EURYTHENEIDAE, Stoddart & Lowry, 2004

Family EURYTHENEIDAE View in CoL 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 .