Shell implosion depth and implosion morphologies in three species of Sepia (Cephalopoda) from the Mediterranean Sea

Abstract

The maximum habitation depths of chambered cephalopods are dictated by the mechanical properties of the shell. All chambered cephalopods have a depth at which ambient pressure is sufficient to implode the gas-filled shell portions. Experiments on Sepia elegans D'Orbigny, S. officinalis Linne and S. orbignyana Férussac from the Mediterranean Sea show these three species to have differing depth limits and modes of shell implosion. Large S. officinalis implode between 150 and 200 m, whereas newly hatched and advanced embryonic specimens implode between 50 and 100 m. The larger S. officinalis are occasionally caught at depths greater than the implosion depth of the juvenile shell parts. They apparently avoid implosion of the early shell portions by refilling these first-formed chambers with cameral liquid later in life. Implosion in S. officinalis generally resulted in the crushing of all or most of the septa in a band extending from the embryonic region to the anterior part of the shell. Implosion was generally accompanied by fatal rupture of underlying tissue into the implosion zones. Implosion of S. orbignyana occurred between 550 and 600 m and was very different in form from that in S. officinalis, occurring mostly within the smooth zone of the last-formed several chambers, and rarely extending back into the siphuncular region (striae zone). Increasing depth caused episodic implosion of sequentially older chambers in the smooth zone. The shallower implosions were accompanied by little soft-tissue damage. Massive internal injury only occurred in the deepest implosions (700 m or greater). Implosion in S. elegans occurred betw en 400 and 600 m. Too few specimens were available to allow generalizations about morphology of implosion in this species.