Mesothuria verrilli (Théel, 1886)

Gebruk, Andrey V., Solis-Marin, Francisco A., Billett, David S. M., Rogacheva, Antonina V. & Tyler, Paul A., 2012, Review of the genus Zygothuria Perrier, 1898 and the Atlantic group of species of the genus Mesothuria Ludwig, 1894 (Synallactidae: Holothuroidea) with description of the new species Mesothuria milleri sp. nov., Journal of Natural History 46, pp. 265-348 : 268-274

Mesothuria verrilli (Théel, 1886)

( Figure 2 )

Holothuria verrilli – Théel, 1886b: 6 (partim).

Mesothuria verrilli (Théel, 1886) – Deichmann, 1930: 93–94, plate 6, figs. 1–8; 1940: 192–193; 1954: 385.

Non Holothuria verrilli Théel, 1886 – Marenzeller, 1893b: 7–9, pl. 1, fig. 2, pl. 2, fig. 2. (= M. intestinalis ).

Non Mesothuria verrilli (Théel, 1886) – Östergren, 1896: 345; Perrier, 1902: 307–312, pl. 16, figs. 22–31; Hérouard, 1923: 10–13; Mortensen, 1927: 381–382; Grieg, 1921: 4. (= M. milleri ).

Non Allantis intestinalis var. verrilli (Théel, 1886) – Hérouard, 1902: 18–21, pl. 1, fig. 306. (= M. intestinalis ).

Material examined

See Table 1.

Type material

Holotype, MCZ cat. no. 448 (230 mm long), Blake, Station number unknown, 18 ◦ 20 ′ 30 ′′ N, 79 ◦ 14 ′ W, 1100 m . Paratypes, MCZ cat. no. 450 (two specimens): the same Blake station .

Type locality

Caribbean Sea, Belize, off Ambergris Cay, 1100 m.

Diagnosis

After Théel (1886b) and Deichmann (1930). Body cylindrical, with relatively thick skin, outer layer often much wrinkled. Maximum known size in preserved specimens 300 mm. Mouth almost terminal, slightly directed ventrally, anus terminal. Pedicels minute, in large specimens hard to observe, except near ends, especially on ventral side near anus; uniformly distributed, absent only on anterior part of ventrum. Tentacles about 20, relatively small, usually retracted.

Ossicles quadriradiate tables ( Figure 2A–E ), similar on dorsal and ventral sides, except in young specimens, <80 mm, where ventral ossicles are slightly smaller than dorsal. Main form of ossicles in type specimen have rounded or almost square disk ( Figure 2A,B ), 0.10–0.12 mm in diameter, surrounded by single crown of 10–12 marginal holes, rarely up to 14, or as few as eight; holes almost equal in size, but some variations exist ( Figure 2A–C ). Spire low, usually about half disk diameter in height, but may equal disk diameter, built up of four rods and one, more rarely two, transverse beams; it terminates with four distinct teeth, often spinous at ends ( Figure 2D,E ). In tubefeet small plate usually present and a number of abortive small tables with disk resorbed and often partly resorbed spire.

Considerable variation exists in form and size of ossicles between larger and smaller specimens ( Table 2). Compared with ossicles in type specimen, 70-mm long specimen has ossicles with obviously smaller disk diameter and fewer marginal holes. In addition, ossicles on ventrum smaller than on dorsum in 70-mm long specimen. This difference gradually disappears with size and is not observed in animals> 90 mm long. Dorsal ossicles known to exceed ventral ones in size as in M. intestinalis ( Heding 1942a) . No major differences were found between 90-mm long specimen and type specimen, whereas in larger specimens (> 250 mm) (1) ossicles diminished in size and (2) number of marginal holes decreased to eight. Hence, there seems to be a peak state in this species in ossicle size and shape in individuals ca. 100–200 mm long. The size series available in this study was not enough to establish precise body size at which ‘peaking’ of ossicles size occurs. In smaller specimens ossicles in general of ‘mature’ type, but smaller. In largest specimens, 250–300 mm long, disks and spires partly resorbed, teeth of spire short and smooth, relative size of holes diminishes and ossicles more robust than in smaller individuals ( Deichmann 1930).

Remarks

As previously indicated ( Marenzeller 1893b; Perrier 1902; Deichmann 1930) the original description of M. verrilli (Théel, 1886) includes under this name at least two or three species of Mesothuria , namely verrilli , gargantua and possibly intestinalis . This has resulted in some confusion. The original morphological description of M. verrilli was based on a 110-mm long specimen ‘dredged at St Vincent’, corresponding apparently to M. gargantua . Ten other specimens from the type series were sampled from different localities in the Caribbean. A specimen from the west Caribbean, off Ambergris Cay, 230 mm long, was later recognized as the type of M. verrilli ( Deichmann 1930) . In the description of variation of ossicles by Théel (1886b), some characters correspond to M. verrilli (‘...in two of the specimens from Dominica, the spire often, but not always, terminates in four smooth teeth’), and others to M. intestinalis (‘...in others, for instance those from Barbados and Grenada, the top of the spire has a very irregular aspect, from numerous longer or shorter teeth, which are placed not only round the opening of the top itself, but also on a cross-like rod which covers the opening’).

In addition to the poorly described characters of ossicles, the type description lacks illustrations of ossicles, and therefore the name ‘ verrilli ’ was widely used later by Marenzeller, Perrier and other authors for the specimens from the east Atlantic having a spire with four smooth teeth. The difference, however, between M. verrilli from the Caribbean and all the east Atlantic specimens attributed to this species is obvious and is discussed in detail under M. milleri sp. nov.

A number of specimens from the Mediterranean and off the Azores described as Allantis intestinalis var. verrilli ( Hérouard 1902) , and attributed later to M. verrilli ( Perrier 1902; Deichmann 1930) most likely belong to M. intestinalis . Such characters as the number of circles of holes on the disks, which may reach three or four according to Hérouard, and the size of disks reaching 0.137 mm in diameter in juveniles, are unknown for M. verrilli , even in the youngest specimens. The illustrations of ossicles given by Hérouard (1902) are confusing, presenting only one extreme type of ossicle from the range of variation typical for M. intestinalis , and this extreme type resembles M. verrilli .

In view of the confusion with the name verrilli , all other records of this species in the West Atlantic, not provided with morphological details, cannot be considered reliable. These records include samples off the Bahamas ( Pawson 1982), the Gulf of Mexico ( Miller and Pawson 1984) and records from the Caribbean ( Suchanek et al. 1985). If the last record corresponds to true M. verrilli , this would increase the bathymetric limit of this species to 3720 m.

Relationships

The species is most closely related to M. milleri sp. nov. Details are given under M. milleri sp. nov.

Distribution

Reliable records in the Caribbean ( Figure 3 ). Type locality, as indicated by Deichmann, off Ambergris Cay ( Belize, former British Honduras), depth 1100 m. Distribution data in the eastern Atlantic for M. verrilli are expected to belong to M. milleri sp. nov.

Bathymetric range

Reliable bathymetric range from 700 to 1850 m, but may be deeper.

Figures

Figure 2. Mesothuria verrilli (Théel, 1886). Dorsal ossicles from type specimen (MCZ cat. no. 448). (A–C) table disks; (D,E) spires. Scale bars 0.06 mm for A–C; 0.1 mm for D,E.

Figure 3. Distribution of Mesothuria verrilli, Mesothuria milleri sp. nov. and Mesothuria intestinalis.