Feeding, nutrition, & growth
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  Nutritional requirements
  Considered in this section are nutritional requirements, while related topics of FEEDING, DIETS, TEST GROWTH, and SPINE REGENERATION & SPINE DISEASE are considered elswhere.
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Research study 1

Nutritional requirements of sea urchins are not known.  In fact, precise nutritional requirements are unknown for invertebrates other than nematodes, certain insects, and a few protists.  One difficulty in determining the nutritional requirements of sea urchins (and other seaweed-eating invertebrates) is that their guts contain high concentrations of microorganisms (bacteria, protists, actinomycetes) that, through their own growth and metabolism, provide nutrients such as amino acids and fatty acids to their hosts.  It is therefore difficult to identify the source of the nutrients, that is, whether they are from the food or from symbiotic microorganisms.

photograph of red urchins eating a moribund sea star Pisaster brevispinusEarly studies on the nutrition of purple sea-urchins Strongylocentrotus purpuratus in the Monterey area of California indicate the presence of abundant gut flora, including bacteria capable of digesting agar in vitro.  Further in vitro tests with these bacteria show that they are able to digest samples of red algal (Iridophycus) completely within a few days.  The authors note that dissected pellets of algae from the second gut-loop of urchins are each surrounded by translucent membranes.  Upon close examination the coatings prove to be bacterial films.  Examination of defecated pellets reveals that these enveloped pieces are largely digested and are no longer recognisable as algal pellets.  Counts of bacteria indicate numbers as high as 2.5 x 108 . ml-1 of gut contents.  The authors note that while it seems likely that purple sea-urchins obtain nutrition from the activity of their intestinal flora, direct evidence for this is lacking.  Lasker & Giese 1954 Biol Bull 106: 328.

Red urchins S. franciscanus scrape the skin from a
still-living pink sea star Pisaster brevispinus 0.25X


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Research study 2


This aspect of microbial1 contribution to nutrition has been investigated in a population of green sea-urchins Strongylocentrotus droebachiensis in St. Margaret’s Bay, Nova Scotia.  The method involves injecting radiolabeled glucose into 2 test groups, one that has been treated with antibiotics and the other, not treated (CONTROL).  The antibiotics, a mixture of penicillin-G and streptomycin-sulphate, are dissolved in the seawater bathing the test group photograph of 2 Petri dishes showng bacterial growths from stomach contents of sea urchns Strongylocentrotus droebachiensisof urchins.  After 2d exposure to the antibiotics the platable2 gut bacteria are reduced from 2.5 x 108 . ml-1 (measured in the untreated CONTROL group) to 1.0 x 103ml-1 (measured in the antibiotic-treated group).  U-14C-glucose is then injected into both groups.  After a periods varying from 3-14d after injection the animals are dissected and the gonads examined for presence of labelled amino acids in the proteins.  In the CONTROL group containing a normal complement of microbes, all amino acids bear the label.  In the treatment group, however, which experienced a 5 order-of-magnitude reduction in microbes, only the non-essential (for the rat) amino acids bear the label.  None of the essential3 amino acids bear the label.  The authors conclude: 1) intestinal bacteria can synthesise essential amino acids (there is nothing new in this but we need to know it for the next point), 2) amino acids of microbial origin are available to and used by green urchins, and 3) green urchins cannot synthesise essential amino acids, and in this regard they resemble most other animals.  Fong & Mann 1980 Can J Fish Aquat Sci 37: 88.

NOTE1  lit. “small life form”, but generally refers to bacteria; the term microorganism also means “small life form”, but generally includes such things as bacteria, protists, actinomycetes, and the like

NOTE2  a simple count of the number of different bacterial colonies that appear on agar culture plates, without regard to types or species

NOTE3  these are valine, methionine, isoleucine, leucine, tyrosine, phenylalanine, tryptophan, lysine, histidine, arginine, and threonine, as determined for white rats and a few marine invertebrates, including sea urchins and abalone

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Research study 3

Like other herbivores, sea urchins consume a lot of cellulose which is not digested because a cellulase enzyme is apparently lacking.  If labelled cellulose is substituted for labelled glucose in the foregoing experiment on Strongylocentrotus droebachiensis, similar results are obtained.  Thus, in the experimental group treated with antibiotics to reduce the complement of gut bacteria, no label is found in protein amino acids in the gonads.  However, in the CONTROL group, which has a normal complement of gut flora, both essential and non-essential labelled amino acids are found in the gonads.  This suggests that sea urchins are able to digest cellulose with the help of their gut bacteria.  Fong & Mann 1980 Can J Fish Aquat Sci 37: 88.



