Feeding, nutrition, & growth
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  Spine regeneration & spine disease
  Considered in this section is spine regeneration & spine disease, while other related topics, including FEEDING, DIETS, NUTRITIONAL REQUIREMENTS, and TEST GROWTH are considered in other sections.
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Research study 1
  photograph of cross-sectional view of a sea-urchin spineAs an urchin grows the spines multiply in number and extend in length. The spines are covered with skin, and it is the dermis component of the skin that secretes the protein scaffolding and calcite during growth and regeneration. Broken spines are regenerated in all 3 Strongylocentrotus species, although apparently less readily in larger-sized S. franciscanus than in smaller-sized individuals, or in the other 2 species. Sea-urchin spines are comparatively strong along their longitudinal axes, but weak along their tranverse axes.  This is because they are comprised of numerous longitudinally orientated calcite components, each a single crystal.  The illustration shows these crystals in cross-section in a spine of a red urchin, with 10 cycles or rings of growth being evident. The ring structure is reflective of cycles of growth, but they are not necessarily annular as in a tree (see following Research Study). A spine will penetrate flesh easily, but readily breaks under transverse shear stress. Swan 1952 Growth 16: 27; Ebert 1968 Ecology 49: 1075; Drawings modified from photos in Ebert 1967 Biol Bull 133: 141
 

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Research study 2

 
drawing of sea-urchin spine showing break areas used to determine regeration patterns

A study on spine regeneration in purple urchins Strongylocentrotus purpuratus collected at Sunset Bay, Oregon shows not only that broken spines are regenerated, but entire spines are able to be replaced when lost or  broken off near the base.  During regeneration the dermis layer of skin first manufactures a 3-dimensional protein lattice or scaffolding, then secretes calcite into this.  As noted above, the rings are not necessarily annular.  A completely regenerated spine has actually fewer rings than an unbroken one, because the spine is created de novo. For example, the spine on the left has been broken twice. After the first break, the entire spine is replaced during the growth cycle indicated in yellow. At this time a cross-sectional view of the spine will reveal only one growth cycle (from one side of the yellow to the other). After the second break, the entire spine is again replaced, this time shown in blue at the top.

In the first illustration on the Right, a spine with 5 growth rings has been experimentally broken off at the tip. Two months later (next illustration) it is completely regenerated, and a new growth ring, No. 6, has been added. Note that the entire section replaced on top is just the single growth ring No. 6. Ebert 1967 Biol Bull 133: 141.

NOTE  in order to view the internal structure of a spine, it is dried, soaked in xylene and then Canada balsam, heated, then ground on either side to create a thin, transparent section.  Both radial and longitudinal preparations can be made in this way.  Ebert 1965 Ecology 46: 193

photograph of spine of a purple urchin Strongylocentrotus purpuratus broken off in order to study regeneration photograph of a spine of a purple urchin Strongylocentrotus purpuratus showing regeneration after an experimental breaking
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Research study 3
 

photograph showing spine erosion in a purple sea-urchin Strongylocentrotus purpuratusA study in southern California on regenerating spine tips in purple urchins Strongylocentrotus purpuratus shows that the site of a fracture plane soon is a zone of intense mitotic activity.  Within 1d after a spine is experimentally broken, the epidermis has almost entirely regenerated over the fractured radioautogram of a regenerating spine tip in a purple sea-urchin Strongylocentrotus droebachiensissurface of the spine. The cells are calcium-secreting and lead to the formation of a new spine. The cells actually migrate distally in spines that are regenerating.  Heatfield 1971 J Exp Zool 178: 233. 


Above: radioautogram of a fractured spine treated
with tritiated thymadine.  The black dots indicated
by arrows are areas of intense mitotic activity

Purple sa-urchin Strongylocentrotus purpuratus
showing spine erosion and perhaps regeneration 6X

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Research study 4
 

photos showing stages of regeneration in a spine of a purple sea-urchin Strongylocentrotus purpuratusphoto/schematic showing 4 growth cycles in a regenerating spine of a purple sea-urchin Strongylocentrotus purpuratusFurther details on spine regeneration in purple sea-urchins Strongylocentrotus purpuratus are provided in a follow-up study by the same author.  Regeneration of a broken spine starts as a conical growth of microspines which, by 25d of growth, attains a diameter equal to the original.  By 60d the regenerating spine is the same length as the original (see photos on Left). 

The spine regenerates by a series of concentric additions of dense calcite in the form of wedges terminating in club shapes, where each “club” represents a ridge running lengthwise down the spine (see photo/schematic upper Right). Each wedge is interconnected to the next by a meshwork of fenestrated calcite.  The middle of the spine is indicated by a letter "M".

drawing of a section of regenerating spine of a purple sea-urchin Strongylocentrotus purpuratusThe drawing on lower Right shows 2 such wedges extending from the middle of the spine outwards, representing 6 growth cycles.  Spines of sea urchins have many holes or fenestrations that endow the spine with great strength. The author provides an excellent series of scanning e-micrographs showing the microstructure of  regenerating spines.  Heatfield 1971 J Morph 134: 57.

NOTE  calcium carbonate

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Research study 5
 

histogram showing effect of spine clippings and starvation on pyramid height in the Aristotle's lantern of purple urchins Strongylocentrotus purpuratusStudies on spine regeneration in purple urchins Strongylocentrotus purpuratus in California show an interaction between spine growth and Aristotle’s lantern growth.  Apparently, spine damage in this species is common, especially in high-energy wave areas, and a good ability to regenerate spines is expected.  So, what happens to the size of the Aristotle’s lantern if test animals are not only starved, but also given a spine clipping?  How do the organisms allocate their available energy? The results after 32wk show that significantly more energy is allocated to jaw growth (defined as pyramid height) by individuals fed and clipped, starved, and starved and clipped, in that order, over control animals. Thus, even if an urchin is well fed, the clipping stimulates skeletal growth, perhaps as an overall "toughening" of the skeleton to withstand better the environmental factors that cause spine breakage in the field (e.g., waves). Starvation alone causes even more skeletal growth, as an adaptation to increase lantern size for more effective feeding. The greatest increase in lantern size comes with a combination of starvation and spine clipping, perhaps as a “double-whammy” effect of the 2 treatments.  The authors interpret these results as the combined responses to spine breakage leading to a general “toughening” of the body, and starvation leading to an increase in size of the lantern. Edwards & Ebert 1991 J Exper Mar Biol Ecol 145: 205.

NOTE  all spines are clipped at the start of the 32-wk experiment to a height of less that 1cm

 

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Research study 6

 
Little is reported on diseases of west-coast sea urchins. However, a parasitic amphipod inhabiting the tips of spines of red urchins Strongylocentrotus franciscanus is seen quite commonly, at least in British Columbia. Little if anything is known of the biology of the amphipod, or of its effects on its host. The photographs show spine degradation in a specimen of red urchin. The crustaceans appear to crawl over the surface of the spine, rather than burrow into it, and their activities may kill the skin covering the spine, thus leading to degradation of the underlying calcium carbonate. The inset photos show crustaceans on the spine tips, but it's difficult to make out much detail. The inset photo on the Left appears to show some juveniles lined up along the spine distally, with one or more adults clustered proximally. The inset photo on the Right shows one (or perhaps two) crustaceans of uncertain identification.
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