Feeding, nutrition, & growth
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  This section starts with test growth, but includes other aspects of growth, including size, age, and longevity. Topics of FEEDING, DIETS, NUTRITIONAL REQUIREMENTS, and SPINE REGENERATION & SPINE DISEASE are considered elswhere.
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Research study 1

The internal skeleton of a sea urchin, or test, is made up of many hexagonally shaped calcareous plates known as ossicles. Each ossicle is a single crystal of calcium carbonate in a form known as calcite.  The ossicles are fenestrated (have holes) which, remarkably, increases their relative strength.  In fact, it takes more relative force to crush a spine of a sea urchin than the shell of a clam or snail (neither of which is fenestrated).  Echinoids and other echinoderms fulfill the strength requirements of their skeletons with a minimum amount of material.  Weber et al. 1969 J Ultrastr Res 26: 355.


NOTE  calcite is the most common form of calcium carbonate in marine invertebrates (e.g., corals, molluscs, echinoderms).  It is quite soft, with a hardness of 3 on the Mohs scale (diamond = 10). Calcite is so soft it can be scratched with a fingernail.  Apparently, however, echinoid calcite is made slightly harder by the addition of small amounts of magnesium

photograph of the test of a sea urchin showing ossicles
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Research study 2

graph showing increase in number of interambulacral plates with age/size of purple sea-urchins Strongylocentrotus purpuratus in Californiadrawings showing change in size, number, and array of interambulacral ossicles over the first 3 years of age in a purple urchin Strongylocentrotus purpuratusThe adjoining plates or ossicles in a sea-urchin test are not fused; rather, they are separated by tissue sutures within which crystalline calcium carbonate is secreted for growth.  A newly metamorphosed sea urchin Strongylocentrotus purpuratus has 10 interambulacral plates (5 in each column). The number of interambulacral plates increases wtih age/size from 10 to over 40 in 70-80mm diameter individuals (see graph upper Left). The drawings below Left follow changes in size, number, and array of interambulacral plates over the first 3yr of life. The original 10 plates are indicated.

photographs comparing growth lines on the same interambulacral Plate 20 in the test of sea urchins Strongylocentrotus purpuratusStudies on growth of S. purpuratus in southern California show, not surprisingly, that the test is a dynamic metabolic entity and that growth zones on the test plates reflect many aspects of seasonal change, most importantly, nutrition and reproduction.  During poor food conditions (e.g., winter) and during gonadal growth, growth zones change from being thick and opaque to being thinner and translucent. This produces lines, or checks, that indicate different states of metabolic activity and act as well to demarcate age.  For example, comparison of Plate 20 from 4 sea urchins of different, known ages (0.5-4yr) reveals seasonal growth zones that can be followed from one age group to the other (see photos on Right).  For example, here the authors have tentatively identified growth zones corresponding to the first winter, second summer, second winter, and so on.  Pearse & Pearse 1975 Am Zool 15: 731.

NOTE  a quick and comparatively easy method to view growth lines is to char the plate in a flame (or muffle furnace), then immerse in xylene.  Alternatively, cleaning in sodium hypochlorite (or commercial bleach) before immersing in xylene also works well.  For method as applied to sea urchins including Strongylocentrotus spp. see Jensen 1969 Sarsia 37: 41.

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Research study 3

photographs of a test of a sea urchin and water-filled balloon to show similarity in shapeThe unusual squashed shape of the test of sea urchins has long been of interest.  If sea urchins lived in air, one could visualise force of gravity bulging out the lower part of the test just as it bulges out a water-filled balloon.  However, in water the internal fluid and viscera have no effective mass for a sea urchin.  Another idea is that osmotic pressure might be higher inside than out and this, combined with increased pressure from gut expansion during feeding and continuous downward pull from the tubefeet, might cause the test to bulge downward as it grows. Studies on purple urchins Strongylocentrotus purpuratus at the Bodega Marine Laboratory, California, however, show that osmotic pressure is actually negative for most of the time and that changes in pressure in the coelomic cavity are caused by expansion and contraction of the Aristotle’s lantern, and by withdrawal or extension of the tubefeet, but not by presence or absence of food in the gut.  Pressure increases appear not to be relieved by outward flow from the madreporite because this is negligible; rather, they occur from outward movement of the peristomial membrane.  The peristomial gills appear not to be involved in such pressure relief. In summary, there seems to be no apparent cause for the unusual form of a sea-urchin test (other than the tubefoot idea).  However, its squashed shape does place more tubefeet into close proximity to the substratum than would a globular shape.  Ellers & Telford 1992 Biol Bull 182: 424. 

