Waldo arthuri, Valentich-Scott, Paul, O Foighil, Diarmaid & Li, Jingchun, 2013

Valentich-Scott, Paul, O Foighil, Diarmaid & Li, Jingchun, 2013, Where's Waldo? A new commensal species, Waldo arthuri (Mollusca, Bivalvia, Galeommatidae), from the Northeastern Pacific Ocean, ZooKeys 316, pp. 67-80 : 70-74

Waldo arthuri sp. n. Figures 1 A–H, 2 A–C

Divariscintilla " " sp. A Coan et al. 2000: 314

Description.

Shell extremely thin, fragile, moderately inflated, translucent; equilateral to slightly longer posteriorly, anterior end slightly flared to gently sloping (Figure 1A-C); shell margins only weakly gaping if at all. Prodissoconch non-umbonate, D-shaped, with a greatly reduced PII comprised of a small number of faint commarginal striae bordering the metamorphic prodissoconch/dissoconch boundary (Figure 1D), prodissoconch length ranged from 338 to 357 µm (n=8) (Figure 1B). Dissoconch sculpture of commarginal striae, plus low broad irregular radial ribs; external sculpture variable, radial ribs absent to moderately strong, especially on anterior and posterior ends in some specimens. Beaks low, wide. Hinge plate extremely narrow, edentulous (Figures 1E, F). Length to 5 mm.

Mantle large, reflected, covering approximately 80% of outer shell surface when fully extended, not covering umbones (Figure 1G); mantle can be completely retracted into the shell; reflected portion papillate (Figure 1H); fused posteroventrally; facultative exhalant siphon, trumpet-shaped, non-papillate; anterior end thin, non-papillate.

Mantle tentacles long, extend well past shell margins (Figure 1G). Adult with projecting anterior pair, two laterally projecting pairs just posterior to anterior tentacles (one pair on each side); lateral tentacles not present on individuals less than 1 mm in length; ventral pair of tentacles just anterior of exhalant siphon (largest of all tentacles, in adults up to length of shell); single posterior tentacle projects dorsally to the exhalant opening. When animals are actively crawling, it appears that the tentacles might be used as levers to navigate between the urchin spines.

Foot large, exceeds the length of the shell when fully extended, vermiform, without heel (Figure 1G); long ventral byssal groove extending to end of smooth foot tip. This species is an active crawler, and can also attach to the host by byssal threads.

Ctenidia with one demibranch on each side, comprised of about 12-15 widely spaced filaments in larger specimens.

Development.

The reproduction is typical of galeommatoideans, in that the animal is hermaphroditic, and the young are brooded in the ctenidia. Two brooding individuals sampled in 1989 showed early and mid developmental stages respectively. Fecundity was low; the early developmental stage individual (3.8 mm length) had 160 yolky embryos all at the blastula stage (approximately 200 µm in diameter) (Figure 2A). The second specimen was brooding mid-late stage shelled embryos (~ 270 µm length) with a protruding unciliated velum containing partially depleted yolk reserves, a larger dense mass of yolk present in anterior visceral mass, a papillate mantle that extended outside of the valve margins, and a protruding foot. The smallest non-brooded individual observed (370 µm length) byssally attached to its urchin host, had attained a modest (20 µm) increment of dissoconch growth, but notably still had visible yolk reserves dispersed across its visceral mass (Figure 2C). Although we have not observed early ontogeny, these characteristics, together with the non-umbonate prodissonch, point unambiguously toward a non-pelagic developmental mode.

Type locality.

USA, California, San Luis Obispo County, off Pt. San Luis; 35°05'18'N, 121°00'54"W; 409 m.

Type material.

Holotype, SBMNH 235142, conjoined shell and anatomy, length 2.5 mm, height 1.5 mm. Holotype comprises two conjoined valves, with anatomy, preserved in 70% ethyl alcohol. Given its wet preservation and small size we were unable to capture high quality photographs of the holotype.

