Megadenus holothuricola Rosen , 1910

Gonzalez-Vallejo, Norma Emilia & Amador-Carrillo, Stephanie, 2021, Assessment of Megadenus holothuricola Rosen, 1910 (Eulimidae), an endoparasite of Holothuria mexicana Ludwig, 1875 (Holothuriidae) in the southern Gulf of Mexico and the description a new species, ZooKeys 1016, pp. 49-61 : 49

Megadenus holothuricola Rosen, 1910 Figs 1A-E , 2A-F

Megadenus holothuricola Rosén, 1910:18-49 pl. 1 figs 1, 2 (type locality: Bahamas Islands; Type material lost).

Specimens examined.

four adult specimens ECOSUR-1386, two juveniles ECOSUR-1387 from Lerma, Campeche Bay, Gulf of Mexico.

Distribution.

Bahamas and Lerma, Campeche, Mexico.

Description.

Shell conical to globose, translucent, thin, fragile, sculpture consists of fine striations and growth scars from earlier positions of outer lip, two or three in random position per whorl. Adult size 4.0-5.5 mm long and 2.5-3.5 mm wide range (N = 6) (Fig. 1B, C ); teleoconch 4-6 very convex whorls, body whorl comprising more than 50% of the total shell length, posterior whorls reduced in size, sutures well defined aperture broad, inner lip broken, rounded basally; outer lip thin, slightly convex in lateral view, no operculum or periostracum; protoconch mucronate, 1½ whorls, translucent, rounded apex; sub-adults shell globose, low spiral, 2½ whorls, 1.0-2.0 mm long, 1.0 mm wide; body whorl half as long as body and more globose than in adults.

Body fully retractable into shell, mantle light cream-colored, pseudopallium whitish to light yellow in color; short and rounded tentacles, eyes round, black at the base of each tentacle (Fig. 2D ); foot reduced with a huge marginal gland of unknown function in dorsal position. The anterior region has two structures arising next to the mantle, the pseudopallium surrounding and partially covering the shell forming an extensive fold with pleated edges, and the proboscis. Epithelium fused with the holothurian respiratory tree; proboscis only partially fused with a part of the pseudopallium and foot, protruding from pseudopallium folds; in juveniles, pseudopallium covering lateral areas, leaving the apical teleoconch exposed; in 1 mm long juveniles, a small fold distinguishable (Figs 1E , 2C ).

Proboscis funnel-shaped, forming a flexible, semi-transparent or white non-retractable tube (Fig. 2C ); inside is a series of long and circular muscular packs giving it an elastic morphology and a rough outer cuticular surface; on the distal part is a series of papillae, allowing it to adhere like a suction cup to the epidermis of the host’s respiratory tree (Fig. 2F ). The proboscis size varies with total shell size: in adults, it is 3 × longer than the shell, 7.0 mm stretched to the maximum, in sub-adults or juveniles the proboscis is smaller and smoother (Fig. 2B, C ).

Variations.

The larval shell shows some intraspecific variations in adults. It is glassy, low, and rounded to mucronate. This seems to be related to the protection of the pseudopallium when it is immersed inside the respiratory tube. The juvenile develops the pseudopallium relatively early, when it has 1.5 whorls, and protects the shell from this stage onwards; at the same time, it is attached to an area of the host’s skin respiratory tree (Fig. 1C, E ).

Remarks.

Inside the posterior whorls of the shell, all adults and subadults presented pink oocytes or germinal cells. Two oval-shaped egg capsules were found situated between the shell and the pseudopallial folds, in one specimen (Fig. 2A, E ). Each capsule was transparent, 1.5 mm long × 1.0 mm wide, with 25-30 embryos in each. A juvenile was found strongly attached to the skin layer of the respiratory tube of the host, in the anterior area close to an adult eulimid (Fig. 1C ). Shells were difficult to preserve complete due to their fragility.

Rosén (1910) illustrated an adult and its coiled proboscis. His figures 1 and 2, show an adult with the pseudopallium at the top, and the proboscis of another adult separated from the respiratory tree tube. Our results are similar. The shape and sculpture of our shells correspond to Rosén’s original description; that is, a conical shape in pre-adults to globose shell, wider in the body whorl, two posterior smaller whorls, and with fine axial striations and continuous incremental growth lines in adults. The shells of M. catharelloides and M. voeltzkowi were described and illustrated, but subsequently destroyed to make histological cuts. In M. holothuricola there are no shells preserved as type material, hindering a better comparison, as noted by Warén (1984). However, shell shape of the pre-adults or juveniles of M. holothuricola resemble adults of Monogamus minibulla Olsson & McGinty, 1958 or juvenile specimens of Pelseeneria (Koehler & Vaney, 1908). Therefore, the morphological comparison of shells can be misleading, although these genera parasitize sea urchins ( González-Vallejo 2008; Delongeville et al. 2011).

Figures

Figure 1. Megadenus holothuricola Rosen, 1910 A two adults between filaments of the respiratory tree B, C detached adult shells, SL = 5.5 mm and 4.5 mm; right side juvenile attached SL = 1.0 mm D broken adult shell SL = 2.5 mm E subadult SL = 2.0 mm with everted proboscis L = 2.5 mm. Scale bars: 1.6 mm (A); 1.8 mm (B); 1.4 mm (C); 0.2 mm (D); 1.1 mm (E).

Figure 2. Megadenus holothuricola Rosen, 1910 A adults immersed in the main tube of the respiratory tree B, C two adults showing the mottled skin of the respiratory tree and the pseudopallium covering shell; proboscis slightly rolled, and another adult stretched to the maximum D cephalic view, 40 x E two egg capsules, L = 2 mm F view of the distal part of an adult proboscis. Scale bars: 1.2 mm (A); 2.3 mm (B); 1.65 mm (C); 0.8 mm (D); 0.6 mm (E); 0.5 mm (F). Abbreviations: e: eyes; eg: egg capsules; p: proboscis; pg: pedal gland; pp: pseudopallium; rt: respiratory tree; s: shell; t: tentacle.