Macrodasys imbricatus , Hummon, William D., 2011

Hummon, William D., 2011, Marine Gastrotricha of the Near East: 1. Fourteen new species of Macrodasyida and a redescription of Dactylopodola agadasys Hochberg, 2003, ZooKeys 94, pp. 1-59: 13-15

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Macrodasys imbricatus

sp. n.

Macrodasys imbricatus  ZBK  sp. n. Figure 6

Macrodasys  EgyI Hummon (2009) [E Med & Red Seas Database].


Adult being described Lt 544 µm; PhJIn at U43. Head stepped, narrowing toward the mouth, pestle organs in the step at U03; trunk broadest in the pharyngeal region, narrowing gradually to the long caudum. Glands inconspicuous. TbA 5-6 per side, in transverse rows that insert directly on the body; TbL/TbV 35 per side, the series beginning as TbL, then rotating to TbV and finally along the caudal base the two series duplicating one another; TbL 7 per side, with 0 along the fore and and 4 along the rear pharynx, 2 along the rear intestine, and 1 behind the anus; TbV 28 per side, uneven in size and location, but symmetrical in placement, with 1 along the rear pharynx, and the remainder in the intestinal region, the final tubes duplicating those of the rear of the TbL series; TbP 11 per side, surrounding the elongate caudum. Locomotor ciliature: a single field runs from the oral opening to the tip of the caudum, unciliated in spots surrounding ventral openings. Mouth terminal, of medium-broad width; buccal cavity lightly cuticularized, expanding with depth; pharyngeal pores para-basal; intestine narrows gradually to the rear; anus ventral at U80. Hermaphroditic; testes begin just before the PhJIn, vasa differentia join beneath the frontal organ; ova probably develop rear to front; frontal organ tubular, with cuticular nozzle, lies behind the foremost ovum; long caudal organ with muscular/glandular construction in a half to half ratio, extensively overlapping with the frontal organ.


Adult being described Lt 544 µm (others Lt 541-625); LPh 234 µm (others LPh 200-205) to PhJIn at U43 (others PhJIn at U32-U38) (Fig. 6). Body medium in length as an adult, ventrally flattened, dorsally vaulted; head stepped, narrowing toward the mouth, bearing pestle organs in the step at U03; trunk broadest in the pharyngeal region, narrowing gradually to the long caudum. Widths at pestle organs /pharynx /PhJIn /anus /caudum, and locations along the length of the body are as follows: 40 /60 /56 /31 /22-6 µm at U03 /U26 /U43 /U80 /U84-U98, respectively. Glands not conspicuous.

Adhesive tubes: TbA 5-6 per side (L 7 µm), in transverse rows, which insert directly on the postoral body surface at U02 and project forward; TbL/TbV are unusual and complex, with 35 per side, from U28 to U95, with the first 4 being lateral and the others rotating ventrally, while the last TbL 3 along the caudal base are duplicated by 3 of the last 4 TbV; TbL 7, symmetrically and evenly placed and of similar size, with 2 along the fore and 2 along the rear pharynx, 2 along the rear intestine, and 1 more behind the anal aperture; TbV 28 per side, symmetrically placed but uneven in spacing and size (L 6-18 µm), with 1 along the rear pharynx, and the remainder in the intestinal region, 2 of which occur behind the anus, with numerous cases of side-by-side tubes; TbP 11 per side (L 10-12 µm) symmetrically surround the elongate caudum.

Ciliation: Sensory cilia (L 5-12 µm) occur around and on either side of the mouth; head lacks a ciliary corona, other sensory cilia arise in four columns on either side of the body: lower lateral (L 12-15 µm), upper lateral (L 7-9 µm), dorsolateral (L 15-26 µm) and dorsal (L 15-26 µm), with about 19, 34, 27 and 16 per column. Ventral locomotor cilia form a single field that lies between the TbL/V series back to the tip of the tail, with bare spots surrounding the ventral reproductive and anal openings.

