Lithacrosiphon cristatus cristatus (Sluiter, 1902)
The grooved, conical anal shield, sometimes covered with calcareous deposits or algae, is the most notable external characteristic of L. cristatus cristatus. Considered to be a small to medium-sized species, it is commonly 10 – 20 mm in length, but has been reported to reach 38 mm in a few specimens. Found in tropical shallow waters, the species inhabits borings in hard substrata such as coral rubble.
(From Cutler, 1994)
Lithacrosiphon cristatus lakshadweepensis is a subspecies, which differs from the nominate form in number of grooves on the anal shield, 20-25 (vs. 30-50), number of LMBs, 8-15 (vs. 13-24), and in geographical distribution. This subspecies has been recorded only from the Arabian Sea off western India.
(From Cutler & Jurczak, 1975, and Cutler, 1994)
External anatomy
Introvert shorter than the trunk.
The tentacular crown encircles the nuchal organ.
Bidentate hooks, measuring 19-48 µm in height, are arranged in rings on the distal introvert. Unidentate hooks are scattered on the proximal introvert and range in height from 17 to 40 µm. Bidentate hooks are always larger than unidentate hooks, and both become smaller towards the tip of the introvert.
The anal shield is a calcareous conical structure covered by a cuticle with 30-50 longitudinal grooves; it may have a sharp or blunt apex. Occasionally it is damaged, overgrown, or missing. The anterior end may be covered with a calcareous deposit or algae, which can conceal the grooves.
The trunk is translucent, light brown, and commonly 10-20 mm in length (a few up to 38 mm) by 1-3 mm wide.
The caudal shield is absent.
Internal anatomy
The longitudinal muscle layer of the trunk wall is divided into 13-24 anastomosing bands.
The spindle muscle is attached at both ends.
Nephridia length ranges from 25- 150 % of the trunk length, and open to the exterior at the same level as the anus.
Two retractor muscles are joined for 60-75% of their length.
Lithacrosiphon cristatus cristatus is a widespread tropical species distributed in the Pacific Ocean from Malaya, Timor, southern Japan, and several Micronesia islands to Hawaii, and Panama. It has been collected in the southern Caribbean from West Indies to Curaçao, Venezuela and Brazil.
This species has been reported only from tropical shallow waters (Cutler, 1994). In the Caribbean Sea it inhabits Porites rubble, coralline limestone boulders, pitted and eroded coralline limestone and beach rock in depths from intertidal to 75 feet (Rice 1975). As a rock-boring species it contributes to the erosion of reefs by weakening the supporting structures of the corals, consequently increasing their susceptibility to breakage and destruction by current and wave action (Rice, 1979).