Figures 1–2
Monticellina heterochaeta Laubier, 1961: 601 –604; Çinar 2005: 150; Ergen et al. 2006: 201; Çinar et al. 2014: 748. Tharyx heterochaeta: Laubier 1966: 631 –638; Salen-Picard 1981: 83 –88; Nicolaidou et al. 1989: 29; Salen-Picard et al. 1996: 304 –305.
Monticellina dorsobranchialis: Blake 1991: 24 –26 (In part). Not Kirkegaard (1959). Monticellina cfr. heterochaeta: Dorgham et al. 2014: 640.
Material examined. Mediterranean Sea, France, Golfe de Fos, west of Marseille, Sta. B, 35 m, coll. C. Salen- Picard, 1979, 10 specimens, donated by L. Laubier (MCZ 135295); same location, Sta. B, 35 m, 7 specimens (MCZ 135296); same location, 42 m 1 specimen (MCZ 135294).
Description. Body elongate, thin throughout with thoracic region only weakly expanded and posteriormost segments slightly enlarged. Longest specimens examined 13 mm long, reported up to 35 mm by Laubier (1966); body narrow, 0.3–0.4 mm wide on specimens examined (0.1–0.4 mm wide reported by Laubier 1966) with 50–70 setigers (30–100 reported by Laubier 1966). Body generally colorless in alcohol.
Thoracic setigers each about 5.5x as wide as long for first 12–15 setigers with parapodia shifted dorsally and elevated above mid-dorsal surface forming distinct mid-dorsal groove on thoracic segments (Fig. 1 A); dorsal surface elevated, forming a distinct ridge line inside mid-dorsal groove (Fig. 1 A). Ventral surface without grooves or ridges. Abdominal segments becoming longer and narrower, and with parapodia in lateral locations (Fig. 1 C). Most abdominal segments up to 2x as long as wide (Fig. 1 C), becoming narrower again in expanded posterior most segments (Fig. 1 D). Far posterior segments expanded bearing a large rounded pygidium (Fig. 1 D).
Pre-setigerous area or head region (prostomium + peristomium) very elongate, narrow, about 2.2x long as wide and same length as first 5–6 following setigerous segments (Fig. 1 A–B). Prostomium short, triangular, bluntly rounded on anterior margin (Fig. 1 A–B); eyes absent; nuchal organs not observed. Peristomium very elongate, relatively smooth with a single lateral groove in some specimens producing two annular rings (Fig. 1 A–B); or no rings apparent. Narrow dorsal ridge extends from prostomium about half way along dorsal peristomial mid-line (Fig. 1 A–B). Paired dorsal tentacles located close together, arising from posterior margin of peristomium (Figs. 1 A–B); first pair of branchiae posterolateral to dorsal tentacles at boundary with setiger 1, generally in line with subsequent thoracic branchiae located dorsal to notopodia (Fig. 1 A–B). Segmental branchiae of thoracic region on curved edge of each segment, somewhat projecting into mid-dorsal grove. Most branchiae broken, leaving stubs, best seen in thoracic segments and when stained with Shirlastain A; branchiae continue in mid-dorsal location for as long as observed; branchiae rarely observed in abdominal segments.
Parapodia low mounds with noto- and neuropodia closely spaced (Fig. 1 E). Thoracic parapodia shifted dorsally; abdominal parapodia more lateral.
Setae all smooth capillaries through thoracic region, numbering about 10–12 in notopodia and same in neuropodia, transitioning to denticulate capillaries in anterior abdominal segments with numbers of setae per fascicle reduced to 6–10. Denticulate setae appear in noto- and neuropodia of anterior abdominal segments from about setigers 11–19 depending on size of worm, appearing more anteriorly on smaller specimens. Denticulate setae with broad shaft and short, curved, downwardly directed denticles along one edge (Fig. 1 F–G); notosetae (Fig. 1 G) longer and narrower than neurosetae (Fig. 1 F); denticulated edge directed ventrally in notosetae and dorsally in neurosetae, thus vis-à-vis (Fig.1 E).
Methyl Green stain. A distinctive and intense MG staining pattern evident (Fig. 2 A–D). Stained oval green patch on dorsum of second half of peristomium anterior to origin of dorsal tentacles, extending only weakly down lateral sides of peristomium (Fig. 2 A, C); no stain on prostomium. Strong staining reaction in thoracic segments, with middle and posterior thoracic segments darkly blue, forming broad bands or stripes extending from middorsum down sides around venter and up opposite side; stain very intense from about setiger 3 to end of thoracic region, weaker on 2–3 segments on either end (Fig. 2 A–D); abdominal segments with mid-ventral spot or longitudinal line on individual segments (Fig. 2 B–C); thin lines of stain also present intersegmentally in abdominal segments.
Remarks. The body of Kirkegaardia heterochaeta is elongate and very slender, with only a minimal expansion of the thoracic region. The species belongs to a group of species with: (1) an elongate prostomialperistomial region that is relative smooth and weakly divided by annular rings and, (2) dorsally elevated thoracic parapodia that overlie the dorsal surface producing a conspicuous dorsal thoracic groove that contains a mid-dorsal ridge. Laubier (1966) observed superficial lines on K. heterochaeta when studied with sagittal thin sections, suggesting that two annular rings were present. However, even with Shirlastain A, annular rings are difficult to detect externally.
Kirkegaardia heterochaeta from shallow-water Mediterranean sediments is morphologically most similar to K. brigitteae n. sp. from deep-sea habitats in Antarctica and the Southern Ocean. The two species have a distinct middorsal peristomial ridge that extends from the prostomium to about halfway along the peristomium, leaving the rest of the dorsal peristomial surface smooth. Both species also have a mid-dorsal thoracic ridge and similar MG staining patterns. The two species differ in that in K. heterochaeta the mid-dorsal thoracic ridge encompasses the entire dorsal surface of the channel, whereas in K. brigitteae n. sp. there is a separate and narrow ridge on the surface within the channel. Further, the peristomium of K. heterochaeta has at least one lateral groove producing two annular rings, whereas K. brigitteae n. sp. has a smooth peristomium not interrupted by annular rings. The far posterior pre-pygidial segments of K. brigitteae n. sp. include about 15 segments that are greatly expanded, whereas the same segments of K. heterochaeta, although also expanded, are few in number, usually no more than three. The MG staining reactions are basically similar in having the venter of the thorax retaining stain; however, K. heterochaeta has an intensely Green stained patch on the dorsal surface of the peristomium, whereas K. brigitteae n. sp. has only a weak staining reaction that de-stains rapidly. Ten other species features with K. heterochaeta; these are compared in the Discussion section of this paper and detailed in Tables 1 and 2.
Biology. Kirkegaardia heterochaeta is a relatively common species along the Mediterranean coasts from France to Greece in muddy to muddy-sand sediments and is also reported from the Egyptian coast. Egg diameters of 200–225 µm were recorded for this species by Petersen (1999) from specimens collected in July from the Levantine Basin, eastern Mediterranean, in 65 m by M.E. Çinar. These large eggs suggest that development is direct for this species.
Distribution. Mediterranean Sea, shallow subtidal.