Predators

A.  Predators on mussels
	1.  Green Shore Crab--uses claws (chelae) to crush mussels and to 
		extract meat from shell pieces.
	2.  Starfish--uses arms to open shell, insert stomach and
		digest meat of the mussel.
		a.  In the West Coast intertidal environment the starfish
			is called a "keystone" species.  The name is given
			because ecologists feel that the species "controls"
			the arrangement and number of other species in the
			ecosystem.  Robert Payne of the University of
			Washington has written a number of articles on this
			matter
		b.  Although the mussel attempts to avoid having its shells
			pulled apart by the starfish through using of non-
			fatiquing muscle fibers called catch fibers, the pull
			exerted by the starfish is too constant and too
			strong.
	3.  The Dog Whelk (Thais lapillus)
		a.  Obtains access to the meat of the mussel by drilling
			a whole in the shell.
		b.  Shell of any mollusk is composed of three layers, an
			outer periostracum (composed of a material called 
			conchiolin, a middle prismatics and an inner
			nacreous (mother of pearl).  The material that makes
			up the largest layer, the prismatic, consists of 
			fibers of protein and a mixture of CaC03 and CaPO4.
			The crystals of calcite and aragonite are held 
			together by the protein fibers.
		c.  Snail chooses its driling site carefully
			1.  Only live mussels
			2.  Prefers one size of mussel
			3.  Drills typically in the area of the umbo, the
				area where the shell was first formed.
		d.  Snail actually uses the product of a gland that is 
			associated with the radula.  The gland secretes
			an enzyme that digests the protein fibers.  This
			then allows the radula to scrape out the crystals
			of calcite.
		e.  Because snail stays on mussel once it has begun to drill
			researchers have been able to do lots of research as
			to how the process works.  Much of the work was done
			by Mel Carriker who worked at the MBL in Woods Hole.
B.  Squid as predators
	1.  Structures
		a.  arms-eight in number, derived from foot which has moved
			forward and surrounds the head area. Arms were once
			greater in number.  Perhaps developed to "hold" the
			squid to the substrate.  Fossil arm prints in New
			Mexico found by Russell Flowers show concentric arm
			prints with radula impression at the center.
		b.  tentacles--two in number.  In the very effective predators,
			the tentacles have hooks with suckers (eg., 
			oegopsids).  The tentacles also have an arangement
			that holds them together as they are shot towards
			the prey species.  
		c.  eyes--eyes in predaceous squid are large and well-
			developed.  They have a lens and a retina.  The visual
			cells have fibers that leave from the back side of
			the retina rather than the fronts such as in the
			vertebrate eyes.  They also have ganglia (collections
			of nerve cell bodies) that are not part of the main
			brain.  Thus processing is decentralized rather than
			centralized.
		d.  bucal bulb--an apparatus at the base of the arms that
			contains the parrot-like beaks and a radula.  The
			radula is not really functional, but the beaks are.
	2.  Mode of predation
		a.  Squid swim rapidly fin-end forward to find fish schools.
		b.  When reaching school, they change direction and swim
			arms first with all arms forming a cone to maximize
			aerodynamic shape.
		c.  When they get close enough to prey species (fish), they
			shoot tentacles out.  Tentacles travel as single
			object but separate just before hitting fish.  
			Tentacles attach to either side of fish, pull fish
			towards bucal bulb
		d.  Fish is turned so that region just behind braincase is
			bitten by the beaks.  This bite often cuts through
			spinal cord and thus paralyzes fish.  Squid then takes
			bite-sized chunks out of fish.  No chewing involved.
			Swallows chunks and stores in caecal portion of 
			digestive tract.  Actual stomach is very effective
			in enzymatically breaking down the fish flesh.