Defenses of asteroids include mucus, considered here, and SPINES & OSSICLES, PEDICELLARIAE, ESCAPE BY CRAWLING, RIGHTING RESPONSE, DISTASTEFUL CHEMICALS, AUTOTOMY, and CAMOUFLAGE, considered in other sections. 

One quickly learned lesson on a collecting trip is not to keep slime stars Pteraster tesselatus. in the same bucket as other invertebrates. They release copious amounts of mucus when physically disturbed as by rough handling, when subjected to temperature shock, or when contacted by asteroid-eating sea stars such as Solaster dawsoni.  Pteraster has broad, stubby rays that hold the mucus on the aboral surface. Contact-tests done at Friday Harbor Laboratories, Washington with 15 species of asteroids show that 3 species elicit mucus discharge in Pteraster: the sun star Solaster dawsoni, the sunflower star Pycnopodia helianthoides, and Solaster stimpsoni.  All three predatory species are repelled. The authors do 2 simple experiments to show the effectiveness of Pteraster’s mucus in warding off predatory sea stars.  They first take a butter clam Saxidomus giganteus, force its shells open slightly with a piece of wood, tie the shell valves together, and then insert into the mantle cavity a tube connected to a 50-ml syringe filled with Pteraster mucus.  The clam is now offered to Pycnopodia and is quickly engulfed and swallowed.  If the plunger on the syringe is now depressed,  the predator quickly regurgitates the mucus-filled clam.  Ordinary seawater in the syringe causes no response and the clam is eaten.  In another experiment, the syringe tube is fastened lightly to the arm of a live S. stimpsoni with the end of the tube pointing into the oral region.  The S. stimpsoni is then presented to S. dawsoni and, a few moments after its attacks, the syringe plunger is depressed.  The predator quickly pulls in its stomach and withdraws, a response that is not triggered in control experiments using seawater in the syringe. The authors consider that the mucus, possibly because of its content of saponins or saponin-like substances, provides 100% protection from predators.  Nance & Braithwaite 1979 J Exp Mar Biol Ecol 40: 259.

NOTE  S. dawsoni eats only sea stars, Pycnopodia has a wide-ranging diet including some sea stars, and S. stimpsoni eats mainly sea cucumbers

NOTE saponins or other chemicals in the mucus are the most likely explanation, but effects externally of the mucus could be a simple as smothering or perhaps "gumming up" a potential predator


To release or not to release? On contact
with the slime star, even this tiny Solaster
dawsoni would cause it to release mucus 0.8X

Pteraster produces its mucus in glands on the ventral sides of special supradorsal membranes.  The membranes are raised up from the aboral surface about 2-3mm and the mucus is secreted into this shallow cavity.  Pressure from expansion of the respiratory chambers (nidamental cavity) in the arms below forces the mucus to the outside via many small spiracular openings.  Within a few seconds Patiria can cover itself with a protective coat of mucus up to 7cm thick.  If the supradorsal membranes are removed, then no mucus is produced, and Pteraster easily falls prey to S. dawsoni and Pycnopodia.  Under such circumstances it takes about 4d for a large-sized S. dawsoni to digest completely a medium-sized Pteraster.  Nance & Braithwaite 1981 J Exp Mar Biol Ecol 50: 21.

NOTE  when Patiria is not in defensive mode, seawater is pumped through the nidamental cavities at a rate of 3-4 volumes ^.  min^-1 (at 10^oC) to enable gas exchange. Note in the drawings that, rather than projecting into the outside seawater as in other asteroids, the dermal branchiae in Pteraster project into the nidamental cavities. In both cases the dermal branchiae are expanded and contracted by hydraulic pressure changes in the coelomic cavity to which they are connected.  Seawater for gas exchange enters through openings termed ambulacral pores that line the ambulacral grooves of the arms, percolates through the nidamental cavities, and exits via the large aborally located osculum.  Prior to this publication these pores have not been described

NOTE  these cavities are are more commonly referred to as nidamental chambers or cavities.  However, as Pteraster does not use them primarily for incubation of young, the authors suggest using the descriptor respiratory chambers instead