Light modulation of postural flexor motoneurones in the tail-fan of the squat lobster Galathea strigosa (Crustacea, Anomura)

Photosensitivity in the terminal abdominal ganglion (G5) of an anomuran, the squat lobster Galathea strigosa (Crustacea, Decapoda, Anomura), is described. In contrast to the caudal photoreceptors (CPRs) of long-tailed natantid and macruran decapod crustaceans, the caudal photosensitive elements in G5 in G. strigosa apparently lack the conventional spiking rostral conduction pathways to the thoracic ganglia, and instead make their output connections to a bilateral pair of tonic flexor motoneurones originating within the caudal ganglion itself. These flexor motoneurones modulate the activity of two bilaterally paired uropod coxopodite tonic flexor muscles. This photomodulated motoneurone (PMMN) activity is not abolished by sectioning the abdominal nerve cord anterior to G5. The pattern of photosensitivity, while differing from that shown by other CPRs, resembles instead the pattern attributed to photosensitive interneurones (PSIs) of rostral abdominal ganglia of crayfish and other long-tailed decapod crustaceans. The caudal PSIs in G. strigosa appear to be involved in the postural control of the tail-fan as it is held flexed against the cephalothorax.