Blood circulation in the balanomorph barnacle, Megabalanus californicus (Pilsbry)

Compared to the lepadomorph barnacles, the morphology of hemolymph circulation in the balanomorphs is poorly known. This report on Megabalanus californicus is the first detailed description of blood circulation in a balanomorph barnacle. In Megabalanus hemolymph circulates in a similar pattern to that seen in lepadomorphs. However, Megabalanus shows major differences. (1) The mantle and associated structures ( =peduncle in the lepadomorphs) shows no large sinuses and blood is directed to branchiae via well defined vessels. (2) The gut is entirely invested with vessels from the distributive system (except for the proctodaeum). (3) A large sinus in the thoracic region (thoracic sinus) has no similar structure in the lepadomorphs. (4) The prosomal sinus is reduced and positioned dorsally. (5) The rostral sinus is larger than any found in the lepadomorphs and receives a substantial amount of blood from the peripheral circulation. The smallness of the prosomal sinus in Megabalanus probably is due to lack of a contractile peduncle. In the lepadomorphs the prosomal sinus likely serves as a storage sinus for hemolymph when the peduncle is contracted.