Flat Rock Crab

Percnon planissimum or flat rock crab is a species of Percnon crab found in the Indo-Pacific from Somalia to Cocos Islands, the Philippines and Kermadec Islands.

Carapace thin, slightly longer than broad, covered with short bristles, some symmetrical raised patches naked. Front narrow, 4-toothed, the median pair deflected. Innerorbital angle with 3 teeth. Dorsal orbital border finely denticulated. Ventral orbital border finely serrated. Ventral innerorbital angle blunt, tooth-like. Epistome with 3 spines; the median one large, each lateral one small, about half the length of the middle tooth. Antero-lateral margin with 4 teeth including the outerorbital angle, regularly decreasing in size.
Chelipeds are of various types, depending on individuals and sexes. Merus slender, with a row of short spines, the ventral inner border with 2 subdistal sharp spines and a distal obtuse spine, the ventral outer border with 3 distal spines. Carpus small, with 2 rows of short spines. Palm high and compressed, about as long as broad, its surface smooth. Fingers shorter than palm, spoon-shaped at the tip. Ambulatory legs slender. Coxa of the last legs without spines. Merus of the first legs narrower, the remaining meri broader. All meri with a row of short teeth on their anterior border, the posterior border with a distal sharp tooth. Merus of the first 3 ambulatory legs with another row of short spines parallel with the anterior border, the last leg without this row of spines. Inner border of the dactylus, with 4-5 spinules, getting longer toward the end.
First pleopod of the male with its distal end hook-like, its base hairy. Abdomen triangular, the third to fifth segments fused; telson rounded distally. Abdomen of the female rounded. Carapace length of the male 24 mm, breadth 21 mm; of the female, length 26 mm, breadth 23 mm.

Indo-Pacific: Somalia to Cocos Islands, the Philippines and Kermadec Islands.