Neopentadactyla mixta ( Oestergren , 1898)

Samyn, Yves, Sonet, Gontran & d'Acoz, Cedric d'Udekem, 2021, Exploring the use of micro-computed tomography (micro-CT) in the taxonomy of sea cucumbers: a case-study on the gravel sea cucumber Neopentadactyla mixta (Oestergren, 1898) (Echinodermata, Holothuroidea, Phyllophoridae), ZooKeys 1054, pp. 173-184 : 173

Neopentadactyla mixta ( Oestergren, 1898)

Figures 1A-L , 2A-D

Pseudocucumis mixta Östergren, 1898: 104, 105, 135, fig. 3 (p. 109).

Pseudocucumis mixta : Bedford 1898: 843 (discussion); Östergren 1902: 24, note 1; Östergren 1904: 659; Östergren 1906: 1-24, figs 1-3; Ohshima 1912 54, 59 (discussion); Lieberkind 1929: 14; Massy 1920: 52; Koehler 1921: 168, fig. 124; Mortensen 1924: 236, fig. 116; Koehler 1927: 195, pl. 16, fig 16; Engel 1933: 33-34 (distribution); Cherbonnier 1952: 570, figs 1, 2; Könnecker and Keegan 1973: 157-162 (in situ pictures).

Pseudocucumis Pseudocucumis Cuenoti Koehler & Vaney, 1905: 395, figs 1-6.

Neopentadactyla mixta : Deichmann, 1944: 736; Heding and Panning 1954: 186, fig. 91; Féral 1979: 111, figs A-L; Féral 1980: 42, figs 1, 2; Smith 1983: 301, figs 1-4; Wood 1988: 143, 151 (drawing), 179; Moyse and Tyler 1990: 866 (key), 869, fig. 15.14 (upper left); Hansen and McKenzie 1991: 123 (lectotype and 3 paralectotypes), fig. 126-140; McKenzie 1991: 126, fig. 1; 132, fig. 3a-d; Picton 1993: 78 (colour photograph); McKenzie 1997: 274; Southward and Campbell 2006: 224, fig. 201.

Status and location of types.

Museum of Evolution, Uppsala University, Sweden (UPSZTY 2346): lectotype (249a); 2 paralectotypes (249b) (designated by McKenzie in 1990 according to the database of the Upsala Museum). McKenzie (1991) stated that he did not designate a lectotype and paralectotypes. In another publication Hansen and McKenzie (1991: 123) did designate and describe the lectotype ("Typsamlingen Nr. 249a") from the four syntypes as present in the Uppsala Museum’s collection; the three remaining syntypes therefore become paralectotypes. Hansen and McKenzie (1991: 123) stated that they have maintained the original division of the four syntypes over two jars (249a and 249b), "each containing two specimens one of which was dissected, the other intact". According to Hansen and McKenzie (1991), the lectotype is thus together with one of the paralectotype in Jar 249a, while the other two paralectotypes are in jar 249b. We did not revisit this type series.

Type locality.

W. Norway (most likely Molde, i.e., about 62°45'N, 7°14'E).

Material examined.

RBINS I.G. 33990, HOL.1736 (4 specimens plus SEM stubs: I.G. 33990/HOL.1736/1-8).

Description

(based on material examined). Body elongate, with central part inflated and anterior and posterior ends more narrow. Length of fixed specimens 50-155 mm (measured along the dorsal face); diameter 30-80 mm at mid-body, 21-43 mm anteriorly and 12-25 mm posteriorly. Color in alcohol after a short period of preservation same as color in life: body light beige, with irregular, brownish spots and patches (Fig. 1A, D ); patches larger ventrally. Tentacles with their shaft light beige and the branches brownish with beige tips. According to McKenzie (1991), 10 large, five small, and five intermediate sized tentacles can be observed in live specimens. Insufficient relaxation of the specimens at hand made it impossible to observe the exact position of the tentacles in the specimens under study. Tube feet predominantly in the radii, in irregular double series anteriorly and posteriorly and in up to six rows, spreading into the interradii ventrally. Tube feet light beige. Body wall gritty to the touch. Body wall thin mid-body, thicker anteriorly and distally, possibly an artefact of fixation. Longitudinal muscles visible through the body wall where the skin is thinnest. Longitudinal muscles thick, undivided, and attached to the body wall along their entire width. Retractor muscles arise about 1/3 of the body length from anterior end, attaching to the radial section of the long and thin tubular calcareous ring, which itself is about 1/4 of the body length (Fig. 2A ). Calcareous ring with five radial and five interradial parts (Fig. 2B ). The radial parts are anteriorly notched, with 1-3 larger plates anteriorly and an irregular meshwork of smaller plates posteriorly. The interradial parts are anteriorly pointed, have 3 or 4 larger plates anteriorly and an irregular meshwork of smaller plates posteriorly (Fig. 2B ). Calcareous ring embedded in a thin layer of tissue, with the calcified elements connected by connective tissue. (Fig. 2D ). No clear posterior ending visible at the end of the comet-shaped, calcareous ring (Fig. 2A ). Internal surfaces of especially the radial pieces are guttered (Fig. 2C ). Single, very long Polian vesicle; single curled stone canal embedded in the dorsal mesentery (Fig. 2D ).

