star patterns (called lick marks) up to a meter in diameter, and depositing their wastes
at the bottom of the burrow. They apparently move to new sites fairly frequently,
since time-lapse photos will show lick marks for a while, then none. In some sites,
usually with fairly persistent currents, there are sessile megafauna, including sponges,
anemones, sea pens, crinoids (Fig. 13.5), and gorgonians or soft corals. Sponges
found in deeper levels mostly are stiffened by siliceous spicules. They vary in form
from flat encrustments to vase shapes. They feed by filtering flow generated by
flagellated cells in the walls of internal passages; particles in the flow stick to collar-
like structures on these same cells. Sea pens are cnidarians. Their bulbous base buries
in the sediment and supports a thick fan of polyps extending into the flow to feed on
impinging particles and plankton. Soft corals, that have branching organic skeletons
extending from a base fixed to rock, feed in the same way. There are also stony corals
in the deep sea with cup-shaped calcite bases several centimeters across, sitting partly
buried in the sediment. The sea squirts, phylum Urochordata, are familiar to tide-pool
visitors, and some deep-living forms are similar: stalked sacs engaged in mucoid
filtering. However, some deep-sea editions of the sea squirt have taken on a predatory
format (Havenhand et al. 2006) (Fig. 13.6). The incurrent siphon has been modified
into a trap, attracting prey from the water then snapping shut around them.
Fig. 13.6 A stalked, predatory sea squirt (Megalodicopia hians, Urochordata,
Tunicata) attached to the wall of Monterey Canyon.
(^) (Photo by Dave Wrobel, © 1995 Monterey Bay Aquarium Research Institute.)