Macrodasys digronus, Hummon, William D. & Todaro, Antonio, 2009

Hummon, William D. & Todaro, Antonio, 2009, Italian marine Gastrotricha: VI. Seven new species of Macrodasyida, Zootaxa 2278, pp. 47-68 : 51-53

Macrodasys digronus new species [Mcd dgrn]

Figure 2 A, B

Macrodasys sp.II (Evans, Todaro & Hummon 1993: Tab. I)

Macrodasys sp. C ( Todaro, Hummon, Balsamo, Fregni & Tongiorgi 2001: p. 101); ( Hummon 2001 – 2009: W Med Database)

Diagnosis: Adult Lt 753 Μm; PhJIn at U41. Head ovoid, with slight swelling behind the pestle organs at U03; trunk broad, but constricting in the regions of the rear pharynx and the frontal organ, before narrowing to the caudum. Glands inconspicuous. TbA 5 per side, in transverse rows that insert directly on the body; TbL 25 per side, symmetrical, but variably spaced, from U37 to the rear, with 1 at the rear of the pharynx, 1 at the PhJIn, and 23 in the intestinal region; TbD and TbV absent; TbP 7 per side surrounding the caudum. Locomotor ciliature: complete ventral covering. Mouth sub-terminal, rim bears a corona of fine projections; buccal cavity lightly cuticularized; pharyngeal pores are sub-basal; intestine broadest in front, narrowing to the rear; anus at U94. Hermaphroditic; testes extend rearward from the PhJIn; an undeveloped egg and a germinal vesicle were seen; caudal organ has a narrow glandular forward portion and a broader rear portion, sinuous at its base, with an internal canal, but no coarse spiral or longitudinal musculature being seen; frontal organ has a double apical section, the one somewhat more ventrolateral to the other, with a bulge in the rear that contains a ventral opening, the interior not containing active sperm, and a basal cell to the rear; caudal organ overlaps part of the basal cell but not the frontal organ itself.

Description: Adult Lt 753 Μm; LPh 310 Μm to PhJIn at U41 ( Fig. 2 A, B). Body medium in length as an adult, ventrally flattened, dorsally vaulted; head ovoid, with a slight swelling, behind the pestle organs at U03; trunk broad, but constricting in the regions of the rear pharynx and the frontal organ, before narrowing gradually to the caudum. Widths at pestle organs/pharyngeal swelling/pharyngeal pores/mid-testes/midFO/midCO/anus/ caudum (min.) and locations along the length of the body are as follows: 62/86/65/93/75/79/29/20 Μm at U03/ U10/U30/U48/U64/U76/U94/U96, respectively. Epidermis unarmored, glands not conspicuous.

Adhesive tubes: TbA 5 per side (L 12 Μm), in transverse rows, which insert directly on the postoral body surface at U01 and project forward; TbL 25 per side (L 16–22 Μm), symmetrically placed, but with variable spacing, from U37 rearward, with 0 before the pharyngeal pores, 1 at the rear of the pharynx, 1 at the PhJIn, and 23 in the intestinal region; TbD and TbV are absent; TbP 7 per side surrounding the caudum posterior to the anus, all as long as the largest TbL.

Ciliation: A few hooked sensory cilia (L 28–30 Μm) occur on either side of the mouth, with a circlet of cilia surrounding the head in front of the pestle organs; other sensory hairs arise in three columns on either side of the body, lateral and dorsal (L similar), and ventrolateral (L 12–15 Μm) with 25–28 per column. Ventral locomotor ciliature: the entire ventral surface is covered with cilia (L 18–22 m), with bare spots beneath the frontal organ opening and the anus.

Digestive tract: Mouth slightly subterminal, 24 um in diameter; mouth rim bears a corona of a dozen fine projections (L 8 Μm); buccal cavity is lightly cuticularized; pharynx has sub-basal pharyngeal pores at U30; intestine is broadest in front, narrowing slowly to the rear; anus is at U94.

Reproductive tract: Testes extend rearward from the PhJIn, vasa deferentia not seen; eggs develop from rear to front, one undeveloped being seen in this specimen, with an additional germinal vesicle immediately to its rear; caudal organ is two parted, the fore part narrow and glandular, the rear part broader, sinuous at its rear with an internal longitudinal canal, but with no coarse spiral musculature or longitudinal musculature being seen; frontal organ has a double apical section, the one somewhat more ventrolateral to the other, with a medial bulge in the rear that contains the ventral opening, and a basal cell to the rear; sperm were not seen internally; caudal organ overlaps part of the basal cell but not the frontal organ itself.

Ecology: Sparse in frequency of occurrence (fewer than 10% of samples), rare in abundance (less than 1% of a sample); littoral in very fine, medium to medium-well sorted clean sand at 0–10 cm depth MLWN-MLWS.

Geographical distribution: MED: EUROPE: ITALY: Veneto {Caorle W^ 45°,35’N/12°,52’E [video]}.

Remarks: The description of Macrodasys digronus n. sp. is taken from a single specimen, the only one found (WDH video #261, a holotype, ICZN Articles 73.1.2). Unusual is the frontal organ with its double apical section, one somewhat more ventrolateral to the other, with a medial bulge in the rear that contains the ventral opening, and the caudal organ that overlaps part of the basal cell of the frontal organ, but not the frontal organ itself.

Etymology: The species is named for the double (Greek: di -) apical section of the cavernous (Greek: gronos) frontal organ.

Taxonomic affinities: No other species in the genus Macrodasys has the combination of characters that occur in M. digronus n. sp.: a frontal organ having a double apical section, one more ventrolateral than the other, with a bulge containing the ventral opening, and a caudal organ that overlaps part of the basal cell of the frontal organ, but not the frontal organ itself, PhJIn at U41, TbA 5 per side, TbL/TbP 25/7 per side, occurring regularly from the rear pharynx back.

Figures

FIGURE 2. Macrodasys digronus n. sp. A—dorsal and B—ventral views of an adult (Lt = 753 µm, LPh = 310 µm), from Caorle W, Italy, dorsal with digestive and reproductive tracts; ventral with adhesive tubes and the ventral locomotor ciliary band.