Dysponetus joeli Olivier 2012

Dysponetus joeli Olivier et al., 2012

Figs 1 A–D, 2A–D

Dysponetus Joeli Olivier et al., 2012: 989–996, figs 1–3.

Chausey, France, Sta. SSMM01 (48° 55.570’ N, 001° 48.270’ W), maerl, 10.0 m, holotype (MNHN POLY TYPE 1533), paratype (MNHN POLY TYPE 1534), 18 Apr. 2006; off St Mary’s, Isles of Scilly, Sta. 3b (49° 54.20’ N, 006° 18.94’ W), coarse sand/shell/gravel with some silt/clay, 25 m, 2 specimens (NMW.Z.2009.027.0001–0002), 25 Jun. 2009; west of St Martin’s, Isles of Scilly, Sta. 14b (49° 57.86’ N, 006° 15.21’ W), shell gravel in muddy sand, 35.1 m, 2 specimens (NMW.Z.2009.027.0003–0004), 27 Jun. 2009; west of Isles of Scilly, Sta. 24b (49° 55.22’ N, 006° 23.91’ W), silty coarse sand/shell gravel, 47.1 m, 1 specimen used for SEM (NMW.Z.2009.027.0005), 29 Jun. 2009.

Holotype in 4 pieces, posteriorly incomplete, 3.1 mm long for 19 chaetigers. Paratype in 3 pieces, anterior dissected fragment of 5 chaetigers with 2 further detached chaetigers. 3 non-type specimens up to 6 mm long, 0.35 mm wide (between segments, not including parapodia or chaetae) with 25–33 chaetigers. Two other incomplete non-type specimens examined. The following description is based on entire specimens from the Isles of Scilly, except where specified as pertaining to the type material.

Body shape cylindrical, ventrally flattened, tapered slightly at posterior. Body pale cream in colour (fixed, unstained), eyes dark reddish brown (Fig. 2A) but pigment may degrade in alcohol. Eyes of type specimens already barely discernable.

Prostomium oblong, wider anteriorly (Fig. 1A). Four large, rounded eyes visible in fixed specimens, anterior pair larger and more widely separated than posterior. Median antenna small, bottle-shaped, arising anterodorsally from prostomium. Lateral antennae bottle-shaped, slightly smaller than median, arising immediately dorsal to palps. Antennae without distinct ceratophores. Palps directed posteriorly, stout, oval, more than twice as long as wide. No antennae or palps remaining on type specimens examined. Nuchal organs not observed.

Distinct single mouth appendage on lower lip, anteriorly directed, digitiform with blunt tip (Fig. 1B, 2B). Single pair of stylet-shaped jaws, visible through body wall with methyl green staining. Proboscis not observed.

First two segments slightly elevated dorsally with four pairs tentacular cirri, longer than but with same shape as dorsal cirri of third and following segments, anteriorly directed. First segment achaetous, second segment with notochaetae only, situated slightly anterior to dorsal tentacular cirrus. Third segment biramous; dorsal cirri present, ventral cirri absent (Figs 1A, B, D, 2B). Holotype lacking all cirri on anterior three segments, but presence indicated by cirrophores (Fig. 1D). Following segments all biramous with both dorsal and ventral cirri. Single noto- and neuroacicula present in each parapodium.

Notopodial lobes reduced. Dorsal cirri long, slender, longer than chaetae (210–430 µm, longest on median chaetigers), cirrophores present. Styles slightly proximally swollen, distally tapering, tips blunt. Notoacicula difficult to detect. Notochaetae inserted dorsal to cirrus, densely packed, directed posteriorly leaving middle part of dorsum exposed. Chaetae D-shaped in cross-section (Fig. 2C) with denticles sharply pointed, in two parallel rows, 15–20 on each side. Notochaetal count, mid-body segments, up to 26.

Neuropodia well-developed, conical mounds. Compound neurochaetae, with heterogomph shafts and fine bidentate falcigerous blades (Fig. 2D). Neurochaetal count, mid-body segments, at least 20–26 (chaetae densely packed and difficult to accurately count). Up to two accessory simple chaetae, similar to but smaller than notochaetae, inserted distally and anteriorly on neuropodial lobe (Fig. 1C). Ventral cirri fusiform, shorter than dorsal cirri (length 110–270 µm, longest on median chaetigers), arising posteroventrally on neuropodial lobe (Fig. 1C).

