Pilumnoides perlatus

Pilumnoides perlatus is a small crab: the specimens gathered in Peru that Rathbun described in 1910 measured 9 to 20 mm in carapace width ^[1]. It lives in coastal waters of Peru and Chile, from the intertidal zone down to about 54 m deep, and reaches the Strait of Magellan ^[2].

It is the type species of the genus Pilumnoides ^[2], and the key to the decapod crustaceans of Peru places it in the family Pilumnoididae ^[3]. Its larvae have been reared in the laboratory since 1973, from specimens collected in southern Chile ^[4],[2].

Poeppig described it in 1836 as Hepatus perlatus, and the combination Pilumnoides perlatus, credited to Milne Edwards and Lucas, already appears in a work of 1843 that Rathbun cites in his synonymy; the same author treats Kinahan's Pilumnoides danai as a synonym ^[1].

The World Register of Marine Species currently keeps Pilumnoides perlatus (Poeppig, 1836) as the accepted name, within the family Pilumnoididae ^[5].

In Rathbun's description the carapace is convex, suborbicular and broader than long; the front two thirds bear tubercles and the rear third is nearly smooth. The anterolateral margin has 5 or 6 irregular teeth, and the bilobed front projects most at the middle ^[1].

The claws are equal, stout and tuberculate, with the tubercles in rows on the lower half of the palm; one of the rows ends in a large tooth on the outer face of the fixed finger. The fingers are brown with white tips, and the slender legs are furry on their terminal half, with dactyls ending in long, curved, horny tips ^[1].

In the identification key to the decapods of Peru, the family is recognised by a carapace that is not much wider than long, is subcircular, and by very rough claws ^[3].

Similar species

• Pilumnoides rotundus: Pilumnoides rotundus Garth, 1940 lacks the oblique or transverse ridge on the lateral angles, its carapace is granulate both in front and behind, and the merus of the third maxilliped is prominent at the outer distal angle; Pilumnoides perlatus has a nearly transverse branchial ridge and short granulate transverse ridges on the front part of the carapace ^[3].

Rathbun summarised its range as running from Panama to Chile ^[1]. A later review describes it in coastal waters of Peru and Chile, from the intertidal down to 54 m, with records reaching the Strait of Magellan ^[2].

In central Chile it was found in patches of the brown alga Glossophora kunthii, at Punta de Tralca (33º35' S), where it was significantly less abundant than the crab Taliepus dentatus; the surveys were made at depths of 7 to 12 m below mean low water ^[6].

Its larval development was described in the laboratory from specimens from southern Chile ^[4]; according to the later comparison by Luppi and colleagues, before becoming a juvenile crab it passes through 5 zoeal stages and one megalopa ^[2].

From measurements that Luppi and colleagues took on the drawings of that study, the first zoea has a carapace 520 micrometres long, and the megalopa measures 1700 micrometres in length and 1350 in width ^[2].

Compared with the Atlantic species Pilumnoides hassleri, the zoeae of Pilumnoides perlatus have finely denticulate lateral spines, dorsolateral knobs on abdominal somites 2 and 3 (the other species only on 2) and 6 pairs of setae on the posterior margin of the telson (the other, 5); in the megalopa, the antennular endopod has 2 segments, whereas in the other species it is unsegmented ^[2].

In a field study from May 1999 to November 2000 at Punta de Tralca, in central Chile, the species was one of the 3 brachyuran crab species found in patches of Glossophora kunthii, along with Cancer setosus and Taliepus dentatus, and it was significantly less abundant than the latter ^[6].

In the adjacent cobble patches, 15 decapod species were counted (5 anomurans, 8 brachyurans and 2 carideans), and the list of those species includes Pilumnoides perlatus ^[6].

The oldest Peruvian data this text cites come from the collections Rathbun described in 1910, made during the investigations of Peru's fishery resources from 1906 to 1908: the species turned up in April 1907 in a trawl in Sechura Bay, west of Matacaballa, at about 5 fathoms (around 9 m) deep, and also in a dredge haul near San Lorenzo Island ^[1]. These are trawl and dredge collection records, not diving sightings ^[1].