Feeding, nutrition, & growth
  Sea urchins eat mainly seaweeds, although animal protein such as dead fishes, invertebrates, and the like, is readily eaten.
 
photograph of a red urchin Strongylocentrotus franciscanus eating the float of a bull kelp Nereocystis luetkeana photograph of a red urchin Strongylocentrotus franciscanus eating a dead fish

 

 


Red urchins Strongylocentrotus franciscanus eating bull kelp Nereocystus luetkeana (left) and a dead fish (right)

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  Feeding
  Considered in this section is feeding, while topics of DIETS, NUTRITIONAL REQUIREMENTS, TEST GROWTH, and SPINE REGENERATION & SPINE DISEASE are considered in other sections.
 

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Research study 1

 
The feeding device of an adult purple urchin Strongylocentrotus purpuratus is a complex assembly of muscles and calcareous ossicles known as an Aristotle’s lantern. Note that it has a five-fold radial symmetry. The "working" parts are 5 extendable calcareous teeth.

NOTE  2400 years ago the Greek philosopher and teacher Aristotle described the feeding apparatus of a sea urchin and, because of its shape, likened it to a lantern
photograph of the mough of a purple urchin Strongylocentrotus purpuratus
Mouth of S. purpuratus from below showing the 5 tooth-tips retracted. Several spine-tips and 2 open pedicellariae are also visible
drawing of side view of an Aristotle's lantern of a sea urchin
The teeth enclose a space or buccal cavity that communicates with the esophagus and thence to the rest of the gut
drawing of the teeth with dental sacs of a sea-urchin Aristotle's lantern
Each tooth is produced in a dental sac at the top at the same rate as it is worn away at the other end by scraping
NOTE  also referred to as pentaradiate (lit. “five radius”) symmetry.  The five-fold pattern seen in many echinoderms, such as sea stars, brittle stars, and basket stars is, of course, how the star part of the names originated. SYMMETRY is considered briefly for sea urchins in another section drawing of 5 teeth and one pyramid of a sea-urchin's Aristotle's lantern
The teeth are supported by pyramids. As they are secreted and grow in length, the teeth slide along in sheaths on the inner faces of the pyramids
drawing of parts of a sea-urchin's Aristotle's lantern including teeth, pyramids, rotulas
The 5 pyramids plus 5 supporting pieces called rotulas are joined by muscles and connective tissue, enabling expansion and contraction of the lantern
drawing showing retractor and protractor muscles of a sea-urchin's Aristotle's lantern
Contraction of protractor muscles pushes out the teeth and makes the bite. When retractor muscles pull in the teeth, they open, and release the food
 
 
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drawing showing retractor and protractor muscles of a sea-urchin's Aristotle's lantern taken from a video

CLICK HERE to play a 3-D video of an Aristotle's lantern taking a bite. Animation courtesy Cindy Young, University of British Columbia.

NOTE video replays automatically

 

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Research study 2
 

photograph of parts of an Aristotle's lantern of a sea urchin

This view looking down on the Aristotle's lantern of a purple sea-urchin Stongylocentrotus purpuratus shows parts of the pyramids, large muscles joining the pyramids, rotulas, dental sacs, and the severed end of the esophagus. Note the groove in the pyramid that accommodates the top end of the tooth. Expansion and contraction of the lantern not only facilitates biting and feeding, but may also cause special gills around the mouth, the peristomial gills, to deflate and inflate. There is some thought that the combined metabolic activities of the lantern, including continuous production of teeth, and practically non-stop activity of the complex musculature during feeding, may require the auxiliary gas-exchanging surfaces of the peristomial gills.

The pyramids are sometimes referred to as "jaws", in the sense that they support the teeth that do the actual biting and scraping.

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photograph of the mouth and teeth of an Aristotle's lantern of a sea urchin taken from a video

 

CLICK HERE to see a video of the feeding action of the Aristotle's lantern in a green urchin Strongylocentrotus droebachiensis. Note that the lantern is quite mobile and, even constrained by the glass wall of the aquarium, it is easy to see that it can be readily moved in different directions and extended to and fro.

