Gymnonereis tenera, Darbyshire, Teresa, 2014

Darbyshire, Teresa, 2014, Intertidal and nearshore Nereididae (Annelida) of the Falkland Islands, southwestern Atlantic, including a new species of Gymnonereis, ZooKeys 427, pp. 75-108 : 78-82

Taxon classification Animalia Phyllodocida Nereididae

Gymnonereis tenera sp. n. Figures 2 A–I, 9 A–B

Material examined.

East Falkland: Teal Creek, Stn 35d (51°49.248'S, 058°55.561'W), muddy sand, midshore, holotype (NMW.Z.2011.039.0102), 09.12.2011; Sand Bay, Port Harriet, Stn 34d (51°44.231'S, 058°00.585'W), fine sand, mid–low shore, 11 paratypes (9 –NMW.Z.2011.039.0093– 0095; 1-USNM 1231433; 1-ZMH p-27694), 08.12.2011; Teal Creek, Stn 35b (51°49.231'S, 058°55.573'W), sandy mud, midshore, 18 paratypes (NMW.Z.2011.039.0096), 09.12.2011; Teal Creek, Stn 35c (51°49.236'S, 058°55.563'W), mud, low shore, 22 paratypes (NMW.Z.2011.039.0097-0101), 09.12.2011; West Falkland: Crooked Inlet, Roy Cove, Stn 55b (51°32.546'S, 060°20.562'W), fine sand, high-midshore, 4 paratypes (1-AM W.46477; 1-NHMUK ANEA2014.31; 2- NMW.Z.2012.082.0001), 30.01.2013; Crooked Inlet, Roy Cove, Stn 55c (51°32.595'S, 060°20.367'W), fine sand, midshore, 2 paratypes (NMW.Z.2012.082.0002), 30.01.2013; Crooked Inlet, Roy Cove, Stn 55d (51°32.664'S, 060°20.255'W), fine sand, low shore, 3 paratypes (NMW.Z.2012.082.0003-0004), 30.01.2013; Crooked Inlet, Roy Cove, Stn 55e (51°32.688'S, 060°20.244'W), fine sand, low shore, 2 paratypes (NMW.Z.2012.082.0005), 30.01.2013.

Description.

Holotype complete, 98 mm long, 1.5 mm wide (excluding parapodia; measured at widest part of anterior - approximately chaetiger 8), for 160 chaetigers. Complete paratypes 3-143 mm long, 0.15-2.53 mm wide (excluding parapodia) for 28-166 chaetigers. Description based on observations of the holotype and a dissected paratype (NMW.Z.2011.039.0098) used for illustrations. Variation shown by other paratypes described in later section.

Body depressed dorso-ventrally, widest anteriorly on chaetigers 8-10 (more pronounced in smaller specimens), then mostly uniform in width before tapering posteriorly. Colour pink/orange or grey/white in alcohol with black aciculae. Neurochaetae and subacicular notochaetae dark golden in anterior chaetigers, supracicular chaetae pale amber; all chaetae pale amber from chaetiger 14. Live animals bright red on each side of body, including the parapodia, in region of vascularized, enlarged cirrophores; rest of body often with bright white dorsal bands centrally either side of central blood vessel from end of vascularized cirrophore region, fading in posterior. Where white colouration absent, body transparent, coloured only by visible gut and blood vessels. Methyl green staining of preserved animals shows glandular areas on tips of cirri and parapodial lobes but not on cirrophores or main body. Cuticle very soft when animals alive as well as post-fixation, body breaks easily when handled.

Prostomium with 2 pairs small, black (dark red when alive) eyes, often difficult to discern when preserved (Figs 2A, 9A). Anterior pair smaller, more laterally placed than posterior pair; crescent-shaped with additional small spot in far corners. Posterior pair darker, rounded. Prostomium subrectangular with deep cleft between antennae (Fig. 2A). Palps with large squat palpophores and short triangular palpostyles (0.4 mm long, 0.27 mm wide). Antennae equal length to or slightly longer than palps, more slender in form. Four pairs tentacular cirri, ventral pairs of equal length, 2/3 to 1/2 length of dorsal pairs; 2nd pair dorsal tentacular cirri marginally longer than 1st pair, reaching to chaetiger 4.

