Hydroides elegans Haswell, 1883
Hydroides elegans secretes a calcareous tube, as do other serpulid polychaetes. Serpulids have a feathery crown of modified prostomial palps, called radioles (the prostomium is the first segment, projecting above the mouth). The radioles can be folded and withdrawn into the tube. One of the radioles is modified to form an operculum, which acts as a plug when the animal contracts. The peristomium (segment behind the mouth) is folded back to form a collar, which bears uniramous parapodia, with a distinctive set of collar chaetae, with spines or serrations. The collar is the first of seven thoracic chaeta-bearing segments (chaetigers). The subsequent segments have biramous parapodia. The dorsal branch of the parapodium is called the notopodium; the ventral is the neuropodium. Chaetae in the two branches and along the body can vary greatly in their morphology, which can be critical in the taxonomy of serpulids. Description from: Barnes 1983; Bastida-Zavala and ten Hove 2002).
The tube of H. elegans is white, and 1.3-2.5 mm in diameter. The tubes are fragile and variable, sometimes having transverse ridges and occasionally having longitudinal ridges, but are usually smooth. The tubes lack peristomes (flared openings). The tubes are usually flattened on the dorsal (upper) surface. The branchial (gill) crown consists of about 10 radioles each on the left and right sides of the mouth. It comprises about 1/4 of the worm's length. The operculum is roughly funnel-shaped, with 23-24 radii, each with rounded tips, a concave distal surface, and a circular row of 13-15 terminal spines, usually barbed with up to 4 spinules. The peduncle is cylindrical. The grooves between the radii are usually about 1/3 of the funnel length. The spines are longer than the radii, and T-shaped, with expanded tips, and have a single spinule at the base. The verticil (ring of distal spines) may have or lack a central tooth. The thorax consists of 7 segments. There are two kinds of collar chaetae: (1) thicker bayonet chaetae, with two teeth at their base, a reap of denticles below the teeth, and fine, saw-like denticles on the distal edge; (2) hair-like (capillary) chaetae. The subsequent thoracic segments bear short, rasp-like setae, called uncinae, and limbate chaetae. The abdomen has about 41 segments (35-57, n=5). The overall length is about 8.5 mm (5-13, n=5). The worm is yellow to light brown. (Description from Bastida-Zavala and ten Hove 2002; Cinar 2006; Ben-Eliahu and ten Hove 2011).
Historically, the cosmopolitan tropical-subtropical harbor serpulid, H. elegans was frequently misidentified as H. norvegica, a Northeast Atlantic species associated with clear, cold waters, found from Norway to Morocco. Zibrowius (1971) clarified distinctions between these species. The collar chaetae in H. elegans have a saw-like row of denticles on their distal portion. Additional features distinguish the two species. Early records of H. norvegica (before the 1970s) from tropical or subtropical areas usually refer to H. elegans (Zibrowius 1971; ten Hove 1974; Bastida-Zavala and ten Hove 2002; Ben-Eliahu and ten Hove 2011).
Northeast Atlantic species (Norway-Morocco), historically confused with H. norvegica (See text)