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

Pedicellariae occur on the skin surfaces and are found mainly in a single Order of sea stars, the Forcipulatida.  Examples of west coast forcipulatids are

Evasterias troschelii
Leptasterias hexactis
Leptasterias aequalis
Orthasterias koehleri
Pisaster ochraceus
Pisaster brevispinus
Pycnopodia helianthoides
Stylasterias forreri

The small jaws are used in defense and, one species, in food acquisition (see Research Study 3 below).  Shown here is a so-called straight pedicellaria on the oral surface of an ochre star Pisaster ochraceus. By the appearance of the tube feet around the mouth, this individual may have just been pulled roughly off a prey, and the pedicellariae are responding to the disturbance. Most of the pedicellariae visible are open and ready to bite. Like other pedicellariae in asteroids, these straight ones are non-toxic, and rely on snapping and crushing to ward off predators, and to kill small settling organisms. Drawing adapted from Verrill 1909 Am Nat 43: 542.

NOTE lit. “small foot units” L.  More information on pedicellariae can be found elsewhere in the ODYSSEY: LEARN ABOUT SEA URCHINS: DEFENSE: PEDICELLARIAE

The sun star Solaster dawsoni is a major predator of other sea stars.  Studies aat Bodega Marine Laboratory, California show that most intended asteroid prey run from contact with Solaster, including small specimens of ochre stars Pisaster ochraceus and even large specimens of sunflower stars Pycnopodia helianthoides, as shown in the photo on the Left.  Larger-sized P. ochraceus and all P. brevispinus apparently will stand their ground and use their pedicellariae against Solaster whenever contact occurs. Van Veldhuizen & Oakes 1981 Oecologia 48: 214.

There must be more to it than this, however, as Pycnopodia has abundant pedicellariae of its own, of 2 distinct types, but still runs from Solaster (see photographs on Right). Do the pedicellariae of Pycnopodia function differently than those of Pisaster spp.? or do different types of asteroid pedicellariae have different effects on Solaster? Further research is needed on this topic.

Sunflower star Pycnopodia helianthoides avoiding
contact with the nematocysts of a sea anemone.
The inset shows skin detail ofPycnopodia's arm:
many soft brown gas-exhanging organs or dermal
papulae are interspersed with spines bearing extended
rosettes ofsmall pedicellariae, all open, and a few
larger vice-like pedicellariae, mostly open 5X

Pedicellariae in sea stars either operate like a pair of scissors or a pair of forceps.  An example of the former are the so-called crossed pedicellariae of the sea star Stylasterias forreri which are  structured in such a way as to be able to catch small fishes. The primary diet of Stylasterias seems to be snails, chitons, and other invertebrates, so perhaps the catching of fishes, although well documented, may be incidental to a primary defensive function of the pedicellariae. 

The pedicellariae are positioned in groups of 40 or so around spines on the upper arm surfaces.  At rest, the pedicellariae lie in rosettes at the bases of the spines.  On stimulation, such as the presence of a fish coming to rest on the spines, the rosettes are elevated and the pedicellariae opened (see drawings upper Right). 

There are 4 main muscle groups that operate the pedicellaria (see drawing lower Left): 2 sets of abductors whose contractions open the jaws, 1 set of adductors which contract to close the jaws, and 1 set of stringy adductors that have multiple functions of closing the jaws and positioning the pedicellaria at different angles.  Note also that pull on these stringy muscles, such as by a prey struggling in the jaws, will lock the jaws even more firmly. The means of elevating the pedicellariae is not well understood, but may be by contraction of longitudinal muscles in the skin covering the spine. Chia & Amerongen 1975 Can J Zool 53: 748. Photograph of Stylasterias forreri courtesy Chris Gunn, North Island Explorer, Campbell River, British Columbia northislandexplorer.com.

NOTE  with over 200 clusters per arm, there are more than 40,000 pedicellariae on the sea-star’s surface, certainly enought to hold a small fish.  The pedicellariae bite at and hold the fins. The fish is slowly moved around to the mouth by the tube feet and twisting of the arms, and ingested.  More on this topic can be found at FEEDING, GROWTH, & REGENERATION: PREY RESOURCES: STYLASTERIAS