Predators & Defenses
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  Defenses
 

The unique biting jaws known as pedicellariae are the last line of defense for a sea urchin. They only occur on sea urchins and some sea stars.

NOTE  lit. “small stalk-like structure” L. First lines of defense are, of course, the spines and calcareous test

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Pedicellariae

  Topics on defenses of sea urchins include pedicellariae considered here, and HIDING/SHELTERING/COVERINGS and SPINES, considered elsewhere.
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Research study 1
 
There are 4 types of pedicellariae in west-coast sea urchins.  Three smaller ones, tridentate, opicephalous, and trifoliate (triphyllous), are used for crunching invertebrate larvae and algal spores that are seeking a spot to settle and attach. These respond mainly to mechanical stimuli, while the larger globiferous ones respond mainly to chemical stimuli.

NOTE  the pedicellariae are not drawn to scale

drawings of the 4 types of pedicellariae in a sea urchin
photogrph showing some of the pedicellariae on a green urchin Strongylocentrotus droebachiensis Pedicellariae are positioned on moveable stalks and are distributed between the spines and tube feet. Visible in this photo of a green urchin Strongylocentrotus droebachiensis are spines, tube feet, and globiferous pedicellariae (globular-shaped, pigmented). Three other types of pedicellariae are present, but are harder to make out owing to background clutter.   drawing of and ophicephalous pedicellaria of a sea urchin
The 3 smaller types, including the ophicephalous shown here, act by physical means and are not toxic. The most active of the 4 types are the trifoliate pedicellariae - they are continually biting and snapping.
  drawing of a globiferous pedicellaria of a sea urchin showing poison glands and ducts
The larger 4th type, the globiferous pedicellaria, is also 3-jawed, but is toxic and is employed in defense against predators such as sea stars.  These pedicellariae have sharply pointed tips on the jaws, and each jaw contains a poison duct leading to a poison gland at its base.

NOTE  referring to their round, globular shape

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drawing of a globiferous pedicellaria of a sea urchin taken from a video

CLICK HERE to see a video of a 3-D rendered globiferous pedicellaria biting. Note the deflation of the poison sacs once the flesh of the "prey" is penetrated, indicating injection of toxin. Video courtesy Cindy Young, UBC.

NOTE the video replays automatically

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photograph of sea-urchin pedicellariae in close view taken from a video

CLICK HERE to see a video of the continuous snapping behaviour of trifoliate pedicellariae of a purple urchin Strongylocentrotus purpuratus. The field of view is a bit busy, but includes the following: spines (whitish or purplish), tube feet (distinguished by their suckers), globiferous pedicellariae (closed, large, reddish), and small trifoliate pedicellariae (whitish, snapping-type jaws). Other small pedicellariae are either tridentate or ophiocephalous.

NOTE the video replays automatically

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Research study 2
 

In red sea-urchins Strongylocentrotus franciscanus, but not in other congeneric west-coast species, a few pedicellariae develop during the late larval stages at about the time the adult rudiment begins to form. They are fully functional.  After settlement and when the body parts are being reorganized at metamorphosis, these few pedicellariae shift to a location around the anus.  Burke 1980 Can J Zool 58: 1674.

 

 

8-arm pluteus larva of Lytechinus
pictus
, a southern California species

drawing of a late-stage pluteus larva of a sea urchin Lytechinus pictus showing developing pedicellariae
 

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Research study 3

 

The pedicellariae appear to operate independently from the main nervous system of the sea urchin and have their own reflex responses to external stimuli. On approach of a sea-star predator to an intended sea-urchin prey as, for example, the sunflower star Pycnopodia helianthoides approaching a purple sea-urchin Strongylocentrotus purpuratus, the spines and tube feet on the side closest to the predator collapse, along with the smaller pedicellariae.  This exposes the globiferous pedicellariae that are then raised and opened.  The open pedicellariae are white and easily seen. Contact of the predator's tube feet to soft pads on the inner side of each jaw triggers the pedicellariae to bite and the poison is released at the same time.

