Ucides occidentalis

Ucides occidentalis is a mangrove crab of the family Ocypodidae ^[1]. It occurs in mangroves along the American Pacific shore, from Espíritu Santo Island in Mexico to the San Pedro estuary in Peru ^[2]. In Peru it is called cangrejo del manglar and has commercial importance in the Tumbes region ^[3]. It is the most heavily harvested crab of the Peruvian mangrove, and a study cited in 2020 noted a population decline of 35.8 % in 11 years ^[4].

The carapace is oval, with strongly projecting gill regions; the distance between the outer edges of the orbits is less than two thirds of the carapace width and the lower margin of the front has a median notch ^[3]. It can exceed 7 cm of carapace width and is greyish red or bluish grey ^[3].

There is sexual dimorphism: males are larger and heavier; females have a broad, oval pleon with 4 pairs of fine-haired pleopods, and males a nearly triangular pleon with a single pair of hard pleopods ^[2]. In adult males the merus of the cheliped is so long that the animal has to walk sideways. There are 2 morphotypes, with equal or unequal claws, each with 2 varieties ^[2].

Similar species

• Cardisoma crassum: Cardisoma crassum has straight or slightly arched, toothless lateral carapace margins, a narrow front, a third pair of maxillipeds that almost closes the mouth and a distance between orbits greater than half the carapace width; its carapace is blue with greyish tones ^[3].

Its range runs from Espíritu Santo Island in Mexico to the San Pedro estuary in Peru ^[2]. Until 2016 the known southern limit in the country was the Tumbes River; on 6 August 2016, 3 specimens were collected in the San Pedro de Vice mangroves, Sechura province, Piura, extending the range southward ^[3].

In Peru the mangrove is almost entirely in the Tumbes region and the crab population is found mainly there ^[4]. According to the literature that Alemán and Ordinola review, it lives in muddy mangrove areas near river mouths and often repairs its burrows; for Ecuador they cite burrows up to 2 m deep ^[3]. At San Pedro, by contrast, the burrows were 33 to 53 cm deep, at densities of 1 to 2 individuals per m2, in sandy-silty and clayey, relatively hard substrates, associated with white mangrove, black mangrove and the shrub Batis maritima ^[3].

It is a herbivore that specialises in mangrove leaves ^[2]. It breeds as a seasonal iteroparous species with a long reproductive period: in Ecuador spawning takes place between December and May, and in Peru it has been observed between December and April ^[2].

In the Gulf of Guayaquil, Ecuador, morphometric maturity is reached at 63.5 mm carapace width in males and 57.8 mm in females; reported mean fecundity is 195697 eggs per female, ranging from 12847 to 385792 ^[2]. In Ecuador moulting has been reported between July and September, though it probably occurs all year, and individual growth has no asymptotic limit ^[2].

In mangroves dominated by red mangrove it is a keystone species that recycles between 75 and 84 % of the ecosystem's organic matter ^[4]. It retains nutrients in the sediment and aerates the soil as it digs its burrow ^[2].

Each burrow is individual; if another crab invades it, the occupant may withdraw or display its merus sideways, and the latter usually ends in a fight; females tend not to take part in these ritualised fights, mainly in the breeding season ^[2]. In Peru densities of 4 to 9.6 crabs per m2 were reported, with more males than females (1.5 to 2 males per female) and mean sizes of 66.6 mm in females and 74.3 mm in males ^[2].

In the Tumbes mangrove, 42 sequences of the COI gene showed 30 haplotypes and only 4 % genetic variation between populations, unrelated to geographic distance ^[4].

In Peru it is the most heavily harvested crab of the mangrove, and a survey by the Instituto del Mar del Perú, cited in 2020, recorded a population reduction of 35.8 % in 11 years ^[4]. A fall in its landings in the country between 1980 and 1994 has also been described ^[2].

The authors of the genetic study suggest that rearing larvae to restock the mangrove may become necessary, and that its genetic diversity should be known first so as not to induce an artificial population structure ^[4]. In Ecuador the fishery is regulated with a minimum size of 75 mm carapace width, a ban on taking females and an annual closure in March ^[2].

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

A moulting crab is not fit to eat, since it can cause intestinal damage and allergic reactions; for this reason fishing is banned in Ecuador between 15 August and 15 September ^[2]. In Ecuador heavy metals (cadmium, lead, chromium, mercury and arsenic) have also been reported in its exoskeleton, perigastric organ and muscle ^[2].

In crabs from the sales points of Tumbes, 6 types of microplastics were recorded in the gills and in the digestive tract; fibres were the most abundant, at 59.64 to 61.05 % depending on the tissue ^[5]. The author regards the finding as a warning signal about the safety of what is sold in the city's markets ^[5].

In the Tumbes region it has commercial importance ^[3], and crabs from its sales points were the source of samples for a microplastics study ^[5]. In Ecuador it is the most important artisanally harvested crustacean, with 3383 registered crab catchers in 2014 ^[2].

In Ecuador it is caught by hand, using a U-shaped metal rod to bring the crab to the burrow entrance, and is sold in bundles of 12 individuals ^[2].

It is found in the mangroves of the Tumbes region, where a genetic study took specimens at 9 stations, among them Zarumilla, El Jelí and Corrales, in a study published in 2020 ^[4]. To the south, on 6 August 2016, 3 specimens were collected in the San Pedro de Vice mangrove, in Piura ^[3]. There its burrows were 33 to 53 cm deep ^[3].