(G.O. Sars, 1877)
? Generic features. General form compact and robust. Carpus large. Propodus divided into two to three subsegments. Posterior margin of carapace transverse. Integument covered with very small bristles or tiny spinules which are particularly noticeable on the dorsal surface of the telson and on the large basal segments of the appendages. Pleural plates of first abdominal somite only slightly produced. Eyes large with thick short eyestalks. Telson very long with small apical cleft. Male pleopods biramous well developed, natatory, third pair with exopod slightly elongated. Spines on outer margin of exopod of uropod small; large spines on distal portions of inner margin of endopod.
? Species. Carapace large, covering the whole of the thorax and more than half the first abdominal somite in dorsal view, widening considerably posteriorly, posterior margin straight and transverse. Rostrum large, narrowly biangular, covering part of the bases of the antennular peduncles, the anterior tip is usually bent downward between the antennules and so appears to be shorter than it really is.
Antennular peduncle in both sexes short and thick, first segment only slightly longer than the eyes, second segment very short and hollowed out at its anterior- end forming a kind of basin in which the third segment rests; inner margins of second and third segments in the male armed with a few long plumose setae.
Antennal scale very small, extending only slightly beyond the first segment of the antennular peduncle, outer margin naked, terminating in a small spine beyond which the apex extends considerably, small distal suture present. On the inner distal corner of the sympod there is a large serrated spine projecting forward.
Eyes thick, cornea occupying about a third of the whole eye, golden brown in colour. Labrum produced anteriorly into a long spine serrated along the margins.
First thoracic limb larger in the male than in the female, without lobes, dactylus small, bearing a long, strong, curved nail, very large axe-shaped vibratory lamella in both sexes. Second thoracic limb short, basal segment large, carpus in the male enlarged irregularly along its inner and distal margins. Third thoracic limb basal joint large, carpus distinct, propodus divided at distal end by two or three transverse articulations, dactylus very small, without nail. distal end of limb in the male furnished with seven long, strapshaped appendages with oblique markings. Eighth thoracic limb, carpus long armed with setae and with three or four strong barbed spines at the distal end; propodus divided into four subsegments armed with strong barbed spines; genital appendix in the male large, armed with ovate, long setae.
Pleopods in the male well developed, natatory, biramous (except the first pair in which the endopod is lacking), large oval pseudobranchial lamella borne at the base of endopod; third pair (not the fourth as described by Sars, 1877, P. 86) with exopod much longer than endopod and its distal segments and their setae modified; fourth pair with exopod longer than endopod but not modified. Pleopods in the female, first pair small, reduced to a simple stiliform segment. The remaining pairs are reduced to very delicate lamellae attached side by side to the posterior ventral margin of the somite and projecting backwards. The free distal margin is angular and bears a few plumose setae with two or three small spines.
Telson very long, nearly half as long as the whole abdomen, three times as long as broad at the base; lateral margins straight, armed with 25-30 closely set, plumose spines, terminal ones much larger than the rest, cleft small, about one-seventh of the length of the telson, armed with a close row of about 25 teeth on each side. Apex of each lobe armed with a strong spine.
Uropods with endopod longer than exopod, bowed inward at its distal end, inner margin armed with a graduated row of large plumose spines with small spines closely set between them. The two strong spines arming the distal end are very characteristic of the species, statocyst small; exopod with outer margin naked proximally, then armed with a close regular row of spines of which the distal ones are larger and may be slightly plumose.
Adult males up to 9 mm, females 7-8 mm long.
Hyperbenthic, coastal to upper slope. A. agilis is found as a rule at or near to the bottom during daylight but it is capable of very rapid movement from one layer of water to another and can also be taken at the surface and in varying depths during the day. It has not been taken in depths of more than 85 m, but has been collected at the surface and in mid-water over 150 m in Irish waters. It has been most frequently taken at the surface at midnight when it occurs in large numbers.
This species can easily be recognised by the robust compact form and large carapace with transverse posterior margin; the large serrated spine at the base of the antennal peduncle; the very small antennal scale; the swollen carpal segment in the second thoracic leg in the male; the peculiar appendages borne at the distal end of the endopod of the third thoracic limb in the male, the small epimeral plates of the first abdominal somite and the very unusual modified pleopods of the second to the fifth somites in the female; the armature and proportions of the uropods with the inward curve of the endopod and the large telson with its small cleft.
Distribution in the North Sea:
Eastern North Sea, maybe in the SW North Sea entering from the E English Channel.
E North Atlantic: <26 - 56°N; Mediterranean.