Hirondelleidae, Lowry & Stoddart, 2010

Lowry, J. K. & Stoddart, H. E., 2010, The deep-sea scavenging genus Hirondellea (Crustacea: Amphipoda: Lysianassoidea: Hirondelleidae fam. nov.) in Australian waters, Zootaxa 2329 (1), pp. 37-55 : 37-38

Hirondelleidae fam. nov.

Diagnostic description. Head exposed, much deeper than long, not extending much below insertion of antenna 2, without cheek notch. Antennae calceoli present in male, absent in female. Antenna 1 with callynophore in male and female; accessory flagellum article 1 forming a cap partially covering callynophore. Antenna 2 peduncular article 3 without distal hook. Epistome and upper lip separate. Mouthpart bundle subquadrate. Mandible incisors well developed, symmetrical, convex, smooth; left lacinia mobilis rod-like, right lacinia mobilis absent; accessory setal row with 5 or less robust setae, with distal setal tuft; molar a setose tongue, occasionally with small triturating surface, or large flap-like, weakly setose; palp inserted approximately midanteriorly. Maxilla 1 inner plate with 2 apical pappose setae, one very broad at base; outer plate with setal-teeth in 7/4 arrangement (or rarely in 8/3 crown arrangement); setal-teeth large; setal-tooth 6 slender, setal-tooth 7 slender, slightly or strongly displaced from setal-tooth 6; palp large, with apical robust setae and subterminal lateral notch. Maxilla 2 inner plate significantly shorter than outer plate, without oblique row of facial setae. Maxilliped coxa and basis normal; outer plate medial setae small, blunt or beadshaped, outer plate without apical setae; palp 4-articulate, article 4 well developed.

Gnathopod 1 subchelate or parachelate; coxa large but shorter than coxa 2 and tapering distally, or reduced; merus and carpus not rotated; ischium short; carpus short; propodus large; dactylus slightly curved. Gnathopod 2 coxa large, subequal in size to coxa 3; carpus rectolinear or rectangular, with palmate setae; propodus rectangular, with palmate setae; dactylus minute. Pereopods all simple; distal spurs absent. Pereopod 4 coxa with well developed posteroventral lobe. Pereopod 5 coxa anterior and posterior lobes subequal. Pereopod 6 coxa posterior lobe slightly deeper than anterior lobe, or much deeper than anterior lobe.

Uropod 2 inner ramus with or without constriction. Uropod 3 rami biramous. Telson cleft.

Type genus. Hirondellea Chevreux, 1889 .

Generic composition. Monogeneric.

Remarks. Most, if not all, hirondelleids are deep-sea scavengers. Seven of the 16 known species have been taken in baited traps. Significant modifications among scavenging lysianassoids include a fully developed 2-field callynophore in males and females, a setose tongue molar, a reduced inner plate on maxilla 2, bead-like setae on the outer plate of the maxilliped and a reduced first coxa. Most hirondelleids have a basic 7/4 setal-tooth arrangement on maxilla 1 outer plate. This is unusual among lysianassoid scavengers. Other lysianassoid scavengers have a modified 6/5 setal-tooth arrangement (tryphosines), an 8/3 crown arrangement (eurytheneids) or a 7/4 crown arrangement (uristids). Outside the lysianassoid group hirondelleids share the 7/ 4 setal-tooth arrangement with stegocephalid amphipods. The only other scavenging lysianassoid family with this type of arrangement is the scopelocheirid amphipods. Among non-scavenging lysianassoid families the 7/ 4 setal-tooth arrangement is found in the Aristiidae , lepidepecreellids and pachynids.

Family level taxa with a setose-tongue molar and bead-shaped setae on the medial margin of the outer plate of the maxilliped include the Alicellidae , cyclocarids, Endevouridae , Eurytheneidae , Hirondelleidae , Lysianassidae , Opisidae and Uristidae . Among these families the alicellids, hirondelleids, opisids and uristids have a reduced first coxa and of these only the hirondelleids and uristids have the inner plater of maxilla 2 shortened.

Apomorphies found only in hirondelleids (except H. sindhusagar ) include the enlarged seta on the inner plate of maxilla 1 and the subterminal lateral notch on the inner margin of the palp.

Hirondelleids appear to be the least specialised lysianassoid scavengers. As such, they could be the basal scavenging lysianassoids without close relationships to other extant lysianassoid groups. The 8/3 crown setaltooth arrangement shared between some hirondelleid species ( Hirondellea gigas , H. abyssalis , H. sindhusagar ) and species of Eurythenes appears to be a homoplasy, independently derived in each group, and indicates a selective pressure among flesh-eating lysianassoids to develop a crown arrangement.