NOTE video replays automatically

 

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Research study 3
 

histogram showing jaw length in sea urchins Strongylocentrotus purpuratus from individuals in high- and low-food areas
An interesting feature of the Aristotle lantern is that in times of food shortage the pyramids and other components grow larger.  The response is thought to increase the efficiency of food-gathering, as bigger lanterns relative to body size are able to scrape and gather in more food.  The calcium carbonate required for jaw formation is taken up from the surrounding seawater. As an example, measurements of pyramid sizes in populations of purple urchins Strongylocentrotus purpuratus in Oregon collected from areas with known low and high food availability show a 20% greater jaw dimension in the former.  Ebert 1980 Bull Mar Sci 30: 467.

NOTE the measurement is the linear extent of a half-pyramid from where it attaches to the epiphysis (this is the part at the top that links the 2 halves of the pyramid) to the oral tip

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Research study 4
 

Within a sea-urchin species, it is generally assumed that feeding capacity is directly related to size of Aristotle’s lantern.  A comparison at the Bamfield Marine Sciences Centre, British Columbia of lengths of demi-pyramids of the Aristotle’s lantern against diameter of test in the sea-urchin species Strongylocentrotus franciscanus, S. droebachiensis, and S. purpuratus reveals significant differences in scaling.  Thus, whereas demi-pyramid length and test diameters scale linearly in S. purpuratus and S. droebachiensis (0.96 and 0.93, respectively), in S. franciscanus they scale allometrically (1.56).  This means that as an individual grows in size, its pyramids grow disproportionately larger. The authors note that this should translate to a greater relative capacity for feeding in S. franciscanus, leading to greater relative growth and gonad production, which seems to be the case.  Lawrence et al. 1995 J Nat Hist 29: 23.

NOTE  equivalent to a half-pyramid. The length of a "demi-pyramid" is identical to the length of the complete pyramid. The same relationships as described above hold true for rotule lengths plotted against test diameters in the same species 

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Research study 5
 

photographs of tip of tooth of a purple sea-urchin Strongylocentrotus purpuratus showing partsIt is well known that sea-urchin teeth are self-sharpening, and recently a consortium of 17 scientists throughout the U.S. and Israel have provided precise details of how this works.  By use of a variety of methodologies1 the authors identify minute plates of calcite2 separated by organic fibres or “fault lines” that fracture under pressure as the tip of the tooth bites and scrapes (see photographs). Similar to the soft- and hard-part layering of a beaver tooth, this fracturing exposes a fresh portion of the central part of the tooth (termed the “stone” by the authors) that becomes the new sharp grinding tip.  Both radial (in and out during biting) and especially lateral (rotating and tilting with the teeth closed during boring into rock) movements2 of the teeth cause the tips to be sharpened. This is an excellent article and tells us everything we would want to know about the sharpening process of a sea-urchin tooth...and more.  Killian et al. 2011 Adv Funct Mater 21: 682.

NOTE1  X-ray photoelectron emission spectromicroscopy, scanning electron microscopy, EDX analysis, nanoindentation, and X-ray micro-tomography

NOTE1  a crystalline form of calcium carbonate commonly found as limestone and marble, and a principal component of shells and tests of sponges, echinoderms, some molluscs.  Hardness of calcite is greatly increased in the teeth of sea urchins by replacement of up to 45% of the Ca atoms by Mg (most notably in the “stone” or tip part)

NOTE1  the authors have provided supplementary drawings of these movements of the teeth, but these seem not to be available in the PDF format of the article

 

 

As the tooth is worn off at its oral end, new calcite material is formed in the
dental sac at the aboral end (see Research Study 1 above). The calcite is secreted
into an organic system of fibre strips or layers, and it is the breakdown of these
layers along their organic "fault lines" during wear that sharpens the tooth

 
close views of the fracturing (sharpening) process of the tooth of a purple sea-urchin Strongylocentrotus purpuratus
This X-sectional series shows the fracturing process. It also occurs on the outer tip surface to further sharpen it The fracturing occurs along fault lines represented by organic matter laid down in parallel strips Close view of the sloughing off of calcite crystals along the fault lines
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