Spio symphyta, Bick, Andreas & Bastrop, Ralf, 2011

Bick, Andreas & Bastrop, Ralf, 2011, On the identity of Spio filicornis (O. F. Müller, 1776) — with the designation of a neotype, and the description of two new species from the North East Atlantic Ocean based on morphological and genetic studies, Zootaxa 2815, pp. 1-27 : 19-23

Spio symphyta sp. nov.

(Figs 11, 12)

Spio filicornis ( O.F. Müller, 1776) . – Böggemann, 1997: 120, fig. 95. Spio cf. filicornis (North Sea) Bick et al., 2010: 166 –168, figs. 3, 9d, 10.? Spio sp. nov. Sikorski, 2001: 282, text figure.

Holotype. North Sea: 55° 13.060N, 06° 51.297E, September 2002, 29 m ( ZSRO P- 2064).

Paratypes. North Sea: 55° 13.060N, 06° 51.297E, September 2002, 29 m, 11 specimens ( ZSRO P- 1862); 54°26.00ʹN, 08°10.00ʹE, 23.08.2006, 18 m, 3 specimens ( ZSRO P- 2063); 54° 35.04N, 8° 07.50E, 22.08.2006, 14 m, 1 specimen ( ZSRO P- 2065); 54° 59.70N, 7° 06.40E, 15.10.2003, 26 specimens ( ZSRO P- 2069), 7 specimens ( ZMH P- 25912), 7 specimens ( SMF 19524); 54° 54.700N 3° 14.000E, 18.03.2009, 36 m, 1 specimen ( ZMH P- 25911).

Non-type material. 54° 33.00N, 2° 56.00E, 35 m, 19.03.2009, 1 specimen ( ZMH P- 25788).

Diagnosis. Anterior part of prostomium bluntly rounded, anterolateral part slightly expanded, with shovel-like appearance; posterior part of prostomium fused with peristomium; branchiae of chaetiger 1 slightly shorter and narrower than those on following chaetigers, absent on last two to three chaetigers; U-shaped nuchal organ; doublepaired metameric dorsal ciliated organs present between transverse ciliated bands (tcb) of two consecutive segments, starting between tcb of chaetigers 4 and 5, i.e., after third tcb; proximal part of neuropodial postchaetal lamellae on first two chaetigers usually distinctly shorter than transverse row of corresponding capillaries; notopodial postchaetal lamellae on very last chaetigers cirriform.

Description. Holotype complete specimen (without palps) with 58 chaetigers, about 13 mm in length and 1.2 mm wide. Other examined specimens with 36 to 60 chaetigers, 4 to 16 mm in length and 0.5 to 1.3 mm in width.

Anterior part of prostomium bluntly rounded, without median incision, anterolateral part slightly expanded giving it a shovel-like appearance (Figs 11A, 12A, C, F); posterior part of prostomium sometimes with papilliform elevation, beginning in front of posterior eyes and terminating on chaetiger 1 ( Fig. 12 A); 2 pairs of black eyes arranged in trapezoid, anterior pair crescent shaped or oval, widely spaced, posterior pair round, closely spaced (Fig. 11A); posterior part of prostomium fused with peristomium (Figs 11A, 12F).

Nuchal organs U-shaped due to posterior fusion of median and lateral ciliated bands, pair of median ciliated bands reaching 1st tcb on chaetiger 2; pair of lateral ciliated bands long and recurved, going back to 1st tcb, outwards curved in the range of 1st tcb on chaetiger 2 (Figs 11A, 12A, C); metameric dorsal ciliated organs doublepaired, beginning from between branchiae 4 and 5 (i.e., after the third tcb), posterior extension depending on size of specimens: up to chaetiger 24 in specimens with about 40 chaetigers and about 45 in 58-chaetiger specimens; dorsal ciliated organ accompanied by ciliated tufts and short transverse ciliated bands on middle chaetigers ( Fig. 12 B).

Branchiae from chaetiger 1, continuing to almost end of the body, only last two or three chaetigers without branchiae (holotype: three chaetigers without branchiae); branchiae on first chaetiger usually slightly but sometimes distinctly shorter, narrower than those on following chaetigers; branchiae reaching midline dorsally and touching on first third of chaetigers, becoming thinner and shorter posteriorly: usually only last, sometimes last two branchiae shorter than notopodial postchaetal lamellae (Fig. 11B); branchiae with narrow base, tapering distally; on anterior chaetigers basally fused with notopodial postchaetal lamellae, distinctly separated from lamellae in posterior chaetigers (Figs 11D–I).