Digestive tract: Mouth terminal, of medium-broad width (16 µm diameter); inner mouth rim bears a series of longitudinal ridges, extending half the depth of the buccal cavity; buccal cavity is mug-shaped, expanding slightly from oral opening to base and, is lightly cuticularized; pharyngeal musculature can be seen, with para-basal pharyngeal pores at U36; intestine is broadest in front, narrowing slowly to the rear; anus is ventral at U80.

Reproductive tract: Hermaphroditic; testes begin just before the PhJIn and vasa deferentia extend back to the rear of the frontal organ, though their termini were not seen; ova probably develop from rear to front, but only one small ovum was seen (22 × 18 µm in size); a tubular frontal organ occurs behind the ovum, having a refractile cuticular nozzle and circular muscles in the front three-fifths, as well as containing motile sperm, and cellular material in the accessory chamber, covering the rear two-fifths; long caudal organ appears to have circular muscles over the rear half that lie atop longitudinal muscles, which occur over the entire length of the organ, except for its rearmost glandular sac that lies at an obtuse angle to the rest.


Occasional in frequency of occurrence (10-30% of samples), scarce to prevalent in abundance (3 to greater than 30% of a sample; occasionally a co- [cdom] or dominant [dom]); littoral in fine to very coarse, well to very poorly sorted, silicious or coralaginous sand with small amounts of detritus, at mean low water to extreme low water, 0-15 cm depth, sometimes on a tombolo; sublittoral in fine to coarse, medium well to very poorly sorted sand or coral debris at 1-7 m water depth, sometimes in sand occupying a ‘blowout’ depression on a coral platform.

Geographical distribution:

RED SEA:EGYPT:{16 km S of Ein Sukhna [video], Marsa Bareika W, Sharm el-Arab Inside [video], Hammam Pharoan [dom] [videos], ^Main Beach Ras Mohamed NP (27°44'N, 34°12'E) [video], Ras Nasrani, Tareef el-Reeh [video], Tip Ras Mohamed NP [cdom]}; ISRAEL: {Coral Beach M2 [video], M3 [3-videos], M4 [video], Coral Beach N)}


There are ten video sequences of Macrodasys imbricatus  sp. n., all from the upper Red Sea in Egypt and Israel. Four of these are available as MPEG 2 (and MPEG 1) in Hummon (2009): #1365 a mature adult of Lt=625 µm (LPh=200 µm) from Hamman Pharoan, Egypt; #1347 a mature Lectotype adult of Lt=544 µm (LPh=232 µm), collected in July 1994 from Main Beach Ras Mohamed National Park, S. Sinai, Egypt; #1364 a mature adult of Lt=541 µm (LPh=205 µm) also from Hamman Pharoan; and #1929 a subadult of Lt=242 µm (LPh=97 µm) from the Coral Reserve, Eilat, Israel.


Imbricatus (Latin: imbricatus = meaning ‘overlapping’) refers to the extensive overlap between caudal and frontal organs.

Taxonomic affinities:

Macrodasys imbricatus  sp. n. is the only species in the genus with an stepped anterior, with pestle organs in the step, a long tail, and a PhJIn of U32-U43, which also has TbA 5-6 per side in transverse rows; a TbL formula of 7=0,4/2,1 (0 along the fore and 4 along the rear pharynx/2 along the rear intestine and 1 behind the anus); a TbV formula of 27=0,1/25,1; and TbP 11 per side; but without TbD. Two characters of Macrodasys imbricatus  sp. n. are unique thus far in the genus: one is the rotation of the TbL series to a TbV position and the rearward duplication of the two; the other is the remarkable overlap between caudal and frontal organs. To be compared with Macrodasys imbricatus  sp. n., a species would have to have a tubular frontal organ and an elongate caudal organ that overlaps it by more than half. While there are several with tubular frontal organs, few have overlapping caudal with frontal organs. Only two species may meet these criteria: Macrodasys gylius  Hummon, 2010 and Macrodasys neapolitanus  Papi, 1957. The former has many fewer TbL/TbV than in Macrodasys imbricatus  sp. n. and those that are present do not rotate from TbL to TbV and back, while the latter has few TbL occurring in front of the caudum and no TbV.