In the shore crab, Carcinus maenas (L.), forward ventilation creates negative pulses of hydrostatic pressure while reversed ventilation causes dramatic positive pressure fluctuations in the branchial chamber. These pressures are transmitted via the gills to the haemolymph of the open circulatory system. The branchiostegal sinus, which is a compliant chamber, may function as a reservoir for displaced haemolymph and may operate as an accessory pump driven by the action of the dorsoventral (DV) muscles.
A band of dorsoventral muscles controls the volume of the branchiostegal sinuses. The muscular activity is coordinated with ventilatory activity and may assist in regulating pressure fluctuations caused by ventilatory pressure pulses. During a ventilatory reversal, the haemolymph displaced from the gills is added to the volume of haemolymph in the open circulatory system and this haemolymph may be accommodated in the branchiostegal sinus by relaxation of the DV muscles.
Artificially regulating the pressure either in the branchial chamber or in the branchiostegal sinus reflexively alters DV muscle activity, which suggests the occurrence of baroreceptors in this crab.
The branchiostegal nerve that innervates the DV muscles contains five neurones identified by cobalt backfills. Three of them are median and two are contralateral. The dendritic field of each neurone is confined to its respective hemiganglia.
The electrical activity of one of the motoneurones in the branchiostegal nerve corresponds to the activity of the DV muscles. In vitro observations of the activity of branchiostegal motoneurones in relation to ventilatory motoneurone activity indicate that both are centrally coupled and support the hypothesis that the branchiostegal motoneurones are influenced by the ventilatory central pattern generator.