7 Paratypes, SBMNH 149934, same locality as holotype (Figures 1 A–E), specimens mounted on SEM stub; Figure 1A length 2.45 mm, height 1.45 mm; Figure 1B length 2.55 mm, height 1.45 mm; Figure 1C length 2.61 mm, height 1.63 mm.

3 Paratypes, SBMNH 235142, same locality as holotype (preserved in 100% EtOH).

4 Paratypes, SBMNH 149933, Canada, British Columbia, Sanford Island, Barkley Sound; 48°51'28"N, 125°08'57"W; 80 m, attached to Brisaster latifrons .

34 Paratypes, UMMZ 303919, Canada, British Columbia, Imperial Eagle Channel; 48°55.052'N, 125°13.657'W (preserved in 100% EtOH).

Distribution and habitat.

Canada, British Columbia, Barkley Sound, Sanford Island, 80 meters, and Imperial Eagle Channel in soft sediments; and United States, California, from Monterey Bay to La Jolla, from 113 to 444 meters [SBMNH].

Ten juvenile specimens from the intertidal zone of Smeaton Bay, Alaska (55.4°N, 130.6°W) [SBMNH 149330] are too small to be identified to species, but might also be Waldo arthuri .

Commensal relationship.

Crawling on the oral surface of the heart urchin Brisaster latifrons , primarily near the peristome. In 1989, most Barkley Sound heart urchins examined had a single bivalve although up to 3 specimens were collected on a single host. In 2011, the commensals were more plentiful: 22/33 urchins bore at least 1 commensal (mean = 2.7 clams/urchin); the maximum number on an individual host was 23 clams.

Discovery.

Independently discovered in the late 1980's by Arthur Fontaine and Diarmaid Ó Foighil in British Columbia and Paul Valentich-Scott and Donald Cadien in southern California.

Etymology.

This species is named after Dr. Arthur Fontaine, Professor Emeritus of Biology at the University of Victoria, British Columbia, Canada.

Comparisons.

Table 1 provides characteristics to separate Waldo arthuri from other members of the genus. The Antarctic Waldo parasiticus is subequilateral, has a distinct anterior gape, and lacks the elongate anterior and posterior tentacles. Waldo trapezialis , has a strong saddle shaped internal ligament, is subequilateral, and lacks strong radial sculpture. Waldo digitatus Zelaya & Ituarte, 2013 lacks the radial sculpture and has a large number of mantle tentacles ventrally. Waldo arthuri is closest to Waldo paucitentaculatus Zelaya & Ituarte, 2013, which has wider, stronger radial ribs, a strongly crenulate ventral margin, and a much narrower anterior end.

Scintillona bellerophon Ó Foighil & Gibson, 1984 is the only other galeommatid from the northeast Pacific that has been recorded as an epibiont on echinoderms. This species attaches externally to the sea cucumber, Leptosynapta clarki (Heding, 1928). Scintillona bellerophon has cardinal teeth in both valves. The shell is much thicker, and not transparent, when compared with Waldo arthuri .

A species from Japan and Hawaii, Scintillona stigmatica (Pilsbry, 1921), has been collected on the heart urchin, Brissus latecarinatus (Leske, 1778). Yamamoto and Habe (1974) illustrate this bivalve on the ventral surface of the urchin, in an arrangement very similar to Waldo arthuri . However Scintillona stigmatica , as with Scintillona bellerophon , has a cardinal tooth in each valve. In addition, Scintillona stigmatica has a red-brown stripe of color running laterally from the umbones to the posteroventral margin.

In the eastern Atlantic Ocean, Gage (1966) documented two species of " Montacuta " attached to spantangoid urchins. Both species have a dentate hinge, and are easily separated from the new species.

Other similar North American species include those belonging to Divariscintilla . Mikkelsen and Bieler (1989, 1992) describe five species of Divariscintilla from Florida. Externally these species all have a papillate, reflected mantle, and long mantle tentacles, similar to the new species. However members of Divariscintilla have distinct cardinal teeth.