NOTE a soft membrane surrounding the mouth, bearing 5 pairs of symmetrically placed gills. The gills are inflated and deflated by pressure changes within the coelomic cavity housing the Aristotle's lantern

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Research study 4

Purple sea-urchins Strongylocentrotus purpuratus not only grow in size but, under certain conditions, they also shrink.  A study in Sunset Bay, Oregon in which animals are measured, tagged, kept in a tidepool for 1yr, and re-measured, show that individuals of 5-6cm test diameter have graph showing shrinkage in the tests of purple urchins Strongylocentrotus purpuratus confined in a tidepool for 1yractually decreased in size.  The decrease is small, representing only about 3% loss in diameter, but is statistically significant, and must involve resorption of calcite.  The author remarks that negative effects of handling and tagging do not seem to be involved because the smallest individuals (1-2cm test diameter) in the test group did grow.  However, casual observation of the graph shows that these represent less than 10% of the test group, and one could argue that they may have grown despite the possibly debilitating effects of the tagging procedure (see note below).  Food availability in the tidepool is not mentioned but, even if it were in short supply, shrinkage of the test is not easily explained.  The first “casualty” of food shortage in a sea urchin is usually loss of reproductive output and/or decrease in mass of other soft tissues, but data for these are not available in this study. Based on graphical analysis of modal size-classes the author suggests that an individual 5cm in diameter is about 10yr old.  Ebert 1967 Science 157: 557.

NOTE  it is difficult to tag or mark a sea urchin.  Most externally applied tags, such as rubber bands, wrapped-around wire, rubber sleeves or straws on spines, and the like, soon fall off or are disruptive to normal behaviour.  Injections of latex or dyes are not effective.  The method used here is effective, but invasive, and involves drilling a hole through the test and soft tissues, threading monofilament line through the holes, and affixing a coloured rubber marker-tube.  Within a period of 2wk the sea urchin secretes calcite to seal off the holes around the monofilament line.  Mortality from this procedure is not mentioned although some tag loss is noted.  In another research publication the author remarks that the tag has “no interference with normal behaviour or growth”.  Such tags last for at least a year.  Ebert 1965 Ecology 46: 193.

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Research study 5

graph showing effect on growth of tetracycline injection in purple urchins Strongylocentrotus purpuratusAnnual growth lines on ossicles in sea urchins are easily distinguished if growth is rapid.  However, if growth is slow, there may be 2 or more lines deposited per year.  Thus, growth rates in larger, slow-growing individuals may be underestimated. Injection of tetracycline1 can be used to calibrate natural growth lines, as is done in a study on growth of purple urchins Strongylocentrotus purpuratus at the Scripps Institution of Oceanography, California.  One part of the research assesses whether the antibiotic has any effect on growth or survival in a test individual.  Results show that growth2 over a 7-mo test period is unaffected by dose3 administered, over a reasonable test range (the negative slope of the data is a predicted scaling effect of size on growth), and survival is similarly unaffected (data not shown here).  Ebert 1985 p.435, In Echinoderm biology (Burke et al., eds.) AA Balkema, Rotterdam.

NOTE1  the tetracycline is incorporated into the test elements and the marks are revealed on later collection (after killing and cleaning) under UV light.  The antibiotic is in the form tetracycline-HCl, and is administered at a dosage of 1mg in small-sized species (such as S. purpuratus) and 2-10mg in large-sized species (S. franciscanus)

NOTE2  for some reason, the author expresses growth as change in diameter in ln (natural logs) rather than simple arithmetic terms

NOTE3  each symbol on the graph indicates a treatment: replicate injections of 0.1ml seawater ("H2O"), replicate injections of 1,2,4, and 8mg tetracycline in 0.1ml seawater, and an untouched CONTROL (Con)

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Research study 6

photogaph of an ossicle plate of a sea urchin showing fluorescent band from tetracycline injectionphotograph of a sea-urchin ossicle marked with tetracycline and showing zero growth after one yearUse of growth lines in ossicles of sea urchins as natural indicators of age and growth rates is quite common. However, few studies have evaluated whether these bands are actually added annually.  In fact, a 1yr study in Maine on fluorescent dye-injected green urchins Strongylocentrotus droebachiensis shows that most individuals, especially those >55mm test diameter, add less than one band to their ossicles annually. The photo on the Right shows an example of an ossicle plate showing zero growth over 1yr (the fluorescent band is indicated by the red arrow). In fact, the authors find that out of a test group of 30 individuals, only 7 (23%) add a complete band to all 4 ossicles monitored (see photo on Left), and 6 (20%) add more than 1 complete band to at least one ossicle.  The authors conclude that band formation and growth are seasonally episodic, but not predictable on an annual basis. They caution that if the growth-band aging method is used in echinoids, it must be demonstrated that one band is added annually for all sizes in a field population.  Other assumptions may lead to serious underestimating of age in S. droebachiensis, especially in the largest size classes. As an example of the variability that can arise from strict interpretation of growth-line data, age estimates of S. droebachiensis range from 9 to >100yr, a discrepancy thought by the authors to owe to the aging methods used.   Russell & Meredith 2000 Invert Biol 119: 410.

