Predators, Interlopers and How Glass Sponges Protect their Turf
With the exception of two or three predators, only mobile species of crustaceans and fish have been found on living sponges. Many sponges produce chemicals which may be toxic, distasteful or act as a repellant. Ray Anderson, a biochemist at Univ. of British Columbia has been unable to find any such compounds in Cloud Sponges. Could there be some physical deterrent?
We employed members of three species to examine the surfaces associated with locomotion after contact with the outer sponge surface. Each specimen was placed in contact with the sponge for four to five minutes.
The spines are limited to the outer surface and are oriented vertically with the thorns pointed toward the sponge. The knob like structures are the spiny ends of 6 rayed pinnules. They appear to guard the pores entering the sponge. However, their role in this regard has not been established.
The orientation of the spinelets are reminiscent of those in a porcupine and presumably serve the same purpose. However, these spines do not provide absolute protection against soft bodied animals. We found a brittle star inside a sponge which earlier had been outside. It must have crawled up the outer surface to get in, where it resided for several weeks. Also, there are soft bodied predators on these sponges.
When all or a portion of a sponge dies all the loose spicules, including the spines, are washed out and only the fused main frame remains. These skeletons become available as substrate for a variety of organisms.
The Wrinkled Star Pteraster militaris
We have found only one species which is likely to be a predator. This is Wrinkled Star, Pteraster militaris. It is occasionally observed with the stomach everted over a living Cloud Sponge which is highly suggestive of feeding. It is also reported to feed on another species of sponge and on certain hydrocorals.
The white thin armed sea star Henricia sanguinolenta. Is occasionally seen on living Cloud Sponges but it is unclear whether or not it is feeding on sponge tissue.
Desmacella austini in a Cloud Sponge
Several kinds of sponges take up residence within the skeletal framework of dead Cloud Sponges. At least one of these, Desmacella austini, extends in dead skeleton to the edge of living tissue.
It is possible that it kills the sponge, not for food but for habitat.
Interlopers competing for space
Sea Urchin on sponge
Sea urchins or urchins are small, spiny, globular animals that use a combination of spines and tube feet to move over the sponge. Sea urchins' tube feet are moved by a water vascular system, which works through hydraulic pressure, allowing the sea urchin to pump water into and out of the tube feet, enabling it to move. This likely helps protect them to a degree from the spiny spicules of glass sponges.
Spines were not seen on the feet of a spider crab and presumably cannot penetrate the hard outer skeleton. The same would be true for shrimp and squat lobsters. No spines were found on any of these animals when they were placed on the inner surface of a sponge.