Thylakogaster lobotourus Wilson & Hessler, 1974

Cunha, Marina R. & Wilson, George D. F., 2003, Haplomunnidae (Crustacea: Isopoda) reviewed, with a description of an intact specimen of Thylakogaster Wilson & Hessler, 1974, Zootaxa 326, pp. 1-16 : 10-14

Thylakogaster lobotourus Wilson & Hessler, 1974 ( Figs. 2–3 )

Thylakogaster lobotourus Wilson & Hessler, 1974: 56–58 , fig. 4.

Type material.— NW Atlantic: Bermuda Slope, southeast of northern tip of the Bermuda Islands, 32º19’N, 64º32’W, 1135–1153 m, stn WHOI 118 a, holotype 1 copulatory male ( USNM 141471 ), paratypes 1 immature female ( USNM 141472 ) and 2 copulatory males ( ZMUC) .

Material examined.— Mid­Atlantic Ridge: Lucky Strike, 37º17.36’N, 32º16.49’W, 1695 m, Submersible Nautile , colonisation tray, stn PICO SMAC C6 , 6. VII .1998 , D. Desbruyères, 1 female (illustrated herein) head–pereon length 1.46 mm ( CENTOB). Same locality, 37º17.44’N, 32º16.60’W, 1685 m, RV Prof. Logachev, TV­assisted grab, stn TTR­10 AT­265­ Gr, 10.VIII.2000, M. R . Cunha, 1 manca IV– V head–pereon length 1.24 mm ( DBUA). Same locality, 37º17.30’N, 32º16.56’W, 1709 m, RV Prof. Logachev, TVassisted grab, stn TTR­12 AT­436­ Gr, 29.VIII.2002, M. R . Cunha, 1 female head–pereon length 3.07 mm, and 1 anterior fragment. NW Atlantic: Bermuda Slope , southeast of northern tip of the Bermuda Islands, stn WHOI 118 a (type locality), 14 individuals or fragments, most in poor condition ( AM P64368); 32º16’N, 64º32’W, 2095–2223 m, stn WHOI 119 , 1 individual, pleotelson only ( AM P64369) .

Supplementary description: Female ( Figs. 2–3 ): Head–pereon length 1.46 mm measured laterally. Antennula with 11 articles, article 1 with 2 penicillate setae, article 2 with 1 penicillate seta, article 4 with 1 penicillate seta, all located distally; article 3 length 1.6 article 2 length; article 4 length 0.28 article 2 length (proportions similar to T. peterpauli and T. majusculus ). Antenna length 2.6 head–pereon length; article 5 length 2.5 articles 1–4 length altogether; article 6 length 1.7 article 5 length, both articles bearing numerous long setae; flagellum length 0.30 article 6 length, with 10 articles. Pereopod I basis length 2.3 width; ischium with 5 ventral setae, 2 dorsal setae; merus with 3 ventral setae, 1 dorsal seta, 4 medial setae; carpus length approximately 2 width, slightly longer than basis, with 11–12 setae on ventral margin, with 5–6 short stout setae distal to large central seta, proximal medial surface with 10 setae in two rows; dactylus length approximately 0.85 propodus length. Pereopod II long, length 2 head–pereon length; basis length approximately 3 width; propodus slender and curved, length approximately 15 width; both carpus and propodus with numerous setae arising from short cuticular protuberances, setae largest on occlusive margin on carpus and lateral and medial side of propodus; dactylus length 0.2 propodus length, with 1 claw. Pereopod III shorter than but similar to pereopod II. Pereopods IV– VII less robust and longer than pereopod II, length approximately 1.1 pereopod II length; basis width approximately 0.8 pereopod II width; propodus and carpus approximately two­thirds total pereopod length; propodus slender, length 1.4 carpus length (somewhat longer in pereopod VII); propodus with long robust mediodistal seta at dactylar insertion.

Remarks: Although the Lucky Strike specimens are distant from the original Bermuda Slope type locality of T. lobotourus , we found no differences that require the establishment of a new species. These new specimens suffered some bending and twisting of their thin legs, so the whole body illustration ( Fig. 2A ) was made by merging several views of the limbs to obtain an approximation of the ‘natural’ condition.

Males and juvenile specimens (mancas or females) of Thylakogaster species have usually one or two pairs of setae on the distal margin of pleotelson (between the uropods), and indentations of the margin; mature females lack these setae or indentations. Our observations on Thylakogaster specimens held at the Australian Museum show the following patterns: T. lobotourus WHOI 118a ( AM P.64368), female no setae, male two pairs of setae; T. lobotourus Lucky Strike female, one pair of setae and indentations ( Fig. 2D ); T. majusculus WHOI 242a ( AM P.64371), juvenile female no setae, male two pairs of setae and indentations; T. peterpauli WHOI 155 ( AM P.64372), small male several small setae, female no setae, both with abundant cuticular hairs and no indentations; T. n. sp.1 WHOI 303 ( AM P.64373), manca two pairs of setae. The cuticle of T. peterpauli has fine spinules, especially on the pleotelson, that are missing in T. lobotourus .

We have observed several differences between species of Thylakogaster : i) The pereopod I carpus setae distal to large central seta on ventral margin differ: 5 or 6 in T. lobotourus , 7 in T. peterpauli ; 7–9 in T. majusculus .

ii) The antenna of T. lobotourus is longer, with a longer flagellum and smaller setae than in T. peterpauli (antenna ~3.1 and 2.5 head–pereon length for T. lobotourus and T. peterpauli , respectively).

iii) The antennula of an adult male is slender with fewer aesthetascs in T. peterpauli and more robust in T. lobotourus and T. majusculus .

iv) Spines on body and pleotelson differ among the species. Relative to each other, number of spines varies as follows: T. majusculus > T. peterpauli > T. lobotourus . The length of the spines shows a different trend: T. lobotourus > T. majusculus > T. peterpauli . The pleotelson in T. lobotourus has fewer spines (10–15% less for the same area) but longer spines (1.1 times) than in T. peterpauli .

Figures

FIGURE 2. Thylakogaster lobotourus Wilson, 1974, female; A, habitus, lateral view (scale bar 1 mm); B, base of antennula and antenna, dorsal view; C, tip of antennula; D, left uropod, lateral view; E, pleopod II, ventral view.

FIGURE 3. Thylakogaster lobotourus Wilson, 1974, female; A, pereopod I, lateral view; B pereopod I, mesial view; C, dactylus, pereopod II; D, dactylus, pereopod IV; E, dactylus, pereopod V; F, head, lateral view (scale bars: A–B, 0.1 mm; C–E, 0.1 mm; F, 0.5 mm).