Leptognathiella Hansen, 1913

Larsen, Kim, Błażewicz-Paszkowycz, Magdalena & Cunha, Marina R., 2006, Tanaidacean (Crustacea: Peracarida) fauna from chemically reduced habitats-the lucky strike hydrothermal vent system, mid-atlantic ridge, Zootaxa 1187, pp. 1-36 : 3-10

Genus Leptognathiella Hansen, 1913

Leptognathiella Hansen, 1913

Leptognathia Lang, 1968

Leptognathiella Bird & Holdich, 1984

Diagnosis Female (after Bird & Holdich 1984 and Larsen 2005). Relatively small (rarely over 2 mm in length. Body elongated. Pleon and pleotelson short (never longer than combined length of three last pereonites). Antennule with four or five articles; article 2 frequently with dorsal projection overlapping basal part of article 3. Antenna with five or six articles. Molar thin with few terminal spines. Maxilliped endite often with medial, frequently triangular, process. Chelipeds attached via sclerite. Pereopods 1– often stout, with or without coxa; ischical setae longer than merus; meral spiniform setae as long as carpus.

Pereopods 4–6 without coxa and not stouter than pereopods 1–3; dactylus and unguis not fused. Pleopods absent or present with simple setae only. Uropods longer than pleotelson, biramous; rami with one or two articles.

Male: pleon and pleotelson longer than female often longer than combined length of two last pereonites (but barely so). Pleopods present with simple setae only.

Type species Leptognathiella abyssi Hansen, 1913 .

Gender Feminine.

Remarks

This genus is represented by seven species including the one described below. The genus appears to be restricted to the Atlantic or Gulf of Mexico. A key to the genus has been given by Larsen (2005).

Leptognathiella fragilis n. sp.

( Figs. 1 , 2 )

Material examined

Holotype: non­ovigerous female ( ZMUC CRU­4912 ), inside the vent field, 37°17.289’ N, 32º16.522’ W, 1718 m, RV Prof. Logachev, TV­assisted grab, TTR­12, station AT­428­GR, August 2002 . Paratypes: 1 female, ( ZMUC CRU­4913 ), same data . 1 female (dissected), same data. Other material : 1 female, 37º17.300’ N, 32º16.563’ W, 1709 m, RV Prof. Logachev, TV­assisted grab, TTR­12, station AT­436­GR, August 2002 .

Diagnosis

Female. Body fairly elongate relative to most other species of the genus (length:width ratio 9.5). Cephalothorax as long as combined lengths of first two pereonites. Pereopods 1–3 carpus and propodus without small ventral spines. Uropods twice as long as pleotelson but shorter than combined pleon; basal article shorter than pleotelson and biarticulated exopod half as long as first endopod article.

Male: unknown.

Etymology. Named after the fragile cuticle of this species.

Description Adult female. Body length 1.3 mm. Body ( Fig. 1A ). 9.5 times as long as broad.

Cephalothorax. As long as combined lengths of pereonites 1 and 2. Eyelobes absent.

Pereonites. Pereonites 1, 2 and 6 wider than long. Pereonite 3 longer than wide. Pereonites 4 and 5 as wide as long.

Pleon. Short (less than 0.25 times as long as total body length). All pleonites subequal. Pleotelson as long as combined length of two pleonites, apex blunt.

Antennule ( Fig. 1B ). Longer than cephalothorax. Article 1 as long as rest of antennule, with two simple distal and three pinnate subdistal setae. Article 2 shorter than half of article 1, with two simple­ and three pinnate distal setae. Article 3 shorter than article 4, with two simple distal setae. Article 4 about as long as article 2, with one aesthetasc, four simple, and one tiny setae, all distal.

Antenna ( Fig. 1C ). 0.7 times as long as antennule. Article 1 not broader than following articles, naked. Article 2 longer than article 5, naked. Article 3 longer than other articles, with fusion line, with one simple seta proximal to fusion line and two distal simple setae. Article 4 longer than article 1, with one distal seta. Article 5 minute, with two long thick distal setae.

Mouthparts. Labrum ( Fig. 1D ) slightly pointed, naked. Mandibular molar pointed and longer than incisor, with small distal spines. Right mandible ( Fig. 1E ) incisor slender and terminating in a small spine. Labium not recovered. Maxillule ( Fig. 1F ) with eight spiniform terminal setae. Maxilla ( Fig. 1G ) slightly elongated and naked. Maxilliped ( Fig. 1H ) endites apparently naked, narrower than basis; palp article 1 naked; article 2 and 3 with three setae on inner margin; article 4 only half as wide as article 3, with four setae. Epignath not recovered.

Cheliped ( Fig. 1I ). Basis divided unequally by sclerite, shorter than carpus. Merus triangular with one ventral seta. Carpus longer than propodus including fixed finger, with two ventral setae. Propodus with one seta near dactylus insertion on inner margin. Fixed finger with two ventral setae, three on inner margin, and with four prominent sharp denticles on inner margin. Dactylus as long as fixed finger.