Notice in the last photograph that some of the pedicellariae are torn off and remain attached to the tube feet of the sea star. It is clear from the response that the sea star does not like what it feels.  However, it often persists with its attack and may eventually catch and eat the sea urchin.  Rosenthal & Chess 1972 Fish Bull U.S. 70: 205.

photo 1 of a series showing defensive function in globiferous pedicellariae of  purple urchin Strongylocentrotus purpuratus photo 2 of a series showing defensive function in globiferous pedicellariae of  purple urchin Strongylocentrotus purpuratus photo 3 of a series showing defensive function in globiferous pedicellariae of  purple urchin Strongylocentrotus purpuratus photo 4 of a series showing defensive function in globiferous pedicellariae of  purple urchin Strongylocentrotus purpuratus photo 5 of a series showing defensive function in globiferous pedicellariae of  purple urchin Strongylocentrotus purpuratus
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photograph of a pin being used to stimulate a globiferous pedicellaria of a purple urchin Strongylocentrotus purpuratus taken from a video
CLICK HERE to see a video of a steel pin touched to globiferous pedicellariae of a purple sea urchin
NOTE all videos replay automatically
photograph of a tube foot of a sea star being used to stimulate a globiferous pedicellaria of a purple urchin Strongylocentrotus purpuratus taken from a video
Now CLICK HERE to see what happens when a tube foot from a sea star is used in place of the pin (same spot, same urchin).
photograph of a purple urchin Strongylocentrotus purpuratus beginning its escape from a sundlower star Pycnopodia helianthoides taken from a video
CLICK HERE to see a video of a purple urchin's escape from a sunflower star. Note the flattening of spines to expose the toxic pedicellariae.
   
 

The toxic globiferous pedicellariae do not respond to inanimate objects like a pin. However, chemical emanations from a sea-star's tube foot cause it to open. When the pedicellaria opens, 3 white areas or "hillocks" are exposed. If touched, they cause the jaws to close. Could this response be used as a kind of "bioassay" system to test responsiveness of different sea-urchin species to different sea-star species in a future research study?

 

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Research study 4

 

The ophicephalous pedicellariae on sea urchins attach to the test, not by the usual ball-and-socket joint, but by elastic collagen fibers. The elastic fibers attach to handles on the jaw components, which themselves rock back and forth on a simple toothed hinge. How do you think this works?  Study the diagram on the Right and see if you can noodle it out, then CLICK HERE for an explanation. Ideas and diagram from Lawrence 1987 A functional biology of the echinodermsThe Johns Hopkins Univ Press, Baltimore.

NOTE a strandlike fibrous protein characterised by its tensile strength and elastic properties.  Collagen in human skin imparts  elasticity; related proteins in human tendons provide springiness

drawing showing mechanism of action of an ophicephalous pedicellaria of a sea urching
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Research study 5
 

Some of the collagen in the pedicellaria is of the “catch” type, that is, it is at one moment stiff and supportive and, at the next, soft and flexible.  Further softening allows for autotomy and the pedicellaria is easily torn free, yet continues to bite a potential predator.  Motokawa 1985 p.69 In, Echinodermata (Keegan & O’Connor, eds.) A A Balkema, Rotterdam.

NOTE the special properties and functions of “catch” connective tissues are considered elsewhere in the ODYSSEY: LEARN ABOUT SEA CUCUMBERS: DEFENSES & PREDATORS

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Research study 6
 
As we have seen from a previous study, the pedicellariae of purple sea-urchins Strongylocentrotus purpuratus can perceive and respond to the scent of a predatory sea star upstream of them. But does the degree of activity of the sea star affect the magnitude of pedicellaria response?  Observations at the Bodega Marine Laboratory, California show that an actively moving sunflower star Pycnopodia helianthoides upstream of a purple urchin will cause almost twice as many of the globiferous pedicellariae to start actively snapping . More locomotory activity means more tube feet being used by Pycnopodia, and this means more surface area being exposed to the seawater flow.  More locomotory activity also requires greater hydraulic pressure, and this may lead to more stimulatory chemicals being released. Phillips 1978 Mar Biol 49: 237.  histogram showing proportion of globiferous pedicellariae of a purple urching Strongylocentrotus purpuratus snapping, depending upon the activity state of the sea star
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What sensory stimuli do you think are involved in the pedicellariae reacting to the upstream presence of a predatory sea star? Think about the answers given, then CLICK HERE to see explanations for them.

