Lecture-11 November 2003

Locomotion in Cephalopods--Jets and blimps

Some different groups of cephalopods:

Nautilus—coiled shell with chambers, siphuncle between chambers allows animal to regulate gas content of the chambers, and thus regulate overall bouyancy. Many arms and mantle cavity with siphon. Creatures regulates vertical position by regulating gas in shell, creature uses mantle contractions and siphon to change horizontal position. Shell can withstand about 50 atmospheres of pressure (roughly about 500 meters of depth).

Coeloid squid (common squid that one eats when having calamari)—Tubular mantle acts as muscular hydrostat. Have fins that can act as foil to produce lift and/or propulsion system to produce horizontal movement. Mantle wall because it is formed of muscular tissue is relatively dense. To prevent sinking creature must swim fairly continuously. When moving between locations, the animal goes fin first, when moving towards prey, the animal goes head first. Arms are held together to reduce turbulence. Siphon can be pointed in different directions to change direction of motion.

Mantle muscle must contract in a coordinated fashion. This is accomplished via impulses generated in the stellate ganglia (paired on either side of body). Radiating from stellate ganglia are nerves of varying diameters. Because these nerves are unmyelinated, the rate of conduction is related directly to cross-sectional area. Thus large diameter nerves carry impulses to the more distant regions of the mantle, smaller diameter nerves to closer regions. Contraction needs to move from fin towards head to avoid bursting the mantle wall.

Body is stiffened by chitinous pen. Fluid passes into the mantle chamber in the space between the mantle wall and the base of the head. When the mantle wall contracts, fluid could pass out through the same space, but doesn’t because flaps on the edge of the mantle as as one-way valves. Thus the only exit is through the siphon. The movement of fluid through and out of the siphon is governed by the Bernouli principle.

Squid locomotion is a costly process. The outcome of the choice of jet propulsion causes them to move more slowly than fish that use the lateral undulation of their body musculature and also use more energy to maintain that slower rate of movement. This is partly because jetting is an on and off phenomenon where as the lateral undulation of the body wall is more or less continuous. Thus squid are constantly accelerating/deaccelerating.

Size of fin and rate of movement are closely associated. Creatures with small fins move much faster than creatures with large fins, as those with large fins produce a horizontal movement by undulating their fins which is much slower than jetting.

Cuttlefish—Cuttlefish have bodies that are oval-shaped in cross-section and have a cuttlebone that has chambers with a siphuncle connecting them. They also have a lateral fin along the edge of their bodies. As noted above, cuttlefish move much slower than coelioid squid, as they use their undulating fins for much of their propulsive force (although they still contract their mantle and use their siphons as jet engines). Cuttlefish can regulate their density by adding or removing gas from their cuttle bone. Interesting research has been done on this aspect. By adding x-ray opaque material to the cuttlebone and subject the creature to different pressures, a researcher can actually the cuttlefish regulate its density by changing the amount of fluid/air in its cuttlebone.