Macrodasys nigrocellus, 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 : 18-20

Macrodasys nigrocellus sp. n. Figures 89

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

Diagnosis:

Adult being described Lt 524 µm; PhJIn at U31. Head stepped, narrowing toward the mouth, with a band of circumcephalic cilia at U01-U02, pestle organs in the step at U03 and black ocelli borne just behind pestle organs at U05; trunk broader in the pharyngeal than in the fore-gut region, broadest in the mid-body region, narrowing quickly in the hind-gut region to the long caudum. Glands 8 per side. TbA 7-8 per side, in arcs that insert directly on the postoral body surface at U02-U03 and project forward to obliquely outward; TbL 14, of similar size, with 3 at U55, U64 and U73, and 12 at U80 and behind (7 along the hind-gut, and 4 behind the anal aperture); TbV 14 per side, even in size, but uneven in spacing, with 1 along the rear pharynx, 1 just behind the PhJIn and the remainder in the intestinal region from U41 to U77, mostly concentrated in the rear, and 0 behind the anus; TbP 10-11 per side symmetrically along the caudum. Locomotor ciliature: a single field lies between the TbV series from the TbA back onto the caudal base, with small bare spots surrounding the ventral openings. Mouth terminal, narrow; buccal cavity shallow, lightly cuticularized, expanding with depth; pharyngeal pores sub-basal; intestine narrows gradually front to rear; anus ventral at U84. Hermaphroditic; testes begin just before the PhJIn, vasa differentia join beneath the frontal organ; ova develop rear to front, with two large developing ova and several smaller bilaterally to the rear; frontal organ Y-shaped, its nozzle in close contact with the most developed ova, with an elongate cavity having a ventral pore lying on the left; caudal organ of medium length, its fore half appearing glandular, its rear half having an internal canal and a spiral muscular covering, except for the rearmost glandular sac, the caudal organ does not overlap the frontal organ.

Description:

Adult being described Lt 524 µm (another Lt 558); LPh 161 µm (another LPh 230) to PhJIn at U31 (another to PhJIn at U41) (Fig. 8). Body medium in length as an adult, ventrally flattened, dorsally vaulted, robust; head stepped, narrowing toward the mouth, bearing a band of circumcephalic cilia at U01-U02, pestle organs in the step at U03 and black ocelli borne just behind pestle organs at U05 (black throughout its development from juvenile to adult); trunk broader in the pharyngeal than in the fore-gut region, broadest in the mid-body regions narrowing quickly in the hind-gut region to the long caudum. Widths at pestle organs /ocelli /PhJIn /trunk /anus /caudum, and locations along the length of the body are as follows: 41 /55 /64 /86 /34 /9-7 µm at U03 /U05 /U31 /U49 /U84 /U92-U97, respectively. Glands 8 per side (5 µm diameter), distributed 3 along the pharynx and 5 along the rear half of the intestine.

Adhesive tubes: TbA 7-8 per side (L 7-9 µm), in arcs that insert directly on the postoral body surface at U02-U03 and project from forward to obliquely outward; TbL 14, of similar size (L 12-18 µm), with 3 at U55, U64 and U73, and the other 11 at U80 and behind (7 along the hind-gut, and 4 behind the anal aperture); TbV 14 per side, even in size (L 12-15 µm) but uneven in spacing, with 1 along the rear pharynx, 1 just behind the PhJIn and the remainder in the intestinal region from U41 to U77, mostly concentrated in the rear, and 0 behind the anus; TbP 10-11 per side (L 12-15 µm), mostly symmetrical along the caudum, 2 at the terminus and 8-9 along the sides.

Ciliation: Numerous sensory hairs (L 10-16 µm) occur on either side of the head; a band of cilia surrounds the forehead (L 10-12 µm) at U01-U02; other sensory hairs (L 12-16 µm) arise in four columns on either side of the body: lateral, lower dorsolateral, upper dorsolateral, and dorsal (in pairs), with about 32, 24, 18 and 11 per column. Ventral locomotor ciliature forms a single field that lies between the TbV series from the TbA back onto the caudal base just behind the anus, but not beneath the caudum, with small bare spots surrounding the ventral openings.