Pereopod 1 ( Fig. 2A ). Coxa naked. Basis as long as three succeeding articles combined, naked. Ischium apparently naked (see remarks). Merus shorter than carpus, widening distally, with one stout ventrodistal seta. Carpus longer than half of propodus, with two stout distal setae. Propodus longer than half the length of basis, with one ventrodistal spiniform seta. Dactylus and unguis subequal, combined shorter than propodus and not fused to a claw.

Pereopod 2 ( Fig. 2B ). As pereopod 1 except: coxa and basis each with one seta.

Pereopod 3 ( Fig. 2C ). As pereopod 1 except: merus with two setae. Propodus with tiny ventrodistal setules.

Pereopod 4 ( Fig. 2D ). No apparent coxa. Basis naked, longer than three succeeding articles. Ischium with one long ventral seta (almost as long as merus). Merus with two spiniform distal setae. Carpus with three stout and one small simple distal setae. Propodus with three stout distal setae. Dactylus and unguis shorter than propodus, dactylus more than twice as long as unguis.

Pereopod 5 ( Fig. 2E ). As pereopod 4 except: basis with one ventromedial pinnate seta.

Pereopod 6 ( Fig. 2F ). Described from holotype. As pereopod 5 except carpus with three stout distal setae.

Pleopods ( Fig. 2G ). Small, rather stout. All pairs similar. Each endite armed with five short simple distal setae only.

Uropods ( Fig. 2H ). Twice as long as pleotelson but shorter than entire pleon. Basal article shorter than pleotelson. Endopod with two articles of subequal length, article 1 with one distal seta, article 2 with two long and three simple distal setae. Exopod half as long as endopod article 1, with two articles of subequal length; article 1 naked; article 2 with one prominent and one small distal seta.

Remarks. This species is rather similar to L. subaequalis Hansen, 1913 and L. clivicola Bird & Holdich, 1984 but can be distinguished from the first by a biarticulated uropodal exopod and from the latter by the cheliped denticles and by the uropodal exopod only being as long as half of first endopod article.

The medial and frequently triangular process on the distal edge of the endite of the maxilliped is reported as a stable character for this genus by Larsen (2005). This structure was not found in this species and this makes genus designation rather weak. However, due to the small size of this species and the poor condition of the material, the lack of maxilliped process might be an artefact.

The ischial setation is described as diagnostic by Bird & Holdich (1984) and seems to be a good character for the genus. However, these setae are fragile and it is often not possible to determine whether they have been broken off or never been present.

Family LEPTOCHELIIDAE Lang, 1973

Genus Mesotanais Dollfus, 1897

Diagnosis (modified from Sieg & Heard 1989)

Female. Eyelobes present but without visual pigment. Antennule with three articles. Antenna with six articles; article 2 and 3 with spiniform dorsal setae. Mandibles well developed with broad molar. Labium with two pairs of lobes. Maxilliped basis and endites not fused; endites with two or three short, flat, unequal­sized setae, basis with one or two long simple setae near palp insertion. Chelipeds attached via sclerite. Pereopod 1 almost twice as long as following pereopods; dactylus and unguis combined longer than propodus, combined dactylus and unguis on other pereopods shorter than propodus. Pereopods 1–3 with coxa; dactylus/unguis not fused. Pereopods 4–6 without coxa; basis thicker than pereopods 1–3, dactylus/unguis incompletely fused to an elongated claw. Pleopods present and well developed, with plumose setae. Uropods biramous; exopod biarticulated; endopod with three or more articles.

Male (after Sieg & Heard 1989). Only known from one incomplete specimen of M. vadicola ( Sieg & Heard 1989). Habitus as female except the carapace is longer and the pleon is wider. Antennule with seven articles and with multiple aesthetascs. Cheliped basis enlarged (rest of chelipeds are missing in only known specimen).

Remarks

This genus is similar to other genera in the family Leptocheliidae but can be separated by the lack of eyes. Previously, the setation of the maxilliped basis was supposed to be stable in this genus but Mesotanais styxis n. sp. conflicts with this character and displays only one such seta. Another leptocheliid genus ( Heterotanais ) is diagnosed by having only one of these setae, but this genus is diagnosed with eyes.

Type species Mesotanais dubius Dollfus, 1897 ; by monotypy.

Gender Masculine.

Figures

FIGURE 1. Leptognathiella fragilis n. sp. A, holotype, lateral view; B, antennule; C, antenna; D, labrum; E, right mandible; F, maxillule, endite; G, maxilla; H, maxilliped; I, cheliped. Scale bars: A = 1 mm. Others = 0.1 mm.

FIGURE 2. Leptognathiella fragilis n. sp. A, pereopod 1; B, pereopod 2; C, pereopod 3; D, pereopod 4; E, pereopod 5; F, pereopod 6; G, pleopod; H, uropod. Scale bar = 0.1 mm.