggs of E. gracile (n = 8). The characteristics of stylet apparatus do not overtly differ in the two species (Fig. 3C, F ).

Figures

Figure 3. Photomicrographs of Emplectonema viride (A-D) and Emplectonema gracile (E-H). Abbreviations: cerebral ganglia, cg; rhynchostomopore, rh; cerebral organ opening, co. Scale bars: 10 mm (A, E); 0.5 mm (B, F); 100 μm (D, H).

Figure 4. Photomicrography of egg (A) and sperm (B) of Emplectonema viride. Scale bars: 25 μm.

Figure 5. Larva of Emplectonema viride collected from plankton in Charleston, OR, on 17 Oct 2013. Same individual is shown in two focal planes to highlight apical tuft (upper left, A) and posterior cirrus (lower right, A) and green epidermal pigment (B). Note paired subepidermal eyes, which are anterior to cerebral organs. Scale bar: 100 μm.