Ditrupa Berkeley, 1835

Ten Hove, Harry A. & Kupriyanova, Elena K., 2009, Taxonomy of Serpulidae (Annelida, Polychaeta): The state of affairs, Zootaxa 2036, pp. 1-126 : 39

8. Ditrupa Berkeley, 1835

( Fig. 15)

Type-species: Dentalium subulatum Deshayes, 1826 = junior synonym of Dentalium arietinum Müller, 1776 ; designated by Bush 1905: 223.

Number of species: 2

Tube free, tusk-like, not attached to substratum, circular in cross-section, open at both ends, broadening anteriorly though exterior tapers just prior to tube mouth. Outer layer hyaline or white, inner layer opaque. Granular overlay of the tube absent. Operculum inverse conical with chitinous endplate. Peduncle cylindrical, smooth, without wings, gradually merging into operculum, no constriction; it is positioned as first dorsal left radiole. Pseudoperculum absent. Radioles arranged pectinately; up to 15 radioles per lobe. Inter-radiolar membrane, branchial eyes, and stylodes absent. Pair of filiform mouth palps present. 6 thoracic chaetigerous segments. Large entire (non-lobed) collar continuous with short thoracic membranes, ending at first chaetiger (second thoracic segment); tonguelets absent. Collar chaetae absent (see Collar segment, p. 22). First thoracic chaetiger biramous (see Collar segment, p. 22) with limbate chaetae ( Fig. 15A, B) and with uncini; sometimes with special chaetae (see remarks). Apomatus chaetae absent. Thoracic uncini saw-to-rasp-shaped (dental formula P:2:2:2:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1:1 or P:3:2:2:1……1) or rasp-shaped; about 25 teeth in profile, with 2 or 3 teeth in a row above peg (P); peg blunt, curved upwards and gouged underneath ( Fig. 15C). Triangular depression absent. Abdominal chaetae and certainly posterior ones thin, almost capillary, with very faint narrow geniculate tip (to completely capillary in D. gracillima ). Abdominal uncini rasp-shaped, with 20–25 in profile, up to 8 teeth in a row above peg; anterior peg blunt, almost rectangular ( Fig. 15D). Achaetous anterior abdominal zone absent; however, anterior half of abdomen with uncini only. Posterior capillary chaetae present. Posterior glandular pad absent.

Remarks. The genus is found living unattached ( Fig. 1A) in soft sediment marine environments around the world. Like many serpulid genera, Ditrupa has a history of taxonomic confusion to the extent that its tubes were included in the Mollusca by some authors. The generic diagnosis was emended by ten Hove & Smith (1990). An unattached free tube similar to that of Ditrupa is known only in two (?three) other serpulids, Bathyditrupa hovei and Serpula crenata (possibly incl. S. sinica ). Unlike the circular in cross-section tubes of Ditrupa , those of Bathyditrupa and S. crenata are rectangular to multi-angular in cross-section (see The tube, p. 7).

Ten Hove & Smith (1990: 113, 115) describe 2 populations of Ditrupa gracillima in which the first thoracic chaetiger shows special chaetae, one almost geniculately terminating in an oblique frayed narrow limbus, the other stoutly acicular. In view of the limited distributions of these two forms they question whether these populations might be in the process of speciation.

1. Ditrupa arietina ( Müller, 1776) , Northern Norway to Azores and Canary Islands, Mediterranean 2. Ditrupa gracillima Grube, 1878 , widely distributed in Indo-West Pacific.