Feeding, nutrition, & growth
  Sea urchins eat mainly seaweeds, although animal protein such as dead fishes and invertebrates is readily eaten.  
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  Considered in this section is the topic of diets. Other related topics, including FEEDING, NUTRITIONAL REQUIREMENTS, TEST GROWTH, and SPINE REGENERATION & SPINE DISEASE are considered elswhere.
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Research study 1

Green sea-urchins Strongylocentrotus droebachiensis strongly prefer brown kelps Laminaria spp.; however, when hungry, they eat most algal species.  On the east coast the species suffers from diseases that wipe them out periodically and cause their numbers to fluctuate widely.  When abundant, green sea-urchins can create large depauperate areas on the sea bottom known as “algal barrens”. Photo courtesy Ron Foreman, University of British Columbia.



Large group of green urchins Strongylocentrotus
create a "algal barrens" area.

photograph of a large aggregation of green urchins Strongylocentrotus droebachiensis courtesy Ron Foreman, UBC

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Research study 2


Algae consumed by red sea-urchins Strongylocentrotus franciscanus in the Santa Cruz area of California are primarily brown kelps Macrocystis pyrifera, Nereocystis luetkeana, Laminaria sp., and Pterygophora californica, in roughly that order of preference.  Monitoring of movements of tagged sea urchins within and outside of kelp beds shows that movements are related principally to feeding. Inside the kelp forest movement is about 7cm per day, and 66% of the animals are feeding.  Outside the kelp forest movement is 50cm per day, and only 15% of the animals are feeding.  Mattison et al. 1977 Mar Biol 39: 25.

NOTE  sea urchins are tagged using numbered plastic drinking straws slipped over the spines, 2 per individual

map showing kelp beds off the coast of Santa Cruz used in a study of red urchins Strongylocentrotus franciscanus

photograph of a red urchin Strongylocentrotus franciscanus eating brown kelp Pterygophora
Above: a red urchin S. franciscanus eats brown kelp Pterygophora californica 0.25X;

Left: kelp beds off the Santa Cruz coast with a 115m transect line indicated

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Research study 3

graph showing correlation between rank of absorption efficiency and feeding preferences rank for sea urchins Strongylocentrotus spp.graph showing correlation between food preferences and "fitness" value of diets for green sea-urchins Strongylocentrotus droebachiensisLaboratory tests of feeding preferences of green sea-urchins Strongylocentrotus droebachiensis and red urchins S. franciscanus in San Juan Island, Washington show the following preference heirarchy: Nereocystis luetkeana > Costaria costata > Laminaria saccharina > L. groenlandica > Monostroma fuscum > Opuntiella californica > Agarum fimbriatum > A. cribrosum.  Similar, but weaker, preferences are shown for purple sea-urchins S. purpuratus.  Absorption efficiencies for the 3 species are 87% on a diet of Nereocystis, 80% on Costaria, 66% on L. saccharina, and less on the other species.  The graph shows that absorption efficiencies for S. droebachiensis and S. franciscanus decline in accordance with rank of feeding preference for 7 algae, with just a single exception for S. franciscanus (see graph on Left). Growth over a 15-mo period is best on the preferred diets of Nereocystis and Laminaria.  Observations of feeding at 7 field sites on San Juan Island show that green and red urchins feed primarily on kelps Nereocystis (27-40% of diet) and green algae Monostroma and Ulva (24-28%).  Overall, brown algae accounts for 64% of the field diets and green algae, 24%. 

When growth and reproduction in juvenile (10-22mm test diameter) S. droebachiensis feeding on 4 diets (2 of the most preferred diets, and 2 of the least preferred) are combined into a “fitness component”, it correlates perfectly with the rank of feeding preference (see graph upper Right). Despite a wide range of potential food items available in the field, urchins feed on a select few.  These preferred algae are highly compatible with the urchins’ digestive and nutritional needs, and consequently growth and reproductive potentials are maximised. Vadas 1977 Ecol Monogr 47: 337.

NOTE  % of what is consumed that is passed through the gut epithelium into the body. In the past, and through international agreement (IBP: International Biological Productivity) the process of absorption was considered to be separate from assimilation, but nowadays the 2 are often used synonymously. In the original IBP definition, assimilation is the incorporation of absorbed nutrient materials into cellular components of the body. As an example, nitrogen fixation in plants is the assimilation of nitrogen into organic cell substances of the plant's body. In the ODYSSEY, these 2 processes are separated

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Research study 4
Not surprisingly, food availability seems to be a prime correlate of degree of aggregation in sea urchins.  Comparison of 2 populations of red urchins Strongylocentrotus franciscanus in La Jolla, California and La Bufadora, Mexico shows that the population with abundant food (California) is more aggregated than the population with limited food (Mexico).  Russo 1979 J Biogeogr 6: 407. photograph of several red urchins eatin a piece of kelp

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Research study 5

photograph of several red urchins eating the float of a bull kelp Nerocystis luetkeana The photo shows a red sea-urchin Strongylocentrotus franciscanus eating the float of a bull kelp Nereocystis luetkeana.  Also eaten by this sea-urchin species are kelps such as Macrocystis, Egregia, and Pterygophora, as well as various red algae.  Red sea urchins, sometimes in the company of green ones, will commonly crawl up kelp stalks until their sheer weight of numbers bears the kelp to the sea bottom.  This does not appear to be organized  “cooperativity” – just the combined mass of many hungry animals.

