GRAVITY AND LIGHT INFLUENCE THE COUNTERSHADING REFLEXES OF THE CUTTLEFISH SEPIA-OFFICINALIS

Rotation (roll or pitch) of a cuttlefish away from its normal orientat ion produces countershading reflexes (CSRs) that consist of chromatoph ore expansion on the ventral body surface. When rotation is in the rol l plane, the CSR has two components on each side of the body. The firs t (component A) consists of a unilateral expansion of chromatophores o n the uppermost latero-ventral edge of the mantle, the underside of th e upper fin and the uppermost side of the head; it occurs when the ang le of rotation is less than 90 degrees. Further rotation (from approxi mately 90 degrees to approximately 180 degrees) adds the second compon ent (component B): a unilateral expansion of the chromatophores on the upper half of the ventral surface of the mantle, funnel, head and arm s. When rotation is in the pitch plane, chromatophores expand on the p osterior part of the ventral mantle and fins when the head is down; wh en the head is up, chromatophores expand on the ventral surface of the arms, head and funnel and on the anterior part of the ventral mantle and fins. The pitch CSR is always bilateral. Destruction of the gravit y or the angular acceleration receptor systems of the statocysts demon strates that it is the gravity receptor systems that drive the CSRs. U nilateral destruction of the gravity receptor systems shows that the p itch CSR is driven bilaterally, whereas the roll CSR is driven unilate rally. Components A and B of the roll CSR are driven by input from the ipsilateral statocyst, but component A is additionally driven by ligh t. Brain lesions provide evidence that the pathways for the CSRs run t hrough the lateral basal lobes in the supraoesophageal part of the bra in.