Streptosyllis biarticulata Hartmann-Schröder 1991
reef, algae, coralligenous sand with coral debris, 26°C
Holotype with 35 chaetigers, 2.5mm long, greatest width with parapodia 0.25mm, 0.18mm without. Colorless. Prostomium transversely oval, twice as wide as long. Four large eyes, in trapezoidal arrangement on posterior half of prostomium. Palps biarticulate, with broad, fused basal articles. Median antenna broken, probably inserting between the four eyes. Lateral antennae fusiform to club-shaped, inserted very near to anterior margin of prostomium, almost twice as long as prostomium. First segment clearly separated from prostomium and from following segment, about half as long as the first chaetiger. Dorsal tentacular cirri articulated, with 6 articles, partly with structures inside. Ventral tentacular cirri smooth, fusifrom, about as long as the lateral antennae. Dorsal cirri: chaetiger 1: broken; chaetiger 2: smooth, as long as width of segment; chaetiger 3: with 7 articles; chaetiger 4: broken; chaetiger 5: with 4 articles; chaetiger 6: with 5 articles; chaetiger 7: with 6 articles; chaetiger 8: with 4 articles; in following chaetigers not always present, with 4 or 6 articles. Articles elongated and differently shaped, only in anterior chaetigers sometimes with inclusions. Parapodia short, blunt. Ventral cirri anteriorly digitiform, not reaching the tip of the chaetae, then becoming longer and pseudoarticulated, reaching tip of chaetae posteriorly. Chaetae posteriorly with much longer blades than anteriorly.
One enlarged aciculae per foot in chaetigers 2-4, especially enlarged at its base, however also the thin aciculae are broadened at base.
First chaetiger with 9 compound chaetae with short but "normal" blades. Chaetiger 2-4 with 7-10 slightly enlarged compound chaetae with very short and stout modified blade.
From chaetiger 5 onwards 5-6 normal compound chaetae, becoming longer towards mid-body, then retaining the length towards the end. Uppermost blade long, the lowest about half this length. All blades finely serrated, distally blunt and unidentate.
Dorsal simple chaeta from first chaetiger, slightly bent upwards, distally with striated membrane.
Pharynx broad, extending over 4 segments, unarmed. Proventriculus over 3.5 segments, with about 35 muscle cell rows. Ripening gametes in body cavity from chatigers 11-28.
Three Streptosyllis species have large palps: S. magnapalpa , S. latipalpa and S. varians . S. varians has enlarged aciculae in 20 chaetigers, S. latipalpa in chaetigers 2-5 and S. magnapalpa in chaetigers 2-6, however in this species only slightly enlarged. S. biarticulata has enlarged aciuclae in chaetigers 2-4 and the palps are biarticulate. The compound chaetae differ in all these species.