First notopodium shifted dorsally. Notopodial postchaetal lamellae on anterior chaetigers oval, becoming smaller and elongated on middle and posterior chaetigers; notopodial lamellae straighten up and resemble branchiae on the very last chaetigers (Fig. 11B). Neuropodial postchaetal lamellae in anterior chaetigers small, base of postchaetal lamellae usually distinctly shorter than transverse row of corresponding chaetae in anteriormost chaetigers; neuropodial postchaetal lamellae becoming larger along anterior chaetigers, oval in middle and posterior chaetigers (Figs 11D–I).

Notopodial chaetae all capillaries with narrow sheaths; capillaries of anterior chaetigers arranged in two rows: chaetae of anterior row short, broad, distinctly granulated; chaetae of posterior row longer, thinner, lacking granulations; additional superior fascicle of very long, thin capillaries without granulations; capillaries of posterior chaetigers arranged in irregular rows, thin, non-granulated, of different length. Neuropodia with rows of capillaries and hooded hooks as well as an inferior fascicle of capillaries; capillaries of anterior neuropodia arranged in two rows, similar to notochaetae (Figs 11L, M); posterior row replaced by single row of about 8 (range 5 to 12, 8 in holotype) hooded hooks from chaetiger 11, rarely from chaetiger 10 (Fig. 11F); third row of very thin, alimbate, finely granulated capillaries on posteriomost neuropodia absent; hooks slightly narrowed subdistally, with short hood, bi- or tridentate, main fang well developed, third tooth hardly visible (Figs 11J, 12D, E); inferior fascicle with 2 to 5 long, thin, limbate capillaries without granulations from chaetiger 1, replaced by 2 to 3 sabre chaetae from middle chaetigers, each distally granulated (Fig. 11K).

Pygidium with four anal cirri; dorsal pair more slender (particular at base) than ventral pair or of same size; dorsal pair more widely spaced than ventral pair; anus terminal (Fig. 11B).

Pigmentation. Faintly pigmented in comparison to S. filicornis and S. arndti sp. nov.; pigment pattern mentioned below usually only visible on well-preserved specimens (holotype without distinct pigmentation). Palps sometimes with light brown pigment on anterior margin of base of food grooves; light brown pigment on anterior part of body as follows: border between anterior part of prostomium and peristomium with longitudinal pigment stripes; transverse stripes laterally between peristomium and chaetiger 1; regular or irregular light brown longitudinal stripes ventrally around mouth opening; pigmented patches in front of and after tcb’s dorsolaterally usually from chaetiger 4 to chaetiger 10 (range from chaetiger 2 to chaetiger 16); posterior margin of branchial base from chaetiger 1 to about 5 with light brown pigment, additionally anterior margin of about first one to four branchiae near base with light brown pigment; borders of chaetigers 1/2 to about chaetigers 6/7 median with continuous or interrupted transverse light brown pigment stripes.

Methyl green staining pattern. Anterior part of prostomium and peristomium as well as papilliform caruncle stain intensively ( Figs 12 C, F); branchiae and margins of postchaetal lamellae of anterior and medium chaetigers also stain intensively ( Fig. 12 C); posterior postchaetal lamellae and branchiae as well as anal cirri completely deep blue after staining.

If specimens are transferred from the methyl green solution into distilled water again, after about 10 minutes 1 to 4 pairs of white dots against the bluish background become visible ventrally on anterior chaetigers ( Fig. 12 G). First pair of dots usually visible on chaetiger 6 (rarely on chaetiger 4) ventrolaterally, about four pairs present from chaetiger 8 to chaetiger 20, diminishing continuously to 1 pair on about chaetigers 25–30 (white dots from chaetiger 6 to chaetiger 30 in holotype). Following pattern of white dots were found on chaetigers 4 to 30 on the investigated specimens (n=10, data for the holotype in parentheses): chaetiger 4–5: 2 (0), 6: 2 (2), 7: 2 (3), 8–9: 3 (3), 10– 14: 3 (4), 15–22: 4 (4), 23–24: 3 (4), 25–26: 3 (3), 27: 3 (2), 28–29: 2 (2), 30: 1 (1). These dots are arranged in a row but the median pair is slightly shifted posteriorly. They obviously correspond to the ventral openings shown for S. filicornis in Fig. 5F (see discussion).

Biology. Spio symphyta sp. nov. was found in fine and coarse sand areas without silt in subtidal regions of the North Sea.

Geographical distribution. So far known only from the North Sea (type locality).

Remarks. Spio symphyta sp. nov. is distinguished from the all other Spio species by the combination of the following characters: fusion of prostomium and peristomium, shovel-like appearance of prostomium, U-shaped nuchal organ, dorsal ciliated organ from chaetiger 4 to chaetiger 25 to 45 (size-dependent), branchiae absent on two or three posteriormost chaetigers.

