Onchidella borealis Dall, 1872

Dayrat, Benoît, Zimmermann, Sara & Raposa, Melissa, 2011, Taxonomic revision of the Onchidiidae (Mollusca: Gastropoda: Pulmonata) from the Tropical Eastern Pacific, Journal of Natural History 45 (15 - 16), pp. 939-1003 : 962-967

Onchidella borealis Dall, 1872

( Figures 11 and 12 )

Onchidella borealis Dall 1872: 135 . — Binney 1876: 184–185, pl. VI, figs BB, E, EE. — Binney 1885: 162–163, figs 148–149. – von Plate 1893: 206–207 [as Oncidiella ]. — Dall 1910: 112–115. — Watson 1925: 301. — Hoffmann 1928: 35. — Labbé 1934: 241. — Abbott 1954: 275. — Marcus 1961: 3–4, pl. I, figs 1–2. — Morris et al. 1980: 342. — Weiss and Wägele 1998: 77–80, figs 22–33, 47, 50–51, 61–63.

Onchidium boreale . – Semper 1885: 282–283, pl. XXI, fig. 13.

Onchidium borealis . – Dall 1910: 112–115.

Type material

Dall did not specify the number of specimens he used for the original description. Also, the type material could not be located and is very likely lost. Type locality: Sitka, Alaska Territory. Type material condition unknown.

Remarks on the original description

The brief original description is reproduced here in its entirety:

Animal small, black with dots and streaks of yellowish white, foot lightcoloured, also muzzle and tentacles. Back regularly rounded, but a little pointed in the middle; smooth or very finely granulose, tough and coriaceous. Eyes globular, blue, on very short constricted tentacles. Muzzle short, rounded-transverse. Head not produced beyond the anterior edge of the mantle. Sexual appendages on the right side, near the head. Foot ovate, narrow, rather roundly pointed behind. Lon.. 3 in. Habitat, Sitka, Alaska Territory, on the rocks near tide marks, especially on the small islets in the Bay. Dall, August, 1866.

Distribution

Most authors have studied only some Alaskan material (type locality; Binney 1876; Semper 1885; von Plate 1893; Weiss and Wägele 1998). However, a wide distribution has been recognized for a long time: e.g. from Prince William Sound to Vancouver Island ( Binney 1885); from Californa to the Bering Sea ( Dall 1910); Alaska and west coast of Canada and the United States ( Watson 1925); from Alaska to Oregon ( Abbott 1954); from Alaska to California ( Marcus 1961). The southernmost record is San Luis Obispo, California ( Morris et al. 1980).

Habitat

Intertidal. On rocks, although they can also be found on algae ( Morris et al. 1980).

Additional material examined and dissected

A total of 213 specimens were examined, 19 of which were dissected to study their internal anatomy. Specimens examined were collected from 11 localities from Alaska to the San Francisco Bay , covering the entire range of the species. ALASKA, Kodiak Island , Three Saints Bay , 57 ◦ 07.8’ N, 154 ◦ 28.7’ W, 29 May 1975, nine specimens 5/3 to 2/ 1 mm preserved [leg. Sears], identified as Onchidella borealis by Mueller , ( CASIZ 013314 ) [two specimens dissected: 5/3 (#1) and 3/2 (#2)]; CANADA, British Columbia, Pacific coast of Vancouver Island , Clayoquot Sound , February 1965, four specimens 6/5 to 5/ 5 mm preserved [leg. A. G. Smith], identified as Onchidella by A.G. Smith, ( CASIZ 021510 ) [two specimens dissected: 6/5 (#1) and 5/5 (#2)]; OREGON, Lincoln County, Road’s End, zone 2, 16 June 1962, five specimens 9/5 (#1) to 4/ 3 mm preserved [leg. Giles], identified as Onchidella carpenteri by M.G. Harasewych, ( NMNH 1102498 ); Oregon, Coos County , Coos Bay , February 1965, 26 specimens 7/4 to 3/ 2 mm preserved [leg. H. Hemphill], identified as Onchidella borealis by A.G. Smith, ( CASIZ 021557 ) [two specimens dissected: 7/4 (#1), 5/3 (#2), and 6/4 (#3)]; Oregon, South Bay , Cape Arago, 24 June 1951, one specimen 8/ 5 mm preserved [leg. J. McCauley], identified as Onchidella borealis [unknown identifier], ( NMNH 574669 ) [not dissected]; Oregon, Cape Arago, [collecting date unknown], nine specimens from 9/5 (#1) to 5/4 (#2) mm preserved [leg. unknown], identified as Onchidium sp. [unknown identifier], ( NMNH 836671 ); CALIFORNIA, Del Norte County , Crescent City, Battery Point Lighthouse, S of lighthouse, N of breakwater, 41.75 ◦ N, 124.20 ◦ W, 27 July 2007, two specimens from 6/3 to 5/3 (#1) mm preserved [leg. T. M. Gosliner, R. Johnson and V. Smith], identified as Onchidella borealis by T. M. Gosliner, ( CASIZ 174474 ); California, Mendocino County , mouth of Buckhorn Creek , 1 mile S of Little River, 4 May 1946, 23 specimens from 9/5 to 4/ 3 mm preserved [leg. A.G. Smith], identified as Onchidella [unknown identifier], ( CASIZ 081437 ) [two specimens dissected: 8/5 (#1) and 5/3 (#2)]; California, [Mendocino County], Noyo, [collecting date unknown], two specimens 4/3 and 3/ 2 mm preserved [leg. Harford], identified as Onchidella borealis [unknown identifier], ( NMNH 047483 ) [not dissected]; California, Humboldt County , two miles S of Cape Mendocino, February 1965, 92 specimens 12/7 to 3/ 2 mm preserved [leg. G.D. Hanna and A.G. Smith], identified as Onchidella by A.G. Smith, ( CASIZ 021519 ) [three specimens dissected: 7/5 (#1), 7/5 (#2), and 6/4 (#3)]; California, San Francisco Bay , Marin County , Angel Island , wind swept point, 1 November 1975, 40 specimens 12/7 to 7/ 5 mm preserved [leg. S. Gray], identified as Onchidella borealis by T. M. Gosliner, ( CASIZ 057732 ) [three specimens dissected: 9/6 (#1), 9/6 (#4), 9/5 (#2), and 6/4 (#3)] .