Final segment lacking noto- and neurochaetae, cirrophores of dorsal and ventral cirri observed although only a single, rounded dorsal cirrus observed on 1 specimen (NMW.Z.2009.027.0003). Pygidium conical with single projection (length 40 µm), cylindrical, slightly distally tapering, inserted posteroventrally (Fig. 1C), anus terminal.

Eggs visible within one specimen (NMW.Z.2009.027.0001), flattened oval in shape, maximum width approx. 50 µm, possibly immature.

The species was originally described from maerl beds in the Bay of Biscay and the English Channel. The new specimens are all from coarse sand and shell sediments collected around the Isles of Scilly.

The original description of D. Joeli was based on small, damaged specimens and this may explain why the absence of the ventral cirri on chaetiger 3 was attributed to loss of the cirri rather than actual absence. Although cirri are easily lost from specimens, the cirrophores can still be seen under light microscopy at x400 magnification or greater. Application of methyl green staining can help distinguish these features. Under SEM conditions the absence of cirrophores on chaetiger 3 is obvious (Fig. 2B).

An additional complication in determining the described characters of D. Joeli arises from the character matrix scores in the original paper (Olivier et al. 2012). In their species description, the authors stated that there were 2 pairs of tentacular cirri on the first 2 segments but made no mention whether segment 3 possessed or lacked ventral cirri. They then later scored ventral cirri as absent for the first 2 segments (no separate score for tentacular cirri) and present for the third in the character matrix However, in the species key itself, D. Joeli is keyed out with ventral cirri being present on the 3 rd segment (i.e. chaetiger 2). It is apparent from the character states scored for the other species in the matrix that the authors have used the term ‘ventral cirrus’ interchangeably with ‘tentacular cirrus’. Earlier authorities have also been variable in their use of the terms ‘tentacular cirrus’ versus ‘cirrus’ with regards to the appendages of the first 3 segments in species of Dysponetus. In this paper, only the cirri of the first 2 segments, which lack parapodia and have cirrophores directly attached to the body wall, are regarded as being tentacular. A later paper will discuss the different characters of Dysponetus species in more detail.

The lack of ventral cirri on segment 3 distinguishes D. Joeli from most other Dysponetus species. The only other species with this characteristic are D. bidentatus Day, 1954, D. bipapillatus and D. macroculatus, although D. bidentatus lacks ventral cirri on segment 2 also (Day 1954). The presence or absence of ventral cirri on segment 3 is unknown for D. hebes (Webster & Benedict, 1887); however, this species differs considerably by having a double mouth appendage as opposed to single, and sphaerical not elongated palps. With the revised character for segment 3, the species key in Olivier et al. would place D. Joeli with both D. bipapillatus and D. macroculatus. The latter two species are distinguished in the key according to the relative size of the eyes; however, this is not a good character as the eye pigments degrade in alcohol and disappear over time – as is already evident in the type specimens. A new revised key to the 12 species of the genus is provided below.

Dysponetus Joeli is a much larger species than both D. bipapillatus and D. macroculatus. Even the type specimens, described as ‘small’ (ranging from 3.5–4 mm in length, incomplete) and smaller than those from the Isles of Scilly, were 2–3 times the size of each of the latter species, respectively.

Dysponetus Joeli is most similar to D. bipapillatus from the Mediterranean, sharing all of the obvious characteristics. The paired papillae on segment 8 described for D. bipapillatus were not observed on any of the D. Joeli specimens; however, these appendages were only seen on a few specimens of D. bipapillatus and are thus not a good character for comparison. In his paper, Dahlgren (1996) stated that the specimens, though small, were considered mature adults due to the presence of the paired papillae, interpreted as genital organs, and the development of the anterior segments. A comparison of the individual characters shows D. Joeli to possess twice as many neurochaetae as D. bipapillatus across all segments (20–26 compared to 9–13), more numerous denticles on the notochaetae (15–20 as opposed to 10–15) and a shorter pygidial projection (40 µm versus 50 µm), the last character particularly noticeable in relation to the larger body size of D. Joeli. Similarly, the number of neurochaetae in D. Joeli is greater than the 19–22 found on D. macroculatus, and, in general, the former species is a much larger animal than the latter, being 2–3 times as long at maturity (as evidenced by the presence of eggs in specimens of both).