FIGURE 11. Spio symphyta sp. nov.: A. Anterior end, dorsal view. B. Posterior end, dorsal view. C. Chaetigers 9–13, lateral view. D–I. Parapodium from chaetiger 1, 10, 11, 20, 18th last, and 9th last, all anterior view. J. Neuropodial hooded hook from chaetiger 20. K. Sabre chaeta from 18th last. L. Anterior neurochaeta from the same. M. Anterior neurochaeta from chaetiger 20. — B, H, I, K, L (ZSRO-P2063), all others (ZSRO-P2065). Scale: A, C 0.5 mm, B 0.3 mm, D–I 0.1 mm, J–M 10 µm.

Closely related species are S. blakei Maciolek, 1990 (syn. S. pacifica Blake and Kudenov, 1978 ), S. decoratus Bobretzky, 1870 and S. pettibone Foster, 1971 all sharing the following characters with S. symphyta sp. nov.: anterior margin of prostomium rounded, large branchiae on chaetiger 1, tridentate hooded hooks first appear on chaetiger 11.

Spio blakei possesses a median furrow on the prostomium, a bilobed caruncle, prechaetal lobes in anterior notopodia and sabre chaetae with partial hood formed by extension of sheath. All these characters are absent in S. symphyta sp. nov.

Spio pettibone differs from S. symphyta sp. nov., in having a variable beginning of hooded hooks (chaetiger 12–16 ( Foster 1971), 11–15 ( Johnson 1984), 11–17 (Maciolek 1990)) rather than starting on a defined chaetiger (chaetiger 11), and the prostomium and peristomium are not fused as in S. symphyta sp. nov.

Spio decoratus differs from Spio symphyta sp. nov., in having a broad transverse pigmented band on the prostomium and peristomium rather than randomly distributed pigmented patches and the metameric dorsal ciliated organs beginning between branchiae 5 and 6 rather than branchiae 4 and 5 ( Bick et al. 2010). In S. decoratus the third apical tooth of the hooded hooks is clearly visible, but hardly discernable in S. symphyta sp. nov. Juvenile specimens of S. symphyta sp. nov. are morphologically very similar to S. decoratus . A character that can be used to separate them is the furrow between the prostomium and peristomium which is present in S. decoratus but absent in Spio symphyta sp. nov. ( Bick et al. 2010).

Intraspecific variation in S. symphyta sp. nov. include the posterior extension of the metameric dorsal ciliated organ, the number of hooded hooks, the length of the first branchiae and the shape of the caruncle. The number of hooded hooks and the posterior extension of the metameric dorsal ciliated organ are size dependent. Smaller specimens with about 35–45 chaetigers possess 6 neuropodial hooks on average whereas larger specimens with about 55–60 chaetigers have 9 hooks. A positive correlation between the posterior extension of the metameric ciliated organs and the size of the specimens could also be found in this species. It reaches chaetiger 20–24 in specimens with about 40–46 chaetigers, and chaetiger 43–48 in specimens with more than 55 chaetigers. The length of the first pair of branchiae varies between two-thirds as long as the second and same length. The caruncle can be elevated becoming a papilliform appearance in some specimens whereas it is inconspicuous in others. Both characters are independent of size.

The pattern of pigmentation has not been found particularly useful for S. symphyta sp. nov., in contrast to S. filicornis and S. arndti sp. nov., because the pigment fades quickly in ethanol.

Spio symphyta sp. nov. might not occur only in the North Sea but this still needs to be investigated. It is quite likely that Spio sp. from the Southern Norwegian Sea mentioned in the key provided by Sikorski (2001) is identical with S. symphyta sp. nov. Sikorski (2001) suggests that this species is morphologically similar to S. decoratus Bobretzky, 1870 and describes the distinct fusion between peristomium and the posterior part of the prostomium.

Figures

FIGURE 12. Spio symphyta sp. nov.: A. Anterior end, dorsal view. B. Anterior dorsum, dorsal view. C. Anterior end, dorsal view; nuchal organ and dorsal ciliated organs well observable in specimen stained with methyl green. D. Neuropodial hooks from chaetiger 20, lateral view. E. Neuropodial hook from 18 th last chaetiger, anterior oblique view; arrow points at uppermost 3 rd tooth. F. Anteriormost region, dorsal view; arrow points at fusion between prostomium and peristomium; specimen stained with methyl green. G. Ventrum of chaetigers 13 – 15, with pattern of white dots (arrows) discernible in specimen stained with methyl green. — A, B (without number, 53 ° 43.941 N, 6 ° 31.310 E, 23.9 m, 23.07.2008), C, F (ZMH P- 25911), D (ZSRO- P 2065), E (ZSRO-P 2063), G (ZSRO-P 1862).