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Research study 6


Sea urchins Strongylocentrotus spp. in Torch Bay, Alaska normally feed on kelps (Laminariales).  At densities of urchins reaching 250 . m-2 most algae are eaten, leaving “barren grounds”.  However, in one summer (1977) the kelps began to increase in number, despite a slight increase at the same time in density of sea urchins.  The author explains the increase as a temporary switch in diet of the urchins from kelps to an unusually rich bloom of diatoms and an enormous incursion of salps Salpa fusiformis into the area.  Both organisms probably responded to warmer water temperatures that occurred at that time.  The author notes that between late May-early June, salps comprised 66% of the diet, and no macroalgae were seen being consumed.  Interestingly, during this period the 2 most abundant kelp species Nereocystis luetkeana and Alaria fistulosa grew large enough to attain size refuge from the sea urchins.   Duggins 1981 Limnol Oceanogr 26: 391.

NOTE  the author does not indicate the units of abundance used on the graph for the algae, but it is assumed to be number of individual plants (note that only 3 species of algae are featured in the graph out of several monitored)

NOTE  this species of pelagic tunicate is rarely seen in Alaskan coastal waters. Densities of Salpa can reach 300 individuals . m-3 in the mid- to upper-water column and 5-10 times that closer to the sea bottom

graphs showing rebound of brown kelps in Torch Bay, Alaska when the resident population of red urchins Strongylocentrotus franciscanus turned temporarily to a diet of salps
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Research study 7
photograph of a tidepool with many purple urchins sheltering in depressions Purple sea-urchins Strongylocentrotus purpuratus, especially those living in protective hollows in sandstone, catch and eat bits of algae that pass by in the waves and currents.  The tiny juveniles tend to eat coralline algae and other surface scrapings. Then, at an age of about 50d and a test diameter of 1mm, they shift to an adult diet of fleshy macroalgae. A comparison of growth of purple sea-urchins in 2 habitats around Santa Barbara shows that growth in kelp-beds is 6-7 times faster than in coralline-algae "barren" areas, presumably because of the greater availability of fleshy macroalgae as food. Rowley 1990 Mar Ecol Progr Ser 62: 229.
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Research study 7.1

histograms comparing consumption of adult vs. juvenile tissues of 8 species of brown algae by purple urchins Strongylocentrotus purpuratusDo food preferences differ for west-coast invertebrate algivores with age of their seaweed foods? This is tested for purple sea urchins Strongylocentrotus purpuratus by researchers at Hatfield Marine Science Center, Newport, Oregon in cafeteria-style feeding experiments using juvenile and adult tissues of 8 species of brown seaweeds.  Results, somewhat unexpectedly, show that for most algal species, juvenile tissues are not preferred over adult tissues (see histograms).  However, note that the sea urchins do show significant preferences for certain seaweed species, but this is true only for adult tissues, not for juvenile tissues. Van Alstyne et al. 1999  Mar Ecol Prog Ser 180: 179.

NOTE  the authors perform parallel experiments with 3 other herbivorous species, the snails Lacuna porrecta and Chlorostoma funebralis, and isopods Idotea wosnesenskii but, as similar results are obtained for all, only the sea-urchin data are considered here

NOTE  seaweeds tested include 6 species of kelps Alaria marginata, Costaria costata, Egregia menziesii, Hedophyllum sessile, Lessoniopsis littoralis, and Nereocystis luetkeana, and 2 species of fucoids Fucus gardneri and F. spiralis



White bars in each set represent mass change ofseaweeds in the
absence of sea urchins. These CONTROL values are subtracted from
each corresponding experimental value before statistical analysis

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Research study 7.2

Although species of the sulphuric-acid secreting brown alga Desmarestia may be eaten by sea urchins, they are not a usual source of food except when hungry.  A study by a researcher from the University of California, Santa Cruz on seaweed community dynamics on subtidal rock pinnacles at Shemya Island, Alaska finds that the brown alga Desmarestia viridis (another congenor D. ligulata is also present in the habitat, but is less dominant than D. viridis) seems to protect itself by physically restricting the movement of grazing sea urchins Strongylocentrotus polyacanthus.  Experimental removals of the alga from pinnacle-top locations leads to an incursion of urchins.  When plastic strips that imitate the swishing movement of Desmarestia are positioned on the pinnacles, they also inhibit sea-urchin movement (or are simply not recognized as food).  Clod cards placed within the range of motion of seaweeds are also eroded, suggesting to the author that it is the physical abrasion on the spines of the urchins by the blades of Desmarestia that protects the alga from grazing activities of the sea urchins.  The author concludes that this particular plant-herbivore interaction plays a strong role in structuring benthic marine communities on the island.  Konar 2000 Oecologia 125: 208.

