A pedicellaria (plural: pedicellariae) is a small wrench- or claw-shaped appendage with movable jaws, called valves, commonly found on

echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

s (phylum Echinodermata), particularly in sea stars (class

Asteroidea Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...

) and sea urchins (class Echinoidea). Each pedicellaria is an effector organ with its own set of muscles,

neuropils Neuropil (or "neuropile") is any area in the nervous system composed of mostly unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies. The most prevalent anat ...

, and sensory receptors and is therefore capable of

reflex response In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

s to the environment. Pedicellariae are poorly understood but in some taxa, they are thought to keep the body surface clear of algae, encrusting

organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...

s, and other debris in conjunction with the

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

ted epidermis present in all echinoderms.

In sea stars

Asterias forbesi ring of pedicellaria around spine

Types

There are two major types of pedicellaria in sea stars: straight and crossed. Straight pedicellaria are typically larger and located on the body surface, whereas crossed pedicellaria are smaller and found more commonly on stalks, raised above the body surface or in clumps circling the spines. The crossed type is connected to the test by an elastic ligament.

Location

Sea star pedicellariae may be located on the

test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...

's surface or mounted on flexible stalks. Depending on the species, pedicellariae may be surrounding the spines, on the surface of the animal's body, in pits on the abactinal, marginal, or actinal surface, and/or within the ad ambulacral plate adjacent to the tube foot furrow. ''Forcipulate'' sea star are so called because each pedicellaria is typically composed of three forceps-like valves. Other asteroids can have pedicellariae composed of only two components.

Function

Pedicellariae in some taxa, such as the deep-sea Brisingida, and the Antarctic ''Labidiaster'' are known to function in food capture.

In sea urchins

Types

Four main forms of pedicellariae are found in sea urchins : tridactylous, ophicephalous, triphyllous and globiferous. There are typically three valves that make up the jaw of sea urchin pedicellaria. The stalk is composed of a skeletal rod and a flexible neck portion.

Location

They are generally attached by a long, inflexible stalk and may be found anywhere on the sea urchin's test.

Function

In some families, globigerous pedicellariae have evolved into venomous structures, used for protection or maybe hunting. This is particularly the case in the family Toxopneustidae, some species such as '' Tripneustes gratilla'' and especially ''

Toxopneustes pileolus ''Toxopneustes pileolus'', commonly known as the flower urchin, is a widespread and commonly encountered species of sea urchin from the Indo-West Pacific. It is considered highly dangerous, as it is capable of delivering extremely painful and me ...

'' being extremely venomous.. Image:Flower urchin by Vincent C Chen.jpg, The dangerous

flower urchin ''Toxopneustes pileolus'', commonly known as the flower urchin, is a widespread and commonly encountered species of sea urchin from the Indo-West Pacific. It is considered highly dangerous, as it is capable of delivering extremely painful and ...

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the northe ...

, with its long flower-like venomous pedicellariae Image:Pédicellaires de Toxopneustes pileolus.JPG, Close-up of flower urchin pedicellariae Image:Red sea-reef 3926.jpg, '' Tripneustes gratilla'' is also covered with venomous pedicellariae, but is less dangerous.

References

Echinoderm anatomy {{Echinoderm-anatomy-stub