Phronima atlantica Guerin-Meneville 1836

Phronima atlantica Guérin-Méneville, 1836 Diagnosis Subadult male. Cephalon shorter than the first five segments of pereon. Antenna 1 (Fig. 4 A) not fully developed; peduncle composed of 3 segments, small and conical; flagellum 4 - segmented, first segment longer than ½ the cephalon anterior margin, with short row of setae on anterodistal margin. Antenna 2 (Fig. 4 A) shorter than antenna 1, peduncle composed of 4 segments, filiform flagellum 5 - segmented. Pereopods 1-4 and 6-7 (Fig. 2) slender, without any subchelate structures. Pereopod 3 shorter than pereopod 5. Pereopod 5 (Fig. 4 C) base straight, with genus-typical spike on posterodistal margin; combined ischium and merus smaller than half the length of basis; posterior margin of ischium and merus slightly convex; carpus enlarged, ratio of length / width 1.5; anterodistal margin of carpus possesses a carpal process which extends beyond the ventral margin of the propodus; carpal tubercle present in the middle of distal margin of carpus, shorter than carpal process, composed of a separate single tooth, followed by a second tooth, finely denticulate on its posterior margin, which also possesses 6 setae; propodus sickle-shaped, 0.7 times longer than distal margin of carpus. Seventh pereon segment longer than first pleon segment. Pleonite 1 ventral margin not strongly indented. Pleonite 1-3 postero-distal corner quadrate, lacking spinose process. Pleopod 3 exopodite 7 - articled and endopodite 6 - articled. Uropods (Fig. 4 E) 1-3 slender and short, with uropod 2 being the shortest; rami of uropod 1 equal in length; inner ramus of uropod 2 is less than half the length of outer ramus; rami of uropod 3 equal in length. Remarks on subadult male compared to adult male Differences between the two life stages are, most noticeably, in the antenna 1 (Fig. 4 A and B). The mature male possesses a well-developed antenna 1, with a 7 - segmented flagellum compared to the 4 - segmented flagellum of the subadult; setation of the first flagellum segment in mature male is lined with long setae on the dorsal and ventral margin, while the subadult only possesses a tuft of stout setae on anterodistal margin. Antenna 2 (Fig. 4 A and B) is also different due to the number of segments of flagellum: in mature male, flagellum is 12 - segmented, in subadult, the flagellum is 5 - segmented. In subadult, male segments of pereopod 5 (Fig. 4 C and D) are less bulbous and longer than wide. Uropod 2 inner ramus (Fig. 4 E and F) in mature male is longer than half the length of outer ramus, while in subadult inner ramus is less than half length of outer ramus. This diagnostic character (Uropod 2 inner ramus) has already been noted to be linked with growth in females of P. solitaria (Shih 1969) and now we report a similar finding for males of P. atlantica. In other congeneric species, P. curvipes, P. dunbari, P. sedentaria and P. stebbingi mature males maintain the uropod 2 outer ramus longer than twice the length of the inner ramus (Table 2). Other remarks Species P. sedentaria and P. solitaria, regarded as morphologically similar to P. atlantica (Vosseler 1901, Shih 1969), require particular attention to avoid misidentification, considering how growth impacts one of the diagnostic characters used to distinguish the three species (Fig. 5). The second antenna in P. sedentaria is particularly reduced, the shape of the pereopod 5 carpus is noticeably different amongst the species, the uropod 2 inner ramus length shows a different proportion in relation to the outer ramus amongst the species and the prominent spinose process on pleonites 1-3 postero-distal corner is absent in P. atlantica. The main diagnostic characters are shown in Fig. 4 and Fig. 5. Identification of P. atlantica subadult male relied on the shape of pereopod 5 (Fig. 4 C) (Shih 1969). Comparison with our specimen of P. atlantica mature male and description of males of the species (Shih 1969, Vinogradov et al. 1982, Shih 1991, Zeidler 2004) resolved which of the taxonomically relevant characters of the key to the species (Shih 1991) could reliably be used to identify subadult males of P. atlantica. It showed that the uropod 2 outer ramus was longer than twice the length of the inner ramus only in the subadult.