Thyonina rasidae, Thandar, 2017

Thandar, Ahmed S., 2017, Two new subfamilies, three new species and a new subspecies of dendrochirotid sea cucumbers (Echinodermata: Holothuroidea), Zootaxa 4365 (4), pp. 410-420 : 416-417

Thyonina rasidae n.sp. Figure 3

Diagnosis. A strongly U-shaped species, up to 50 mm in length along ventral surface; mouth surrounded by fringe of papillae. Colour off white in alcohol. Tentacles 10, ventral two reduced, others of unequal length. Tube feet short, scattered. Anal teeth absent, anal papillae present. Calcareous ring complex, radial and interradial plates short, faintly fused and weakly subdivided into few pieces of calcite; posterior processes of radial plates long, about thrice the length of ring, conspicuously subdivided. Polian vesicles four, one long, others diminutive; stone canal short, madreporite spherical. Body wall ossicles as smooth curved rods, 20–47 µm long, with dichotomously branched and/or perforated ends. Tube feet with similar rods, end-plates reduced. Introvert with closed mulberrylike rosettes. Tentacles with mulberry-like bodies and rods, of which some rosette-like.

Etymology. This species is named after my dear wife Rasida for her patience and tolerance in allowing me to continue with my passion long after retirement.

Holotype. NHMUK 1883.4.19.7, Kurrachee (colonial spelling of Karachi), ex Karachi Museum, Pakistan.

Paratypes. NHMUK 1883.4.19.8-9. Same data as the holotype, 2 specimens.

Description. All three specimens U-shaped, mouth and anus terminal, level of mouth higher than that of anus, anal end attenuated. Length 28–51 mm, mid-body width 5–10 mm. Skin thin, colour off white in alcohol.

Holotype

Length 51 mm ( Figure 3A ). Tentacles 10, white, bushy, in 8 + 2 arrangement, ventral two reduced, others of unequal length, longer ones occurring dorsally, decreasing in size ventrally. Tube feet short, well developed, scattered, longer and more numerous in ventral ambulacra, decreasing in size dorsally, suckers conspicuous. Mouth circular, surrounded by a fringe of papillae. Anal teeth absent but anal papillae present in pairs in the five radii surrounding anus.

Calcareous ring complex ( Figure 3G ), radial and interradial plates short, faintly fused, plates weakly subdivided but subdivisions not obvious. Radial plates anteriorly bifid, but bifurcations not conspicuous; some radial plates somewhat incised posteriorly; posterior processes of radial plates arise at posterior border of the interradial plates; posterior processes long, about thrice the size of the ring itself, all conspicuously subdivided. Polian vesicles four, of variable length, longest one straight, tubular, 10 mm in length, remaining three diminutive, dorsal in position, apparently arising at the same point as the stone canal. Stone canal short, thin, straight, embedded in dorsal mesentery; madreporite spherical. Longitudinal muscles unpaired; retractors arise from longitudinal muscles at about mid-body, dorsal ones more anteriorly.

Body wall ossicles comprise smooth, curved rods ( Figure 3C ) of two types: delicate ones 20–47 µm long (mean 36 µm) with dichotomously branched and/or perforated ends; stouter rods with corrugated surface and also with perforated and/or branched ends, both types somewhat corroded, hence corrugation of the larger rods perhaps a result of corrosion. Anal region supported by fragmented, perforated plates on way to corrosion ( Figure 3B ). Other perforated plates in body wall are perhaps remains of tube feet end-plates but without any regular arrangement of holes. Tube feet supported by similar rods and reduced end-plates ( Figure 3D ), also without regular arrangement of holes and showing signs of corrosion. Introvert supported by closed, mulberry-like rosettes, 12–22 µm (mean 16 µm) and branched rods ( Figure 3F ) and tentacles by rods with few “branches”, thus appearing as open rosettes, as well as closed mulberry like rosettes, 11–29 µm (mean 17 µm) ( Figure 3E ).

Distribution. Known only from Karachi, Pakistan (Arabian Sea).

Remarks. Thyonina articulata from South Africa was the only species in this genus, hence the genus was long thought to be monotypic and endemic to South Africa. The presence now of a similar but not identical form from Karachi, Pakistan ( T. rasidae n. sp.), and another form ( T. bijui n. sp.) with similar ossicles from Kerala, India, also in the Arabian Sea, described above, confirms that the genus is not monotypic after all. The Pakistan species differs from the South African T. articulata not only in the form of the body and the size and form of the body wall rods, but also in the tentacle and introvert deposits and tube feet end-plates. The tentacle deposits in the South African species comprise plates only, compared to rods and rosettes in the Pakistan and Kerala forms. The end-plates of the South African form and T. bijui are very characteristic with minute medial holes surrounded by a ring of larger holes outside these. However, these deposits in T. rasidae , although somewhat corroded show no signs of clearly differentiated holes with regular arrangement. The calcareous ring is also quite different with the new species not showing as much fragmentation of plates as the type species and T. bijui , and the processes are much longer.

Figures

FIGURE 3. Thyonina rasidae n. sp. A. Specimen entire (holotype,NHMUK 1883.4.19.7); B. Anal body wall plates; C. Body wall ossicles; D. Tube feet end-plate; E. Rods and rosettes from tentacles; F. Rods and rosettes from introvert; G. Part of calcareous ring (dorsal).(B–F drawn to same scale).