Description of new specimens

Several anatomical systems of O. borealis are very similar to O. binneyi (nervous system, pallial organs, digestive system except for the radular formulae, and the posterior genital organs). Their description is not provided here again. Given that the type material of O. borealis could not be located, the description below is exclusively based on the non-type specimens examined and dissected here.

External Morphology ( Figure 11 ). Background colour of dorsal notum of live animals quite variable: reddish brown, brown, dark grey, or black, mottled with white and yellowish dots and streaks. In preserved animals, dorsal colour from homogeneously white to dark brownish grey. Hyponotum and pedal sole whitish tan both in live and preserved animals.

Size from 12/ 7 mm ( CASIZ 021519, California) to 2/ 1 mm ( CASIZ 013314, Alaska). Body high, not flattened. Hyponotum horizontal. Dorsal notum oval, longer than wide. Notum finely granular with small papillae of various sizes (usually <0.7 mm high), covering mainly sides of notum; centre covered by smaller papillae, occasionally smooth. Margin of notum not smooth: characterized by warts of various sizes (<1 mm long). In some individuals, larger warts (locations with repugnatory glands) regularly separated by shorter ones (from two to seven). However, distinguishing marginal warts based on their size not possible in many individuals, because most warts similar. Marginal warts hosting marginal glands more prominent in posterior region of body. Papillae with dorsal eyes absent. Dorsal gills absent.

In most individuals, pedal sole wider than the left and right hyponotum added (H <S). Exceptionally, total width of hyponotum (left and right side added) equal to the width of pedal sole (H = S). Exceptionally, left and right hyponotum added together wider than pedal sole (H> S) (e.g. CASIZ 057732). On hyponotum, hyponotal line around pedal sole which separates hyponotum into inner area (close to the foot) and outer area. Distance between hyponotal line and pedal sole from one fifth to one third of width of hyponotum (hyponotal line closer to pedal sole than to hyponotum margin). Openings (pneumostome, male opening, eye tentacles) within smooth area delimited by hyponotal line.

Pedal sole surrounded by two grooves on left and right sides. However, the left groove is shallow. Peripodial groove present on right side, from buccal area (pedal gland opening) to posterior openings (anus and female opening). Anus posterior, slightly on right of median line, very close to pedal sole. Anus occasionally median (e.g. CASIZ 081437, CASIZ 057732). Posterior female genital opening very close to anus. Position of pneumostome on hyponotum relative to notum margin and pedal sole varies among and within lots. However, pneumostome close to anus in most individuals. Generally pneumostome slightly on right side but occasionally median (e.g. NMNH 836651, CASIZ 081437). In anterior region, head covered dorsally by the notum. Head bears pair of retractile, ocular tentacles, with eyes at tip. In most specimens, tentacles deeply retracted. Left and right oral lobes distinct, i.e. not fused medially, superior to mouth and inferior to ocular tentacles. Opening of pedal gland is median, inferior to mouth. Male genital opening on right of lateral side of (or behind) right oral lobe.

Marginal glands. Marginal glands similar to those found in O. binneyi , except between eight and twelve glands on each side.

Digestive system ( Figures 11 and 12 ). Radular maximum size 5/ 2 mm, when flattened. Radular formulae vary among individuals ( Table 1). From 44 to 78 rows, and from 32 to 58 teeth per half row.

Reproductive system ( Figure 11 ). No accessory penial gland. No distinct penis inside penial sheath. No conspicuous papilla at distal end of deferens duct. Penial sheath not covered internally by thick folds. Length of penial sheath rarely> 2 mm. No accessory caecum filled with white concretions opening into penial sheath. Deferent duct convoluted and coiled but loops always easy to count (fewer than 10). The diameter of deferent duct, more or less constant, is ∼ 250 µm maximum. Retractor muscle anchors on penial sheath near where deferent duct joins penial sheath and inserts on floor of visceral cavity between one third and one half of its length, just posterior to nervous system. Length of retractor muscle rarely exceeds 3 mm.

