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DescriptionAtelecyclus rotundatus has a reddish-brown carapace, light brown limbs, and black claws (chelae). This species has an almost circular carapace, up to 5 cm in diameter. The carapace bears short transverse, granular grooves on its upper surface and very hairy margins. There are 9-11 sharp teeth on the front and side margins of the carapace and 3 teeth between the eyes, central one being distinctly longer than the other two. Its chelipeds are robust and equal in size, with males having two large tubercules on the upper surface of the propodus. The legs (pereiopods) are slightly flattened with hairy margins. This species also has a long second antenna.Atelecyclus rotundatus bears eggs from February to September with planktonic larvae present from February to November. This species is eaten by Gadus morhua and Raja spp. Atelecyclus rotundatus may also be mistaken for the less common Atelecyclus undecimdentatus. But Atelecyclus undecimdendatus has a much broader carapace and coarser granulations than Atelecyclus rotundatus. The thumbnail crab, Thia scutellata is also similar, but lacks teeth on the front of its carapace.
Interestingly, Atelecyclus rotundatus buries itself by digging backwards into the substrata, and reverses its respiratory water flow through its branchial chambers to prevent suffocation (Tayor, 1984). In order to maintain sufficient respiratory currents, this species (akin to Carcinus maenas, Cancer pagurus, Crangon crangon, Bathynectes longipes, Corystes cassivelaunus for example) holds its chelipeds close to its body creating a respiratory channel with the larger second antenna. Taylor (1984) noted that when burried, its gill bailer (scaphognathite) beats in the opposite direction to normal, pumping water out at the base of the limbs instead of drawing water in.