Mottled Shore Crab, Pachygrapsus transversus

Mottled Shore Crab, Pachygrapsus transversus. Crab photographed underwater in coastal waters off Cozumel Island, Quintana Roo, March 2021. Photograph courtesy of Marina Sutormina, Stockholm, Sweden. Identification courtesy of Ondřej Radosta, Prague, Czech Republic.

General information: The Mottled Shore Crab, Pachygrapsus transversus, is a member of the Grapsidae Family of marsh crabs, shore crabs, or talon crabs. Their lifespan is unknown, but is strongly affected by food availability and environmental factors. They are known to be preyed upon by the Caribbean Reef Octopus, Octopus briareus and the Snapping Shrimp, Synalpheus longicarpus. They have the ability to swim, utilizing drag power swimming, which is similar to oars rowing a boat. Reproduction occurs on a seasonal basis and is internal with fertilization occurring during copulation. The males transfer sperm filled cases (spermatophores) to the females, fertilizing the eggs, which are then brooded within the female’s body.

Identification: The Mottled Shore Crabs are small in stature and have squarish carapaces that converge toward the posterior end to form a trapezoidal shape and they have a limited number of spines; the area between the eyes is slightly curved and has no teeth. Their shells reach a maximum of 1.7 cm (0.7 inches) in width. The surface of the upper surface of the mobile finger is smooth and the hind margins of the largest leg joints have teeth. They vary in color from transparent to dark green to black, their claws are cream, pink or brownish, and their shell surface is covered with oblique lines of fine tubercles or bumps. The sexes are difficult to separate visually, with the males being larger than the females and the females having larger abdomens than the males. The Mottled Shore Crab is very similar and easily confused with the Spray Crab, Plagusia depressa (larger in stature, red, green or orange in color, nearly circular shell with a toothed front margin and teeth on the front side of the walking legs).

Habitat and Range: The Mottled Shore Crab is a resident of all Mexican waters of the Atlantic Ocean including the Gulf of Mexico and the east coast of the Yucatán Peninsula in the Caribbean. The Mottled Shore Crab is abundant, being found in a wide variety of habitats but generally among rock jetties, on buoys, on pilings near inlets and sea walls.

 Diet: The Mottled Shore Crab is an omnivore that consumes both plant and animals feeding off small invertebrates. They are known to consume the Porcelain Crab, Pachycheles susanae.

Conservation Status: The Mottled Shore Crab has not been formally evaluated from a conservation status.  However, they are common and of limited interest to most and should be considered to be of Least Concern.

Synonyms: Goniograpsus innotatus, Grapsus declivifrons, Grapsus transverse, Leptograpsus rugulosus, Metopograpsus dubius, Metopograpsus miniatus, Pachygrapsus advena, and Pachygrapsus intermedius.