Tentacle shafts with irregular, complex rosettes, 30-45 μm long and 15-25 μm wide (Fig. 1E ); tentacle tips with straight to curved, terminally perforated rods, 30-55 µm long and rosettes similar to those of the tentacle shafts (Fig. 1G ); introvert with 2-pillared tables only, disc smooth, 65-80 µm in diameter, irregular in outline, perforated by four central holes and a variable number of irregularly peripheral holes; pillars 40-65 μm high, with single cross-beam, ending in a narrow, sparsely-spined crown (Fig. 1F ); dorsal and ventral body wall with 4-pillared tables, 80-100 μm in diameter, smooth rim, irregular in outline, perforated by four central holes and a variable number of peripheral holes arranged in multiple irregular circles; pillars 60-76 μm high, with single cross-beam, ending in a narrow, spined crown (Fig. 1H, I ). Dorsal and ventral tube feet with plates, 64-95 μm long and 40-55 μm wide; endplate ± 200 μm in diameter, with uneven sized holes and with some relief (Fig. 1L ). Contrary to Féral’s (1980) observation, no tables could be found in the tube feet. Longitudinal muscles devoid of ossicles.

Distribution.

North and West European coasts: Molde, West Norway ( Ötsergren 1898); Arcachon, Bréhat Island, Wimereux, Roscoff, Chausey Islands, France ( Koehler and Vaney 1905; Koehler 1921; Cabioch 1965; Féral 1979, 1980; this study); Northern British Islands, Faroe Islands ( Östergren 1906; Lieberkind 1929); Tatihou Island, Normandy, France ( Östergren 1906), Ireland ( Massy 1920; Könnecker and Keegan 1973; Féral 1979); Denmark ( Mortensen 1924); Swedish and Norwegian waters ( Hansen and McKenzie 1991).

Bathymetric range.

Intertidal (present study) to 200 m depth ( Southward and Campbell 2006).

Ecology.

Neopentadactyla mixta is most frequently found in maerl beds and coarse shell gravels with fairly strong tidal streams. It is a gregarious species, which may occur in densities of up to 297 individuals/m2 ( Könnecker and Keegan 1973; McKenzie 1991; Picton 1993). Könnecker and Keegan (1973) reported N. mixta to be a rheophilic suspension feeder with diurnal feeding rhythm. Smith and Keegan (1985) demonstrated that N. mixta individuals on the west coast of Ireland stop their suspension feeding from autumn to spring and retire to depths of 30-60 cm in the coarse sediments during that period. As with other phyllophorids (e.g., Massinium maculosum Samyn & Thandar, 2003), this species exposes only its tentacle crown and part of its introvert and the tip of its anus.

The sediment from the beach where the studied specimens were collected consisted of coarse, gravelly sand, characteristic for a wave-beaten environment and harboured a very rich and diverse fauna of other burrowing taxa ( Bivalvia, Polychaeta, Sipuncula, Nemertea, etc.)

Systematics.

The DNA sequence retrieved from GenBank (http://www.ncbi.nlm.nih.gov) most similar to the 18S sequence obtained here was labelled as Neopentadactyla mixta (accession number AY133482). Its similarity with our sequence is 99.16%. This sequence is currently the only DNA sequences available online for this species. The next most similar public DNA records were from Phyrella mookiei Michonneau & Paulay, 2014 (KX856842, 97.18%), a phyllophorid, and Afrocucumis africana (Semper, 1867) (KX856841, 97.18%), a sclerodactylid. The high DNA similarity with N. mixta supports the morphological identification of the specimen studied here as N. mixta , as a species belonging to Phyllophoridae . This DNA-based result is backed up by its ecology, the gross morphology of the specimens, the structure of the calcareous ring, and the ossicle assemblage. However, the high DNA similarity with A. africana is troubling, as Afrocucumis Deichmann, 1944 is characterized by a very different "skeletal structure": the calcareous ring has its radial pieces with two short, unsegmented, projections; the interradial pieces are without posterior projections; and the body wall holds large, 310-400 μm wide lenticular plates ( Massin 1996). In N. mixta , no lenticular plates are present, and the calcareous ring is a much more complex structure (see the description above).

Deposition of images.

SEM images of ossicles and a focus-stacked image of the calcareous ring has been put on the Royal Belgian Institute of Natural Sciences "Virtual Collections" website at http://virtualcollections.naturalsciences.be/virtual-collections/recent-invertebrates/echinodermata#c4=N&b_start=0.

3D mesh files have been put on https://sketchfab.com/3d-models/be-rbins-hol-1736-neopentadactyla-mixta-b013d76558234a84a4b6907132eff93d.

Figures

Figure 1. Neopentadactyla mixta (Oestergren, 1898) A focus-stacked view of the dorsal-lateral view of dissected specimen B focus-stacked view of the ventral-lateral view of dissected specimen C focus-stacked view of the dorsal-lateral view of a non-dissected specimen D focus-stacked view of the ventral-lateral view of a non-dissected specimen E SEM view of the rosettes from the shaft of a tentacle F SEM view of the 2 - pillared tables from the introvert G SEM view of the rods and rosettes from a tentacle tip H SEM view of the 4 - pillared tables from the dorsal body wall I SEM view of the 4 - pillared tables from the ventral body wall J SEM view of the plates from the dorsal tube feet K SEM view of the plates from the ventral tube feet L SEM view of half of an end-plate from a ventral tube foot. Scale bars: 1 cm (A-D); 50 μm (E-L).

Figure 2. Neopentadactyla mixta (Oestergren, 1898) A micro-CT scan visualizing the position of the calcareous ring B lateral view with micro-CT imaging of the anterior part of the calcareous ring (AR: most anterior radial piece; AIR: most anterior interradial pieces; SAR: subsequent anterior radial pieces; SAIR: subsequent interradial anterior pieces; Mesh: meshwork of radial and interradial median to distal pieces) C oblique view with micro-CT imaging showing a guttered internal side of the calcareous ring D focus-stacked view of the calcareous ring and associated structures (T: tentacles; LM: longitudinal muscle with bifurcation point (BfP); PV: Polian vesicle: SC: stone canal). Scale bars: 1 cm (A-D).