histogram comparing attractiveness of various algae to sea urchins Strongylocentrotus polyacanthus in AlaskaNOTE  members of the genus Desmarestia manufacture sulphuric acid and store it in vacuoles in its tissues, presumably for defense from herbivores (see Research Study 8 below) .  The author notes this in passing, but for some reason dismisses the notion that it may be the acidity of the alga’s tissues that is actually doing the inhibiting. Sea urchins that eat Desmarestia spp. end up with badly eroded teeth. 

NOTE  these consist of an ice-cube-sized piece of plaster of Paris with added latex paint glued to a piece of flat plastic.  Its rate of loss gives a measure of erosion from water movement or, in this case, abrasion by moving seaweed fronds

Although tufts of living D. viridis are consumed by food-deprived sea
urchins S. polyacanthus in “barrens” areas near the pinnacles, a field
test of “feeding preferences” by the sea urchins shows that neither
species of Desmarestia is preferred more than "no algae" controls . 
The author does not actually measure consumption rates, but only whether
one algal species or another attracts sea urchins to crawl on it

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Research study 8

histograms comparing amounts of algae eaten of several species by fed vs. starved sea urchins Strongylocentrotus droebachiensisEvery collector of algae is wary of gathering samples of red alga Desmarestia spp. in a bucket, for several or all species are rich in sulphuric acid and moments after collecting the alga turns to pulp.  The function of the acid, which can reach pH’s as low as 2, is unknown, but is generally thought to be defensive.  Tests at Shannon Point Marine Center, Washington show that well-fed green sea-urchins Strongylocentrotus droebachiensis prefer Desmarestia munda significantly less than 4 other species of brown algae in multiple-choice feeding experiments. In fact, D. munda is eaten only by urchins that have been starved for several weeks. Note in the histograms that the algae most preferred are the browns Nereocystis luetkeana, Laminaria saccharina and, to a lesser extent, Costaria costata. Pelletreau & Muller-Parker 2002 Mar Biol 141: 1.

photograph of green and red sea urchins, Strongylocentrotus droebachiensis and S. franciscanusNOTE  the authors report a dry mass content of sulphuric acid in D. munda of 16%

NOTE  anecdotal reports tell of the teeth of the Aristotle’s lanterns of sea urchins feeding on acidic species of Desmarestia being noticeably rounded off

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Research study 9

graph showing availability of attached food algae with depthAround Friday Harbor Laboratories, Washington, as in other west-coast regions, red urchins Strongylocentrotus franciscanus live in shallow to deep subtidal regions.  In shallow areas they feed mainly on attached algae, while in deep areas they subsist mainly on drift algae (see graph showing absence of attached algae at depth).  The extent to which shallow kelp beds provide food for the deeper populations is investigated  by examining drift-macrophyte availability to, and diets and fecundity of, individuals at >20m.  Results of direct feeding observations and gut analyses show that perennial kelps are the main dietary items, particularly the shallow subtidal species Saccharina subsimplex, but also including Costaria costata and S. latissima (and even some bryozoans).  As for potential fecundity, gonad indices of deep-dwelling individuals are not significantly different from shallow-dwelling ones (seasonal maxima of 9-11% in Dec-Feb/Mar in both populations at 5 sites).  The authors provide data useful in the management of harvestable populations of photograph of red urchins Strongylocentrotus franciscanus at depthred urchins.  Britton-Simmons et al. 2009 Aquat Biol 5: 233; for comparative work in California on the same species, see LEARN ABOUT SEA URCHINS: FISHERIES & HARVEST REFUGIA: RESEARCH STUDY 2

NOTE  the researchers use the term detritus for these floating bits, regardless of their size.  However, this term is more commonly used to describe dead organic matter of fine particulate size along with its complement of microorganisms such as bacteria, fungi, and actinomycetes

NOTE  measured as live gonad mass/total live body mass

Red urchins Strongylocentrotus franciscanus occupying
what essentially is a "barrens" area at about 20m depth

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  Research study 10
  histogram showing dietary items of red urchins Mesocentrotus franciscanus in rocky areas of San Juan Island, WashingtonAs part of a larger study on urchin-chiton interactions in subtidal rocky areas of San Juan Island, Washington, researchers at Friday Harbor Laboratories analyse gut contents of red urchins Mesocentrotus franciscanus. Chief dietary items are several macroalgae and a colonial ascidian Metandrocarpa taylori, with smaller representation from a variety of sessile invertebrates and even a few motile ones (see histogram). The ascidian is noted by the authors to be the most common sessile invertebrate in the habitats occupied by Mesocentrotus. Elahi & Sebens 2012 Mar Ecol Progr Ser 452: 131.
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