Petrolisthes violaceus

Petrolisthes violaceus is a porcelain crab of the family Porcellanidae ^[1]. It is an intertidal crustacean of the south-eastern Pacific: a 1997 paper gives its range from Bahía de Ancón, in Peru, to Taitao, in Aisén (Chile) ^[2]. It lives under rocks in the lower-middle part of the intertidal zone and in the shallow subtidal ^[3]. Its larval development has two zoeal stages and a megalopa ^[4]. Ovigerous females clean their brood with the fifth pair of legs ^[5].

Pardo and colleagues, relying on a 1981 review, bound its range from Bahía de Ancón, in Peru, to Taitao, in Aisén, southern Chile ^[2]. Another paper describes it as a species of Chilean rocky shores that extends from northern Peru to Aisén ^[3]. The two texts do not agree on the exact northern limit, so in Peru that limit should be read as approximate. A population genomic analysis between 20.4° S and 41.8° S distinguished two genetic groups along that coast ^[6].

Its habitat is loose rock on the intertidal shore, under which it shelters, in the lower-middle zone and down to the shallow subtidal ^[3]. On a coast in southern Chile Petrolisthes violaceus peaked in numbers lower on the shore, while its congener Petrolisthes laevigatus peaked higher up ^[7].

Its larval development was described from larvae reared in the laboratory: the sequence runs prezoea, zoea I, zoea II, megalopa. At 19 °C and a salinity of 34, the megalopa was reached after 25.2 days on average ^[4].

The first zoea depends strongly on food. Larvae fed daily raised their dry weight by 94.7 %, whereas those kept unfed lost up to 18.9 %. With 1 to 3 days of food at the start they did not complete the moult, and those that did moult had grown by more than 55.4 % over their initial weight ^[2].

Ovigerous females actively tend their brood. The fifth pair of legs is small, bears a pincer and specialised bristles, and the female uses it to clean the embryos, mostly by day. Females whose fifth legs were experimentally removed had higher embryo mortality during incubation ^[5].

Its position on the shore is linked to its physiology. Compared with Petrolisthes laevigatus, which occupies higher levels, Petrolisthes violaceus withstood drying in air and a lack of oxygen during long exposure better, but tolerated high temperatures less well ^[7]. The two species have a similar gill surface; Petrolisthes laevigatus, however, has a larger area of respiratory membranes on its legs, which, according to the authors, could give it an advantage in withstanding emersion ^[3].

The optimal performance temperature of seven populations, spread over about 3000 km of coast, declined toward higher latitudes, and high-latitude populations tolerated a wider thermal range. The authors conclude that low-latitude populations still have a margin against the warming projected for the end of this century ^[8].

The literature describes this species on rocky shores, under stones in the lower-middle intertidal zone and in the shallow subtidal, and places the northern end of its range in Peru, at Bahía de Ancón according to a 1981 review reported by a 1997 paper, or in northern Peru according to another. The texts consulted name no specific Peruvian observation sites and mention no access restrictions ^[2],[3].