NOTE these include 3 interambulacral test plates in a single column (equatorial bulge, mid-top, and mid-bottom), and a rotula from the Aristotle’s lantern.  Thirty individuals ranging in test diameter from 14-77mm (in 2-mm size intervals) are marked (injected with tetracycline or calcein), released into tidepools, and left for 1yr

NOTE this paper provides a good review of plate-growth studies done on Strongylocentrotus spp., including another Atlantic-coast study on S. droebachiensis not included in the ODYSSEY that provides supporting data for the traditional growth-line method: Robinson & MacIntyre 1997 Bull Aquacult Assoc Can 97: 56.

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Research study 7

Studies on growth of Strongylocentrotus franciscanus at two locations in San graph showing growth of red urchins Strongylocentrotus franciscanus at 2 locations in San Nicolas Island, CaliforniaNicolas Island, California show that over 40% of final size is attained during the first year.  Individuals of 8cm test diameter are about 10yr of ageEbert & Russell 1992 Mar Ecol Progr Ser 81: 31.

NOTE  in this study, individuals are monitored with injection of tetracycline 

  Research study 8

Scientists used to think that large red urchins Strongylocentrotus franscicanus are no more than about 10-50yr old (10-14cm test diameter).  However, recent studies using deposits of 14C within the pyramids of the Aristotle’s lantern as markers to show growth, suggest that these large urchins may be much older, with ages in excess of 100yr. Individuals of 16cm test diameter are thought to be 130yr old, while really big specimens of 19cm test diameter may be 200yr old.  Growth in these old-timers slows to a virtual standstill as most of their tissue production is in the form of gametes for reproduction.  Do they become senescent?  Researchers in California provides the following information on this and related topics. The photograph of several red urchins Strongylocentrotus franciscanus, one of which is eating a kelp holdfastauthors note that annual survival does not decline with size and there is no decrease in gonad size with increasing diameter up to the maximum size dissected (17.5cm test diameter).  Thus, based on continued survival and increasing reproductive output with increasing size, the conclusion is that S. franciscanus does not senesce as it ages. In San Nicolas Island, growth is slow at first, then maximal, and then slow again. Individuals of 4cm diameter are estimated to be about 12yr of age, while ones of 10cm diameter are about 12yr of age.  Ebert & Southon 2003 Fish Bull 101: 915; Ebert et al. 1999 Mar Ecol Progr Ser 190: 189; Ebert 2008 Exper Gerontology 43: 734; Ebert & Russell 1993 Mar Biol 117: 79.

NOTE radioactive carbon, present in high and easily detectable concentrations in calcium carbonate that makes up the test and “bones” of the lantern, originates from the dates of first atmospheric testing of nuclear weapons.


Several red urchins Strongylocentrotus franciscanus,
one of which is eating a kelp holdfast. The largest
individual (on the Left) may be several decades old

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Research study 9
photograph of green urchins Strongylocentrotus droebachiensis Green urchins Strongylocentrotus droebachiensis, in comparison, are comparative toddlers when they die at around 25yr of age. Robinson & MacIntyre 1997 Bull Aquacult Assoc Can 97: 56.
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Research study 10

The question arises as to how an animal can avoid being killed by disease at some point in its long life.  Long-term tudies on red urchins Strongylocentrotus franciscanus in British Columbia show that even minor spine loss – sometimes as little as a single spine – may lead to fungal infection and ultimately to death.  How does an animal as apparently fragile as a sea urchin live for +100yr without incurring potential lethal spine loss or damage? Charles Low MSc thesis, Dept Zool, Univ British Columbia.

photograph of a red urchin Strongylocentrotus franciscanus exhibiting some type of disease, possibly fungal

Two specimens of red urchins infected with bacterial or fungal disease (Left: 0.25X; Right: 0.75X)

photograph of a red urchin Strongylocentrotus franciscanus exhibiting some type of disease, possibly fungal.
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Research study 11

histogram of probabilities of red urchins Strongylocentrotus franciscanus living to be certain ages in 4 west-coast statesA survey of ages of red urchins Strongylocentrotus franciscanus in the 4 west-coast states show that sea urchins living in more northern climes (Washington to Alaska) have a much greater probability of living to 50yr of age than ones in more southern areas (Oregon to California). Ebert et al. 1999 Mar Ecol Progr Ser 190: 189.

photograph of red sea-urchin Strongylocentrotus franciscanus eating a piece of kelpNOTE it would have been interesting to see comparable age-data for red urchins in British Columbia in the graph

NOTE the authors provide similar estimates of probability of living to 80 and 100yr of age in the different states (data not shown here). Although the patterns for these resemble what is shown in the figure for reaching 50yr of age, the difference in probabilities become even more pronounced. Thus, a red urchin in California/Oregon has only about 1/285th chance of becoming a centenarian than does an individual in Washington/Alaska

A red sea-urchin Strongylocentrotus franciscanus eats a piece of kelp 0.6X

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So, there are likely to be lots of centenarian sea urchins in more northern parts of the west coast, but only pentenarians in California.  Why is this, do you think? Consider the answers provided, then CLICK HERE for comments. Ideas from Ebert et al. 1999 Mar Ecol Progr Ser 190: 189.

