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Tuesday, August 21, 2012

A veiled threat


On 20 August 2012, we were excited to find a couple of Veiled Chitons (Placiphorella velata) in the low intertidal zone on the Mendocino coast.  In the photo above, the chiton (~4 cm long) is behind a small green sea anemone.  In addition to grazing on algae, Veiled Chitons are very unusual among chitons in that they capture live, mobile prey!  

Look for the greatly expanded head flap, or veil, at the anterior end (in front of the head, left side of photo below).  The fleshy portion surrounding the hard plates is called a girdle (it's mottled with pink and white).  Note the setae (hairs) scattered across the girdle, with longer setae at the edge of the veil.


In a sit-and-wait posture, the Veiled Chiton raises its head flap high above the surface of the rock (see diagram below), waiting for unsuspecting prey such as amphipods (shrimp-like crustaceans) or worms.  Beneath the head flap, there are specialized tentacle-like projections that radiate outward in front of the head.

Figure modified from McLean, J.H.  1962.  Feeding behavior of the chiton Placiphorella.  Proceedings of the Malacological Society of London 35: 23-26.  


When the setae are triggered, the head flap clamps down on the prey in less than a second (somewhat reminiscent of a Venus Flytrap).  The food is then moved back towards the mouth where it is either swallowed whole (if small) or torn into pieces by the radula (if large).
 
The next photo was taken along the Big Sur coast in 2009.  The head flap is elevated revealing the pink, tentacle-like projections below.

 
Veiled Chitons live along the West Coast from Alaska to Baja California.  They are uncommon in the intertidal zone, and seem to prefer shallow depressions and crevices or the undersides of rocks.  They are often associated with coralline algae  the pink and purple crusts in the photos from Mendocino.

Some individuals may be very well camouflaged by the growth of other animals on their backs.  The next photo shows a Veiled Chiton discovered by Eric's sharp eyes (after we had developed a search image for the red girdle).  The upright, branched, golden-colored animals are hydroids growing on the chiton's shell plates.


Although some other chitons are known to graze on sessile animals (such as barnacles, bryozoans, and hydroids), it is rare for a chiton to trap mobile prey.  Interestingly, McLean (1962) suggests that this behavior has allowed some species of Placiphorella to live at great depths (over 500 fathoms or 3,000 feet) where algae is unable to grow.

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