Chemical. 

Sound/vibration. 

Vision.

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Research study 7
 

It would make evolutionary 'sense’ that a sea-urchin’s pedicellariae would respond more to sea stars that actually prey on the urchin than to sea stars that eat other things. Tests in California using globiferous pedicellariae of purple urchins Strongylocentrotus purpuratus exposed to scent of various sea-star species show the following results. Rosenthal & Chess 1972 Fish Bull 70: 205; see also Rosenthal & Chess 1972 Calif Fish Game 56: 203.

POSITIVE results from leather stars Dermasterias imbricata, sunflower stars Pycnopodia helianthoides, and bat stars Asterina miniata. The first 2 species are known predators of sea urchins, but the third, Asterina, is more of an omnivore, eating detritus, and dead animal and plant matter.

NEGATIVE results from blood stars Henricia leviuscula, vermilion stars Mediaster aequalis, pink stars Pisaster brevispinus, ochre stars P. ochraceus, and Orthasterias koehleri. Of these, the last 3 species are certainly capable of eating a sea urchin but, practically speaking, only the last one, Orthasterias, is likely to do so in the field.

Perhaps locomotory speed of the sea stars is an important factor. For example, of the 5 species that elicit no biting response in the pedicellaria, 4 are mostly sedentary.  The fastest species of all, Pycnopodia helianthoides, elicits a strong overall defensive (biting pedicellariae) and escape response in the purple urchin.

Possibly, the pedicellariae respond only to sea stars that themselves have biting pedicellariae (although why this should be is a mystery).  In fact, of the 3 sea-star species eliciting a biting response, only one, the sunflower star Pycnopodia helianthoides, possesses pedicellariae, while the other 2 species, the leather star Dermasterias imbricata and the bat star Asterina miniata, lack them.

Clearly, this is an area deserving of further research.

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Research study 8
 

photograph of a sunflower star Pycnopodia helianthoides apparently attacking a red sea-urchin Strongylocentrotus franciscanusHow effective are pedicellariae as defense against sea-star predators in comparison with, say, spines? This is examined in laboratory studies using specimens of sunflower stars Pycnopodia helianthoides, and purple and red sea-urchins collected from around Bodega Bay, California. Results show a 45:1 preference ratio of purple urchins Strongylocentrotus purpuratus over red sea-urchins S. franciscanus by the sea star.  The authors of the study consider this not so much a “preference” as an imbalance in the effectiveness of pedicellariae versus spines as defense.  The most effective defense, employed by red sea urchins, is not to flee or have the pedicellariae bite, but to employ the long sharp spines, both as a barrier and as lances to poke the predator’s arms.  The short-spined purple urchins are only able to employ pedicellariae against attacks by Pycnopodia, and these seem to be much less effective than are the long spines of red sea-urchins. Moitoza & Phillips 1979 Mar Biol 53: 299.

NOTE an assumption made in the study is that the 2 species of urchins are equally palatable to Pycnopodia. This would need to be established by other kinds of experiments before conclusions could be made of the relative effectiveness of spines over pedicellariae. In fairness to the authors, they do show in Y-tube experiments that Pycnopodia exhibits no distance preference for one sea-urchin species over another, but distance chemoreception is a level removed from actual consumption

Sunflower star Pycnopodia helianthoides reacting to the
spines of a red urchin Strongylocentrotus franciscanus 0.3X

 

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