Settlement of the barnacle Balanus trigonus Darwin, 1854, on Panulirus gracilis Streets, 1871, in western Mexico

Abstract A large number of specimens (2765) of the acorn barnacle Balanus trigonus Darwin, 1854, were observed on the spiny lobster Panulirus gracilis Streets, 1871, in western Mexico, including recently settled cypris (1019 individuals or 37%) and encrusted specimens (1746) of different sizes: <1.99 mm, 88%; 1.99 to 2.82 mm, 8%; >2.82 mm, 4%). Cypris settled predominantly on the carapace (67%), mostly on the gastric area (40%), on the left or right orbital areas (35%), on the head appendages, and on the pereiopods 103. Encrusting individuals were mostly small (84%); medium0sized specimens accounted for 11% and large for 5%. On the cephalothorax, most were observed in branchial (661) and orbital areas (240). Only 40-41 individuals were found on gastric and cardiac areas. Some individuals (246), mostly small (95%), were observed on the dorsal portion of somites. Of the encrusting individuals, 18% were dead with no clear pattern in localization, but less common (7%) on dorsal than on lateral portions (right, 18%; left, 32%). Larger specimens (up to 11 mm) were observed in other lobsters from different catches. Previous presence of Cirripedia in decapod crustaceans, size range in the material examined, settlement patterns, and growth of the barnacles are discussed.