Discussion

Of all the species described here, O. borealis is relatively well known. Dall (1910) described in detail its habitat and colour variation. Binney (1876) illustrated accurately the radular teeth. Its anatomy was described by Marcus (1961) based on a few specimens from Oregon and California, and by Weiss and Wägele (1998) based on a few specimens from Alaska. Semper’s (1885) and von Plate’s (1893) anatomical notes are less detailed. Its dorsal colour variation and its habitat have been mentioned in field guides (e.g. Morris et al. 1980).

The present description of O. borealis is in agreement with past descriptions. However, because it is based on a total of 213 specimens examined and 19 specimens dissected, covering the entire range of the species (Alaska to California), it provides new insight on individual variation, such as, for instance, the anterior male organs. The maximum size found here (12 mm) is the maximum size found in the literature. The range of radular formulae found here of 44/78 × (32/58–1–32/58) is compatible with the literature, although Binney (1876) found a few more teeth per half row (61– 1–61) without specifying the number of rows; Weiss and Wägele (1998) found 47/49 × (40–1–40); Marcus (1961) found 50/52 × (45/53–1–45/53). Von Plate (1893) found a number of teeth per half row that is beyond all other observations: 88-1-88, in a specimen 11 mm long (which is not longer than other specimens dissected).

The taxonomy and nomenclature of O. borealis would be quite simple without O. carpenteri . The latter, originally described from the Strait of Juan de Fuca, i.e. within the range of O. borealis , has remained very confused in the literature because many authors have erroneously claimed that O. carpenteri had been described from the Gulf of California. Ultimately, it led authors to believe that there were two distinct, sympatric species of Onchidella from California to Alaska: for instance, Abbott (1954) mentioned O. carpenteri from Puget Sound to Lower California, and O. borealis from Alaska to Oregon. Onchidella carpenteri is regarded here as a nomen dubium although it was very likely based on young specimens of O. borealis (for more information, see the detailed discussion on O. carpenteri below).

Binney (1876) described the radular teeth of Onchidella borealis from Alaska, but he did not discuss the possible synonymy between his O. carpenteri , which he had described in 1861, and Dall’s O. borealis , described in 1872. Later, Binney (1885) thought there were two species in the Pacific North-west, Onchidella borealis , which he mentioned from Prince William Sound (Alaska) to Vancouver Island (i.e. Juan de Fuca), and Onchidella carpenteri , from the Strait of Juan de Fuca to the Gulf of California. According to Binney, a jaw was present in O. borealis and absent in O. carpenteri . However, this difference might have simply been due to the fact that jaws are quite inconspicuous. In any case, Onchidella borealis remains the valid name because Onchidella carpenteri is a nomen dubium (see below).

All authors have agreed that borealis belongs to Onchidella , with the exception of Semper (1885) who classified borealis in Onchidium . However, Semper (1885) only accepted two genera in Onchidiidae : Onchidina (then a monotypic genus) and Onchidium (for all other onchidiid species). In total, Semper (1885) redescribed five species that are now classified in Onchidella , and he considered them all to be in the same “group” within Onchidium . So, although he did not regard Onchidella as a valid name, he did acknowledge the close relationship of all Onchidella species.

Onchidella borealis was designated by Dall (1910) the type species for Arctonchis , a section of a subgenus ( Onchidella ), of the genus Onchidium . The name Arctonchis has never, to our knowledge, been raised to the generic rank. According to Dall, this section included both Onchidella borealis and O. celtica , both characterized by a thin jaw in the oral tube. Dall’s internal classification of Onchidella has not been adopted, especially after Watson (1925) demonstrated that the main feature used by Dall (the thin jaw) to separate O. borealis and O. celtica from the rest of the Onchidella species , might actually be shared by all Onchidella species (see below, Discussion). However, the supra-specific name Arctonchis will remain available if a name is needed for a clade including O. borealis , when the phylogeny of Onchidella is known.

Figures

Figure 11. External morphology and internal anatomy of Onchidella borealis, CASIZ 057732: (A) ventral view, spm #2, California; (B) jaw-like structure, specimen #4, California; (C) nervous system and male anterior genital organs, spm #1, mature. Scale bar: A, 2.5 mm; B, 25 µm; C, 1.5 mm. Abbreviations: dd, deferent duct; hl, hyponotal line; lc, left cerebral ganglion; lpd, left pedal ganglion; lpl, left pleural ganglion; mo, male opening; pn, pneumostome; ps, penial sheath; vs, visceral ganglion.

Figure 12. Radular teeth of Onchidella borealis: (A) entire radula, CASIZ 021510 #1, British Columbia; (B) rachidian and right innermost lateral teeth, CASIZ 021519 #2, California; (C) lateral teeth, CASIZ 021510 #1, British Columbia; (D) rachidian and innermost lateral teeth, CASIZ 021510 #1, British Columbia, arrow indicates small pointed cusp on outer, lateral expansion of base of rachidian tooth. Scale bars: A, 1 mm; B, 25 µm; C, 10 µm; D, 10 µm.