Digestive tract: Mouth terminal, narrow in width (10 µm diameter), surrounded by sharp tooth-like projections; buccal cavity shallow, lightly cuticularized, expands from oral opening to its base; pharynx has sub-basal pharyngeal pores at U24; intestine is broadest in front, narrowing to the rear and bending around the base of the caudal organ; anus is ventral at U84.

Reproductive tract: Hermaphroditic; testes beginning just before the PhJIn, and extending as vasa deferentia back to the rear of the frontal organ where they join ventrally; ova develop from rear to front, with several large ova seen (two at 106 × 48 and 102 × 32 µm in size) and three smaller ovules bilaterally distributed to the rear; frontal organ Y-shaped, its nozzle in close contact with the most developed ovum, with an elongate cavern with its own ventral pore lying on the left, sperm not seen internally; caudal organ of medium length, its fore half appearing glandular, its rear half having an internal canal and a spiral muscular covering, except for the rearmost glandular sac, the caudal organ does not overlap the frontal organ.

Ecology:

Occasional in frequency of occurrence (10-30% of samples), rare to scarce in abundance (fewer than 1% to 5% of a sample); littoral in very fine, medium-well sorted coralline sand, at mean low water to extreme low water, 0-15 cm depth; sublittoral in very fine to very coarse, medium well to very poorly sorted coralline sand and coral debris at 1-4 m water depth, between patches of healthy or unhealthy fringing reef.

Geographical distribution:

RED SEA:EGYPT:{Wadi 'Araba [video], ^Giftun Island SE (27°10'N, 33°57'E) [video], Moon Valley [3-videos], 'Uyun Musa [video]}; ISRAEL: {Snuba (Eilat) [video]} INDIAN OCEAN:MALDIVES: {Addu-Atoll}

Remarks:

There are seven video sequences of Macrodasys nigrocellus sp. n., all from the upper Red Sea in Egypt and Israel. Four of these are available as MPEG 2 (and MPEG 1) from Hummon (2009): #1175 a mature adult of Lt=558 µm (LPh=230 µm) from Moon Valley, near Hurghada, Egypt; #1171 a mature Lectotype adult of Lt=524 µm (LPh=172 µm), collected in June 1994 from Giftun Island SE, also near Hurghada; #1178 a subadult of Lt=360 µm (LPh=154 µm) from the Wadi 'Arabah, Gulf of Suez, Egypt; and #1886 a juvenile of Lt=209 µm (LPh=104 µm) from the Snuba Dive Shop, Eilat, Israel.

Etymology:

Nigrocellus (Latin: niger + ocellus = meaning 'black eye’) refers to the black ocelli that it bears.

Taxonomic affinities:

Macrodasys nigrocellus sp. n. is the only species in the genus with a stepped anterior, with pestle organs in the step, black ocelli, a long tail, and a PhJIn of U31-U41, which also has TbA 7-8 per side in arcs; a TbL formula of 11=0,0/7,4 (0 along the fore and rear halves of the pharynx /7 along the rear intestine and 4 behind the anus), a TbV formula of 14=0,1/13,0 (mostly aggregated to the rear); and TbP 10 per side; but without TbD. There are three species that have mitten-shaped frontal organs: Macrodasys caudatus Remane, 1924 (though the specimen as originally described did not show a frontal organ), Macrodasys pacificus Schmidt, 1974 and Macrodasys meristocytalis Evans, 1994, none of which has black eye spots. No species yet described in this genus has black ocelli, but Macrodasys sp. Gerlach (1961: p.474, Fig. 3ab) from the Maldive Islands has such ocelli, and shows some characters that are similar (Fig. 9) to those of Macrodasys nigrocellus sp. n., namely the TbA, pestle organs and ocelli, though other characters such as the location of the testes and of the adhesive tubes make one wish that Gerlach had completed the entire drawing, rather than just the front half so that we might have seen the accessory reproductive organs.