EURYTHENEIDAE, Stoddart & Lowry, 2004

Stoddart, Helen E. & Lowry, James K., 2004, The deep-sea lysianassoid genus Eurythenes (Crustacea, Amphipoda, Eurytheneidae n. fam.), Zoosystema 26 (3), No. 3, pp. 425-468 : 427-428

Family EURYTHENEIDAE n. fam.

DIAGNOSIS. — Head exposed, much deeper than long, not extending much below insertion of antenna 2, without cheek notch. Antennae with calceoli present in male, absent in female. Antenna 1 with well developed two-field callynophore in male and female. Antenna 2 peduncular article 3 without distal hook. Mouthpart bundle subquadrate. Epistome and upper lip separate. Mandible incisors present, well developed, symmetrical, convex, smooth; right lacinia mobilis absent; accessory setal row without distal setal tuft; molar a setose tongue, with small triturating surface; palp present, inserted approximately mid-anteriorly. Maxilla 1 inner plate with more than two apical pappose setae; outer plate narrow with setal-teeth in 8/3 crown arrangement, setal-teeth large, ST6 and ST7 slender, ST7 slightly displaced from ST6; palp large, with apical robust setae. Maxilla 2 inner plate significantly shorter than outer plate. Maxilliped outer plate present, medial setae small, blunt or beadshaped; palp four-articulate, article 4 well developed. Gnathopod 1 subchelate to parachelate; coxa vestigial; merus and carpus not rotated; carpus short; propodus large, palm straight to convex; dactylus slightly curved, not hidden by setae. Gnathopod 2 coxa small, shorter than coxa 3. Pereopods all simple; distal spurs absent. Pereopod 3 coxa large. Pereopod 4 coxa large with well developed posteroventral lobe. Pereopod 5 coxa with anterior and posterior lobes subequal. Uropod 2 inner ramus without constriction. Uropod 3 biramous. Telson present, cleft.

GENUS INCLUDED. — Eurythenes Smith, 1882 .

REMARKS

Among the amphipods generally considered to be members of the Lysianassoidea , the Eurytheneidae n. fam. occur in the most basal clade, along with the Hirondellea -group, the Opisidae Lowry & Stoddart, 1995 , the Podoprionidae Lowry & Stoddart, 1996 and the Uristidae Hurley, 1963 (s.s.). The most common lifestyle in all of these groups (except the Opisidae ) is scavenging. In general, species in these groups have callynophores in both sexes, molars in the form of a setose tongue or flap, setal-teeth of maxilla 1 outer plate in a crown arrangement (except in the Podoprionidae ), well developed third uropods and a long, deeply cleft telson.

There are two forms of crown-type arrangements of the maxilla 1 setal-teeth: the 8/3 crown ( Lowry & Stoddart 1995: 140) which occurs fully-formed in the Alicella -group, eurytheneids, opisids and some species of Hirondellea and slightly modified in the Cebocaris -group and Thoriella- group; and the 7/4 crown ( Lowry & Stoddart 1992: 186) which occurs in Cyclocaris , Eclecticus and uristids. We suspect that each of these types has arisen independently on more than one occasion.

The eurytheneids appear to be the sister taxon to the Hirondellea -group. Species of Hirondellea differ from eurytheneids in having a setose flap molar, a distal setose tuft at the end of the accessory setal row, only two apical setae on the inner plate of maxilla 1 and coxa 2 about the same size as coxa 3.

Eurytheneids also appear to be closely related to uristids (s.s.). Most genera in this group have scavenging members. Uristids differ from eurytheneids in having a shorter head, the maxilla 1 setal-teeth in a 7/4 crown and setal-tooth 7 strongly displaced from setal-tooth 6.

Many uristids, Hirondellea species and eurytheneids are scavengers. Although opisids have many of the characters of scavenging lysianassoids, they are generally considered to be ectoparasites/micropredators of fishes – a lifestyle probably only one step removed from scavenging. Apparently they share a common ancestor with the scavenging groups and appear to be related to eurytheneids. In opisids the head is not as deep as in eurytheneids, the inner plate of maxilla 1 has two or less apical setae, the outer plate setal-tooth 7 is not displaced from setal-tooth 6, the coxa of gnathopood 1 is large or reduced, but never vestigial as in eurytheneids, and coxa 2 is as large as coxa 3.

The podoprionid scavengers also occur in the basal lysianassoid group. They have similarities to the eurytheneids, but their strongly serrate mandibular incisors, non-crown-like setal-teeth arrangement on maxilla 1, strongly chelate first gnathopods and strongly serrate posterior margin on the basis of pereopod 5 easily distinguish them from the eurytheneids.

Eurytheneids also share characters with the Alicella -group, but are immediately distinguished from this marginal lysianassoid group by the absence of a lacinia mobilis on the right mandible in Eurythenes .