Peristomium dorsally more narrow than following segments. Jaws with smooth edges, teeth absent (Fig. 2 A–B). Oral ring with triangular papillae arranged as follows (Figs 2A; 9A, B): Area V–VI = 3, VII–VIII = 7; maxillary ring bare.

Chaetigers 1-2 uniramous (Fig. 2C), single black acicula, tip curved, just emergent. Subsequent chaetigers all biramous (Fig. 2 D–I), notoacicula not emergent, neuroacicula thicker, emergent anteriorly only up to around chaetiger 50. Dorsal cirri of chaetigers 1-12 with accessory dorsal cirrus (Fig. 2A, C–E), up to 1/3 length of main cirrus, appearing as extension to cirrophore rather than dorsal cirrus. From chaetigers 16-52 (Fig. 2 F–G), dorsal cirrophores expanded and vascularized, although start and end of region difficult to define. Remaining chaetigers with dorsal cirrus long, narrow, tapering (Fig. 2 H–I).

Double ventral cirri present throughout (Fig. 2 C–I), branches unequal, ventral branch reducing in size posteriorly. Dorsal branch 1.5 times as long as ventral branch in anterior region, 4-5 times as long posteriorly.

Chaetiger 1 (Fig. 2C), neuroacicular papilla small, rounded, posterior and slightly dorsal to digitiform prechaetal lobe. Postchaetal lobe broad, rounded, approximately 2/3 length of prechaetal lobe. Acicular lobe similar shape to postchaetal lobe, approximately 1/2 length. Ventral neuropodial ligule of same size and shape as prechaetal lobe.

Chaetiger 3 (Fig. 2D) with basally swollen, digitiform notopodial prechaetal lobe twice as long as broadly rounded notopodial postchaetal lobe; acicular lobe 1/4 length of latter. Notochaetae in 2 unequal bundles, arranged as a smaller semicircle above and larger semicircle below the notopodial prechaetal lobe. Neuropodium as for chaetiger 1, ventral ligule of same size and shape as neuropodial prechaetal lobe. Neurochaetae in 2 semicircular fascicles of greater density than notochaetae. Superior fascicle arranged around neuroacicular papilla with larger, inferior bundle ventral and posterior to neuropodial prechaetal lobe. Arrangement continues to start of vascularized cirrophores then number of chaetae reduces posteriorly, becoming bundles rather than semicircles. Greatest density of chaetae occurs in chaetigers 6-8.

Posteriorly, neuropodial prechaetal lobe reducing in size, ventral ligule even more so. Neuropodial postchaetal lobes also decrease in size proportionately, becoming more conical.

Noto- and neurochaetae consist of both homogomph and sesquigomph spinigers throughout, no falcigers observed. Accurate numbers of chaetae and distribution of homogomph versus sesquigomph chaetae on anterior chaetigers difficult to identify due to density.

No dorsal flaps connecting chaetigers. Transverse, faintly defined ridges present from chaetiger 11-16.

Pygidium with anus terminal; 2 long cirri ventral to anus. Anal cirri of similar shape to dorsal cirri on body, 1.2 mm long.

Eggs found in 2 specimens, spherical, 120-130 µm diameter.

Variation.

Most characters varied with number of chaetigers and continued to change as the number increased. Accessory dorsal cirri were not observed on animals with less than 95 chaetigers (unless regenerating) although they were absent in one specimen of 103 chaetigers (62 complete specimens examined; 27 with less than 95 chaetigers, 35 with 95 or more chaetigers). As chaetiger number increases, additional anterior dorsal cirri have accessory cirri, with animals of more than 160 chaetigers with accessory dorsal cirri as far as chaetigers 10-14. The variation in this character means that it should not be used as diagnostic for the species on its own but only in conjunction with other characters.

The faint transverse ridges connecting parapodia were mostly visible from chaetiger 11 to 15 or 16 but were occasionally observed as far back as chaetiger 20 on the largest specimens.

Determination of the start and end of the expanded cirrophores was difficult, particularly the former, as the transition was not as abrupt as described for some species. The region generally occurred from around chaetigers 11-18 and continued to chaetigers 22-51 over the range of body sizes observed.

