Platyxanthus orbignyi

Platyxanthus orbignyi is a decapod crab of the family Platyxanthidae ^[1]. In Peru it is called cangrejo violáceo, or purple crab ^[2],[3]. It has separate sexes; a Peruvian thesis, citing a 1967 author, notes a marked sexual dimorphism that makes males and females easy to tell apart, and in its own sampling sex was determined from the shape of the abdomen ^[3]. The same thesis reports that it ranges from Ecuador to San Antonio, in Chile, and has been found down to 50 m depth ^[3]. At Huanchaco it is taken by small-scale fishers with traps, and that 2016 thesis says its fishery was not yet regulated, citing a 1996 report ^[3].

A Peruvian thesis, citing earlier work, describes the species as characteristic of the eulittoral zone of the south-eastern Pacific, ranging from Ecuador to San Antonio (Chile) and present down to 50 m depth ^[3]. The same thesis cites a 1967 study that places it between 13.2 and 22 °C in temperature and between 34.4 and 34.7 in salinity ^[3].

On the beaches of the port of Ilo (Moquegua), the temperature recorded during sampling was 15.5 °C in September and November 2021 and 15.6 °C in March 2022 ^[4]. The authors state that purple crabs are benthic organisms that feed on bottom surfaces ^[4].

Reproduction was studied in 1477 specimens caught at Huanchaco (La Libertad) during 2015, of which 912 were females (61.75%) and 565 males, an overall ratio of about 2 females per male ^[3]. Size at first maturity was 37 mm in females and 39 mm in males, and mean size at maturity was 61 mm and 59 mm, respectively ^[3]. Sexual maturity stage V predominated in females in March (32.91%) and in males in October (20.22%) ^[3].

Egg diameter ranged from 466 µm at stage I to 575 µm at stage IV; atresia averaged 8.52% and mean fertility at stage IV was 105,462 eggs ^[3]. An earlier study cited in the thesis places the spawning season in spring ^[3].

As for growth, the study of the same 1477 crabs distinguished 7 moult stages in females and 9 in males, and concluded that growth in both sexes is of the retrogressive geometric type, fitted to the von Bertalanffy equation ^[5]. An earlier work that thesis cites mentions a moulting period between May and August in both sexes and an additional one for females in summer ^[5].

On its diet, a Peruvian thesis reports two different descriptions from earlier authors: one treats it as an omnivore, eating diatoms as a juvenile and barnacles and small mussels as an adult, and the other as a carnivore, with Semimytilus algosus as its main food ^[3].

A study from southern Peru, citing a 1997 work, presents the purple crab as a filter feeder when juvenile and an omnivorous scavenger as an adult, and as food for octopuses, otters, sea lions, seabirds and fish ^[4]. Its authors conclude that, at Ilo, it acted as a bioindicator of environmental quality ^[4].

The sources consulted contain no assessment of its conservation status. At the cove of Huanchaco a significant share of small individuals was observed, which is why the effect of fishing on its growth was of interest ^[5].

In addition, the 2016 thesis on its reproduction states that its fishery was not yet regulated, citing a 1996 report ^[3].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

At 3 beaches of the port of Ilo, the muscle of 30 commercial-size males (70 mm), 10 per beach, was analysed; they were caught by free diving in September and November 2021 and March 2022 ^[4]. Iron (3.47 mg per kg), copper (1.23) and zinc (1.18) were found, in that order of concentration ^[4]. In the sediments the order was the same, with 28.45 mg per kg of iron, 0.17 of copper and 0.11 of zinc ^[4]. The authors report higher concentrations in summer and on beaches closer to human activities such as mining, and in the abstract they name Fundición beach as the one with the highest concentration of the three metals in summer ^[4]. It is a pilot study of 3 metals, which its own authors acknowledge to be not very robust ^[4].

Its harvest is a source of income for small-scale fishing and it is well received in the national market ^[3]. At Huanchaco it is caught with traps called nasas, made of a metal ring and anchovy mesh, fished from caballitos de totora ^[3]. According to the Ministry of Production, cited in a 2016 thesis, demand for consumption had grown and fishing this species was seen as a high-potential alternative ^[5].

The literature places it at two specific localities. At Huanchaco, 13 km north-west of Trujillo, the 2015 material came from small-scale fishers using caballitos de totora and nasas ^[3],[5]. At Ilo, the crabs of 2021 and 2022 were collected by free diving on the beaches of Tres Hermanas, El Diablo and Fundición ^[4]. These are places of capture and sampling; the sources do not describe places to observe it ^[3],[5],[4].