Sipunculus (Sipunculus) cf. polymyotus Fisher, 1947

Silva-Morales, Itzahí & Gómez-Vásquez, Julio D., 2021, First records and two new species of sipunculans (Sipuncula) from the Southern Mexican Pacific, European Journal of Taxonomy 740 (1), pp. 77-117 : 80-82

Sipunculus (Sipunculus) cf. polymyotus Fisher, 1947

Figs 1–2

Type locality of nominal species

Key West, Florida ( Fisher 1947: 354–358, pl. 10, fig. 24).

Material examined

MEXICO • 1; Oaxaca, Puerto Ángel, Boquilla Beach ; 15°40′58″ N, 96°27′54″ W; 21 Mar. 2016; FRE leg.; depth 5 m, in sand; UMAR-SIPU 106 .

Description

Trunk 80 mm in length ( Fig. 1A ). Introvert 10% of total length; with triangular papillae and associated microbivalves ( Fig. 1B–D ). Transparent and iridescent body. Tentacular membrane with two large ventral lobes and two smaller dorsal lobes with margin greatly subdivided ( Fig. 1E ). Longitudinal and circular musculature in bands, with 49 longitudinal muscle bands (LMBs). Gut with post-esophageal loop. Two pairs of retractor muscles, each ventral retractor muscle attached to six LMBs starting from second band after ventral nerve cord; each dorsal retractor muscle attached to five LMBs starting from 15 th band after ventral nerve cord ( Fig. 2A ). Most LMBs split and double in posterior end. Brain with approximately ten digitate processes, large, conspicuous, leaf-like flaps and long strings ( Fig. 2B ). A pair of brownish nephridia with irregular knobby surfaces, about 10% of trunk length ( Fig. 2C ). Anus with small prominences surrounding its boundary ( Fig. 2D ). Nephridiopore anterior to anus. Spindle muscle not attached to body wall posteriorly.

Morphological remarks

The prominences of the anus are conspicuous. This morphological feature was not described by Cutler (1994), or even by Fisher (1947). The prominences are not related with the elevation of the anus or the fixation process.

Taxonomic remarks

Fisher (1947) described Sipunculus polymyotus from Key West, Florida and later S. natans Fisher, 1954 from Santa Inés Bay, on Gulf coast of Baja California, Mexico. Cutler (1994) synonymized S. natans with S. polymyotus . The principal morphological difference between the two species is the number of longitudinal muscle bands. Sipunculus polymyotus has 53–55 while S. natans has 49–52. The single specimen of S. cf. polymyotus from the Southern Mexican Pacific revised in this study has 49 LMB, i.e., more similar to conditions described for S. natans ( Table 2 ).

Once more specimens of Sipunculus (S.) cf. polymyotus become available, the name S. natans could be revalidated. This hypothesis is based on some recent papers where the use of molecular data has rejected the supposed wide distribution of some species ( Staton & Rice 1999; Kawauchi & Giribet 2010, 2014; Schulze et al. 2012; Johnson et al. 2016; Silva-Morales et al. 2019).

Habitat

Subtidal (5 m); in sand.

Distribution

Boquilla Beach, Oaxaca, Mexico.

Figures

Fig. 1. Sipunculus cf. polymyotus Fisher, 1947 from Oaxaca, Mexico lateral view. A. External morphology. B. Introvert. C. Microbivalve associated with introvert papillae. D. Triangular introvert papillae. E. Peripheral tentacles stained in methyl blue. Abbreviations: Dlob = dorsal lobe; Vlob = ventral lobe.

Fig. 2. Sipunculus cf. polymyotus Fisher, 1947 from Oaxaca, Mexico. A. Internal morphology. B. Digitate processes of brain. C. Nephridium. D. Anus (black row indicates the prominences of the anus). Abbreviations: DRM = dorsal retractor muscles; I = intestine; LMB = longitudinal muscle bands; N = nephridium; PEL = post esophageal loop; VRM = ventral retractor muscles.