Presence and number of the oral papillae did not vary with size although papillae were occasionally lost and a single specimen was identified with 4 papillae in Area V–VI. Relative length of tentacular cirri was also stable with the longest cirri always reaching to chaetiger 4 in all body sizes.

Although jaw teeth were absent in the majority of specimens, juveniles of less than 80 chaetigers (jaws of 26 specimens were examined including 12 juveniles of 33-80 chaetigers in size) were found to have 4-5 small teeth on each jaw with jaws in larger animals becoming more roughly crenated until the largest jaws appeared almost completely smooth.

Etymology.

The specific name tenera is derived from the latin adjective tener meaning 'soft, delicate’, referring to the very soft nature of the worm when alive and its fragility when handled.

Habitat.

Found intertidally from mid to low shore in soft, fine, sand or mud sediments.

Remarks.

With 3 papillae in Area V–VI of the oral ring and the absence of jaw teeth, Gymnonereis tenera sp. n. can be distinguished from all other Gymnonereis species except for Gymnonereis sibogae and Gymnonereis phuketensis . Gymnonereis minyami and Gymnonereis yurieli both have jaw teeth and only 1 papilla in each of Areas V and VI. Gymnonereis crosslandi and Gymnonereis fauveli both lack jaw teeth but Gymnonereis crosslandi has only 1 papilla in each of Areas V and VI, accessory dorsal cirri in only chaetigers 1 and 2 (chaetiger 1 to 12 or further in Gymnonereis tenera sp. n.) and no enlarged dorsal cirrophores, whilst Gymnonereis fauveli has 5 papillae in Area V–VI and accessory dorsal cirri from chaetiger 3 (as opposed to chaetiger 1 in the new species).

Gymnonereis tenera sp. n. is most similar to both Gymnonereis sibogae and Gymnonereis phuketensis and can only be distinguished from each of these through combinations of characters. Although Hutchings and Reid (1990) listed Gymnonereis sibogae as having sesquigomph falcigers, Horst (1918), in his original description, actually stated that "the neuropodial fascicle does not contain true setae falcigerae, but instead of these some faintly heterogomph setigerous bristles, with a short, lanceolate terminal piece", although his figures of the species ( Horst 1924) did not illustrate this. Pettibone (1970) re-investigated and drew all of Horst’s specimens and in her detailed description of the first two chaetigerous segments stated that "a few lower neurosetae of some anterior setigers may have blades which end bluntly" and this was figured accordingly ( Pettibone 1970, fig. 30 c–e). No such short, blunt chaetae were observed on any specimens of Gymnonereis tenera sp. n. A more consistent character is that of the length of the anterior, neuropodial prechaetal lobe. In Gymnonereis tenera sp. n., this lobe is consistently longer than both the neuropodial acicular and postchaetal lobes and of a similar length to the ventral ligule. In Gymnonereis sibogae , the neuropodial prechaetal lobe (termed the prechaetal ligule by Pettibone 1970) is as long as or shorter than the postchaetal lobe and shorter than the ventral ligule for at least the first nine chaetigers ( Horst 1924, pl. XXX, fig. 1; Pettibone 1970, fig. 30 c–d, fig. 31a,d,e,f, fig. 33b), thereafter becoming only slightly longer. Unfortunately, all of Horst’s specimens were incomplete with only 36-56 segments and the species does not appear to have been reported since, making further determination of differences between the two species difficult.

Apart from the character of presence or absence of jaw teeth, the new species is also very similar to Gymnonereis phuketensis , although juveniles of the new species do have a small number of jaw teeth. Hutchings and Reid (1990) listed the character of jaw teeth as being present or absent for Gymnonereis phuketensis , although the original description by Hylleberg and Nateewathana (1988) states only that they are present (adult specimens, no comments on the juvenile form) but that they can be weakly defined. Where jaw teeth are found in Gymnonereis tenera sp. n., however, there are only up to 5 compared to 10 for Gymnonereis phuketensis . Additionally, in Gymnonereis phuketensis the dorsal cirrophores become "abruptly enlarged" from chaetiger 14 ( Hylleberg and Nateewathana 1988) compared to a more gradual enlargement from chaetiger 12 for the new species and the second ventral cirrus is absent from around chaetiger 35 on Gymnonereis phuketensis but present throughout on Gymnonereis tenera sp. n.