Dipolydora giardi Mesnil 1896

Dipolydora giardi (Mesnil, 1896)

(Fig. 2)

Polydora giardi Mesnil, 1896: 195 –202, pl. 13, figs. 1–12. Fauvel, 1927: 50 –52, fig. 17 h–m.

Dipolydora giardi: Blake, 1996: 186. [New combination introduced. D. giardi record uncertain].

Material. FRANCE: La Manche, coll. & id. F. Mesnil, accessioned ZMUC 22 June 1896, ZMUC POL­ 653 (3 spec.). ITALY: Ischia Is., Gulf of Naples, 40°44.8' N, 13°56.7' E, 2 m, from shells of the whelks Stramonita haemastoma (L.) and Hexaplex trunculus (L.), coll. V. I. Radashevsky, 18 Jul 1994, BMNH 2002.620–629 (17 spec.).

Comparative material. Dipolydora trilobata: RUSSIA: Sea of Japan, Vostok Bay of Peter the Great Bay, 42°53.6' N, 132°44.1' E, from shells of the oyster Crassostrea gigas (Thunberg), ZISP 1/46576­3/46578 (holotype + 19 paratypes); IMBV 1/12171 (50 paratypes); USNM 183520 (30+ spec.).

Description. Mesnil’s material consists of three complete individuals 3–4 mm long and 0.3–0.4 mm wide for c. 60, 70, and 75 chaetigers. One individual regenerating two anterior segments. Largest complete specimen from Italy c. 4 mm long and 0.3 mm wide for 60 chaetigers. Body pigmentation absent. Prostomium incised to bifid anteriorly. Caruncle extending posteriorly to end of chaetiger 3. Eyes and nuchal antenna absent (Fig. 2 A). Palps extending posteriorly for 10–15 chaetigers.

Chaetiger 1 with postchaetal lamellae in both rami; both noto­ and neurochaetae short capillaries, fewer than in following chaetigers. Posterior notopodia with only capillaries, without spines.

Chaetiger 5 greatly modified, larger than chaetigers 4 or 6, without postchaetal lamellae, with 2 or 3 dorsal superior geniculate capillaries (Fig. 2 H), 4 or 5 major spines alternating with bilimbate­tipped companion chaetae (Fig. 2 I), and a tuft of 4 or 5 ventral capillaries (Fig. 2 G). Major spines falcate, with lateral tooth on one side and fine spur or protuberance on the other side; spines arranged in a slightly curved diagonal row; lateral tooth occasionally broken in older spines positioned in anterior part of the row.

Hooded hooks in neuropodia from chaetiger 7, up to 5 in a series, accompanied by single, winged inferior capillary chaetae in chaetigers 7–9 (Fig. 2 D) and by 1 or 2 hair­like capillaries situated between hooks in a few posterior chaetigers (Fig. 2 E). Hooks bidentate throughout, with angle between main fang and shaft wider in posterior chaetigers; hook shaft slightly curved, without constriction (Fig. 2 C, F).

Branchiae from chaetiger 9–10, usually from 10 but one specimen with short branchial buds present on chaetiger 9 (Fig. 2 A); branchiae full­sized from chaetiger 11, absent from posterior half of body. Branchiae flattened, with surfaces oriented parallel to body axis, free from notopodial postchaetal lamellae.

Pygidium small, cup­shaped, with one ventral lobe and two smaller dorsal lobes, appearing white because of large number of spindle­shaped mucous cells (Fig. 2 B). Glandular pouches from chaetiger 6, full­sized from chaetiger 7 or 8, diminishing in size after chaetiger 10 or 11.

Gizzard­like structure present at about chaetiger 17. Hind gut white.

Metanephridia opening middorsally via two separate nephridiopores on fertile segments apparent under methyl­green staining in a 40­segment posterior fragment from Italy which had oocytes up to 130 µm in diameter in middle segments. No sperm were recognized in this fragment or in other specimens.

Methyl­green staining patterns. Head and anterior chaetigers do not retain stain. Staining dorsally as paired spots, one on each half of chaetiger, usually from chaetiger 8 or 9, occasionally from chaetiger 7; size of spots and intensity of staining gradually increasing posteriorly.

Remarks. No differences in size, morphology and life style (boring in calcareous substrata) could be detected between the Mesnil’s worms from northern France and those from the Mediterranean and thus we are confident they are D. giardi sensu stricto Mesnil as indicated. Two specimens from the Mesnil’s sample found in Copenhagen have an indistinct pygidium, but one of them clearly has a middorsal cleft and two dorso­lateral clefts that divide the cup into one ventral lobe and two smaller dorsal lobes. The same trilobed pygidium was also invariably present in all specimens collected by us from Italy.

Dipolydora trilobata specimens examined for the comparison resemble D. giardi in that they also have an incised to bifid prostomium, caruncle extending to the end of chaetiger 3, pygidium with three small lobes, major falcate spines of chaetiger 5 with large lateral tooth on one side and a smaller accessory spur on the other side. Maximal size of worms and boring mode of life are also identical. Both species lack eyes, occipital antenna and modified spines in posterior notopodia. However, branchiae in D. giardi begin on chaetiger 10 (19 specimens in 20 examined), rarely on chaetiger 9 (1 specimen), whereas in D. trilobata they usually start on chaetiger 9 (46 specimens in 83 examined), occasionally on chaetiger 8 or 10 (23 and 13 specimens in 83 examined, respectively), and rarely on chaetiger 7 (1 specimen). Hooded hooks in D. giardi are distinctly bidentate in all neuropodia, whereas in D. trilobata they become unidentate in 2–5 posterior chaetigers (Radashevsky 1993: fig. 17I). A more fundamental difference may be that Mesnil's D. giardi were reportedly simultaneous hermaphrodites (Mesnil 1896: p. 200) while D. trilobata is a gonochoristic species (Radashevsky unpublished). The description of gamete arrangement in D. giardi provided by Mesnil is, however, ambiguous and hermaphroditism in the species needs to be confirmed by further studies.

Polydora anoculata Moore, 1907 from Massachusetts is also a similar species. Blake (1971) examined the holotype and emended the diagnosis. Subsequently Maciolek (1984) did not find differences between Moore’s and Mesnil’s species and placed P. anoculata into the synonymy of P. giardi. Blake (1971) and Maciolek (1984) reported that specimens from Massachusetts and North Carolina had the caruncle extending posteriorly to chaetiger 3–5, branchiae beginning from chaetiger 9–10, and the pygidium small and disclike, with only a dorsal notch. However, these characteristics indicate that Dipolydora anoculata is not only validly described, but also distinct from D. giardi and D. trilobata.