Holothuria (Mertensiothuria) hilla Lesson, 1830

Yves, 2003, The holothurian subgenus Mertensiothuria (Aspidochirotida: Holothuriidae) revisited, Journal of Natural History 37, pp. 2487-2519 : 2500-2502

Holothuria (Mertensiothuria) hilla Lesson, 1830

(figures 5A–E, 11C, 12F) Holothuria (Thymiosycia) hilla: Massin, 1999: 55 (synonymy and records before 1999);

Leonardo and Cowan, 1984: 38, text figure; Liao, 1998: 80; Samyn, 2000: 15, table 1;

Lane et al., 2000: 489; Samyn and Vanden Berghe, 2000; Schoppe, 2000: 113, text figure.

Original name. Holothuria hilla Lesson, 1830 .

Name-bearing type. Holotype, MNHNP EcHh542.

Type locality. Society Islands (Borabora) .

Current status. Holothuria (Thymiosycia) hilla Lesson, 1830 .

Material examined. Kenya ( Kiunga Marine Reserve ), April 1999, 2 m depth, coll. Y. Samyn, IRSNB IG 28628 /KKiun/9942 (one specimen) ; Kenya ( Kiunga Marine Reserve ), April 1999, 3 m depth, coll. Y. Samyn, IRSNB IG 28628 /KKiun/9943 (three specimens) ; Kenya (Mombasa), August 1998, 5 m depth, coll. Y. Samyn, MRAC 1665 /KMom/9857 (one specimen) ; Kenya ( Kisite ), August 1998, 6 m depth, coll. Y. Samyn, MRAC 1666 /KKis/98102 (one specimen) ; Madagascar ( Tuléar ), October 1998, Grand Récif at low tide, coll. I. Eeckhaut et al., IRSNB IG 29142 (one specimen) .

Description. Medium-sized species; preserved specimens from 42× 14 mm to 135× 20 mm, live specimens up to 30% larger. Body cylindrical; colour in alcohol similar to the colour in life (figure 12F); bivium brown with yellow spots corresponding to large dorsal papillae; trivium beige-brown with white-yellow spots corresponding to large tube feet which are distributed in three to four rows in the ambulacral areas. Mouth ventral, surrounded by 20 very short, dirty-yellow tentacles. Anus terminal, not surrounded by papillae. Tentacle ampullae short. Single to two Polian vesicles. Single, curled stone canal ending in a spherical madreporic plate. Body wall, smooth, a few millimetres thick. Longitudinal muscles bifid, well developed. Cuvierian tubules numerous.

Ossicles: dorsal and ventral body wall with similar tables and buttons (figure 5A, B), though fewer tables are found in the ventral body wall. Tables (figure 5A, B) 52–67 m m across; rim of disc smooth, circular to quadrangular in outline; disc perforated by four large central holes and nine to 13 smaller peripheral holes; spire consists of four short pillars, single cross-beam, small moderately spiny crown with a small central hole. Buttons (figure 5A, B) with rim smooth but irregular, 70–100 m m long and 25–45 m m wide, with three to six pairs of irregular holes. Tentacles with rods 45–145 m m long, extremities slightly spiny (figure 5C). Dorsal papillae with buttons, button-like plates and rods (figure 5D); buttons similar to the ones in the body wall, but up to 125 m m long; rods up to 200 m m long, often with distal or median perforations. Ventral tube feet characterized by large plates and buttons (figure 5E). Plates 110–160 m m long and 55–75 m m wide, with two to five longitudinal rows of irregular holes; buttons similar to those of the body wall. Longitudinal muscles with rings, C-shape ossicles and irregular buttons with one pair of holes, 17–40 m m long (figure 11C).

Ecology. The fugitive H. (M.) hilla is found under coral debris and under live coral slabs. Massin (1999) notes that H. (M.) hilla often occupies the same habitat as H. (Thymiosycia) impatiens and H. (M.) leucospilota . Depth range 0–30 m (Rowe, in Rowe and Gates, 1995).

Geographical distribution. Rowe and Gates (1995) note a tropical Indo-west Pacific distribution. Massin’s distribution map (1999: 56), however, shows subtropical records ( Japan, Tasman Sea, South-West Australia, Northern Red Sea, Persian Gulf).

Diagnosis. Present note.

Proposed status. Holothuria (Mertensiothuria) hilla Lesson, 1830 .

Comments. Although H. (M.) hilla is a well-known, easily identifiable species from the Indo-west Pacific, its systematic position is changed from the subgenus Thymiosycia to Mertensiothuria in the present note. This taxonomic decision is justified by the presence of rings, C-shape ossicles and small irregular buttons in the longitudinal muscles (figure 11C), a diagnostic character restricted to the subgenus Mertensiothuria . This claim is further confirmed by investigation of the longitudinal muscles of H. (Thymiosycia) impatiens Forskål, 1775 , the type species of Thymiosycia , and of H. (T.) thomasi Pawson and Caycedo, 1980 , as these species do not have ossicles in their longitudinal muscles (see also table 1). The form and distribution of the other ossicles in the different body parts (especially the large plates in the tube feet) agree better with the diagnosis of Mertensiothuria than that of Thymiosycia in which it was formerly placed. Moreover, the ossicles (tables and buttons) of H. (M.) hilla are close to those of H. (M.) albofusca (see Cherbonnier, 1988). The ossicles found in the longitudinal muscles of H. (T.) arenicola Semper, 1868 (figure 11K) are pseudobuttons rather than the rods to C-shape ossicles, rings and small smooth buttons, here considered diagnostic for Mertensiothuria . For now, it suffices to note that further examination of the longitudinal muscles of other species currently classified under Thymiosycia most probably will bring additional insights into the systematics of the Holothuriidae .