Life is simply too frenetic in California. 

More seawater pollution is present in the south. 

There is more disease in warmer water-temperatures. 

There are more predators in the south. 

NOTE  50-59yr old

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Research study 12

photograph of a red sea-urchin Strongylocentrotus franciscanus eating the float of a bull kelpAs an aid  to management of the fisheries for purple urchins Strongylocentrotus franciscanus in northern California, researchers at Bodega Marine Laboratory use tag1-recapture methods to estimate growth over a 1yr period.  Rather than use ossicle plate dimensions, which have been found in other studies to be useful but not as precise as necessary, the authors use change in jaw2 size as an indicator of growth.  Six different growth models3 are tested for best fit to the data, then melded into a composite growth curve of jaw size as a function of time.  The authors suggest that their composite model has broad flexibility and provides robust results.  They confirm that S. franciscanus is a slow-growing species, requiring 7yr to reach a legally fishable size.  Growth rates in shallow (5m) and deep (17m) populations are not significantly different.  Rogers-Bennett et al. 2003 Fishery Bulletin 101: 614.

NOTE1  tagging is done by in situ injections of tetracycline.  In addition to monitoring of natural field populations, several tagging/re-stocking experiments are also done

NOTE2  a “jaw” is one of the 5 pyramids of the Aristotle’s lantern.  See FEEDING, NUTRITION, & GROWTH: FEEDING in this section of the ODYSSEY

NOTE3  the 6 models tested are the logistic, Gaussian, Tanaka, Ricker, Richards, and von Bertalanffy.  Of these, the first 2 provide best individual fits to the data, but a combination of these and the Tanaka and Ricker models give overall best fit

Strongylocentrotus franciscanus eats the float
of a bull kelp Nereocystis luetkeana 1X

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Research study 13

graph showing growth of red sea-urchins in 4 areas of British ColumbiaIn British Columbia red urchins Strongylocentrotus franciscanus are legally harvestable when they reach 90mm test diameter.  Estimates of growth rates in Haida map showing study locations in British Columbia for investigation of growth of red sea-urchins Strongylocentrotus franciscanusGwaii (Queen Charlotte Islands) and other west-coast sites (see map) using tetracycline and calceine marking show that this size is reached at about 10-18yr of age, depending upon area (see graph). Zhang et al. 2008 J Shellf Res 27: 1291.

NOTE  there are several ways to model growth, the most common being the von Bertalanffy function.  However, because the growth curves for S. franciscanus are sigmoidal, the authors use 2 other methods that provide better fits to the data, the Tanaka and Logistic functions.  Results from the former are presented here

NOTE  the researchers tag about 9,000 individuals using in situ tetracycline injections with a syringe or, for small individuals, by bathing them overnight in calceine solution.  Both chemicals bind to calcium in growing portions of the skeleton, thus providing a fluorescent mark visible under UV light. Urchins are resampled from the 4 areas 1yr later

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Research study 14

exploded view of a sea-urchin test showing the location, and parts of, the Aristotle's lanterngraph showing relationship between size of test and growth lines in the rotula of sea urchinsA study in southeastern Alaska on red urchins Strongylocentrotus franciscanus employs transponder tags and growth-ring data from the rotula part of the Aristotle’s lantern to measure growth and age (see illustration on Left).  Data from 3 study sites generally confirm that the rotula is a reliable aging component, but only for younger individuals (see graph).  Note in the graph that beyond a test diameter of about 10-11cm (about 14 rotula rings or approximately 14yr of age), age can no longer be reliably estimated.  The authors discuss the merits of their methodology in comparisons with other aging/growth techniques.  Shelton et al. 2006 Trans Am Fisheries Soc 135: 1670.

NOTE  these passive-integrated transponder tags are injected inside the test

NOTE  the rotulae are dog-bone shaped structures that connect each of the 5 demi-pyramids (each pyramid supports one tooth, and the pyramids are comprised of 2 symmetrical parts) in the lantern.  When cleaned, broken in half, and charred in an alcohol flame, growth rings become visible.  Each ring is assumed to represent 1yr growth


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