A green urchin S. droebachiensis perched on a purple urchin S. purpuratus
are eating a sea star sandwiched between. The common-ness of urchin
carnivory may stem from nutritional need for protein nitrogen 0.5X

photograph of a green sea urchin Strongylocentrotus droeachiensis and a purple sea urchin S. purpuratus eating a sea star

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Research study 4


Green sea-urchins Strongylocentrotus droebachiensis are able to utilise a wide variety of algal species as food.  No comparable study has been done on this species on the west coast, but in a unique field study of the nutritional value of different seaweeds to green sea-urchins S.table showing "fitness correlates" in green urchins Stronglylocentrotus droebachiensis eating a number of seaweed dietdroebachiensis in the Gulf of St. Lawrence, scientists maintain groups of animals (30-35mm test diameter) on 17 different algal diets1 in cages for a 4-mo period over late summer/autumn and measure 4 correlates of fitness2.  The 4 correlates were test diameter, total live mass, test mass, and gonad mass.  Analysis of the data shows that of the 4 correlates, the greatest effect of diet is on gonad mass, and the effect divides the diets into 3 nutritional groups of “low-value diets”, “intermediate-value diets”, and “high-value diets” (see table of data above).  The data shown in the table are gonadal masses in g. Interestingly, the best diet for gonadal growth is the green alga Spongomorpha3 but, when converted to % of total body mass at the end of the 4-mo period (11%), it is still greatly inferior to the mean 20% increase shown by field animals over the same period.  This suggests that mixed field diets are superior to mono-algal diets. Lemire & Himmelman 1996 Mar Biol 127: 73. 

NOTE1 the species in the table are colour-coded in accordance with whether they are red, green, or brown seaweeds. Species and genera that are also found on the west coast are indicated by asterisks.

NOTE2  how well adapted an organism is to its environment; specifically, the liklihood of its genes being passed on to the next generation

NOTE3 the best dietary species Spongomorpha arcta has been re-named Acrosiphonia arcta

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What do we learn from the foregoing study? Consider these possible answers, then CLICK HERE for explanations.

Green algae generally are nutritionally better for gonadal growth of green sea-urchins. 

Algal classification (red, green, or brown) is a poor indicator of how well a certain seaweed promotes gonadal growth in field diet-tests. 

The reason that gonadal growth of animals on the best diet,  Spongomorpha, is less than that of uncaged field animals is that oncoming winter conditions slow growth. 

Field animals show enhanced gonadal growth because they select a nutritionally balanced diet from a range of algae available. 

The sea urchins eat less of some diets; hence, grow less and thus produce smaller gonads. 

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Research study 5

histogram comparing different levels of beta-carotene addition to artificial diets on growth of juvenile green sea-urchins Strongylocentrotus droebachiensisA recent study on nutritional requirements of green sea-urchins Strongylocentrotus droebachiensis tests whether augmentation of ß-carotene to artificial diets will increase somatic and gonadal growth.  Results show that over a period of 4mo, addition of 50mg ß-carotene . dry kg diet-1 significantly increases overall growth, but higher doses have no additional effect (see graph on Left).  The better growth actually manifests as increased test size, but with no significant change in gonad size.  There are also unexpected changes in test morphology, manifested as an increase in ratio of height to equatorial diameter.  The authors conclude that addition of ß-carotene to the artificial diet is overall beneficial to growth of juvenile green urchins.  Robinson et al. 2010 p. 397 In, Echinoderms: Durham (Harris et al., eds.) Taylor & Francis Group, London.

NOTE  for some reason the authors replicate their study at the Pacific Biological Station in Nanaimo, British Columbia and the Biological Station, St. Andrews, New Brunswick, using specimens collected in, and shipped from, B.C., as well as wild-harvested specimens in N.B.  As no differences are found in the growth effects of ß-carotene in the 2 locations, only the B.C. data are shown here

NOTE  this pigment is known from other west-coast studies to increase egg size and subsequent larval size in green sea urchins: see LEARN ABOUT SEA URCHINS: REPRODUCTION: LARVAE: FEEDING, GROWTH, & LARVAL LIFE SPAN.  It also adds colour to the gonads, thereby increasing their market value, and acts in several other ways in echinoid physiology, including as an anti-oxidant

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