Predators & Defenses
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photograph of the dead test of a sea urchin Strongylocentrotus sp.Defenses of sea urchins include hiding/sheltering/covering considered here, and SPINES and PEDICELLARIAE, considered in other sections.



Test of a dead sea urchin
sp. 0.7X

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Research study 1

The first line of defense for a sea urchin is to hide away in crevices or under rocks or, if a juvenile of a certain species, under the sheltering spine canopy of an adult.   Studies in Nova Scotia on survival of different life stages of green sea urchins Strongylocentrotus droebachiensis show that tiny individuals, 3-6mm in diameter, seek shelter from crab and other crustacean predators amid small stones.  At a test diameter of about 30mm, the sea urchins reach a size refuge from most predators save for large crabs and lobsters.  Scheibling & Hamm 1991 Mar Biol 110: 105.

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Research study 2
On the west coast, while juvenile green sea-urchins Strongylocentrotus droebachiensis often shelter under the spine canopies of red urchins S. franciscans as shown in the photograph, they do not normally shelter with conspecific adults. Could the reason for this be that there is more room under the spine-canopies of red-urchin adults? photograph of juvenile green urchins Strongylocentrotus droebachiensis under the spine canopies of 2 adult red urchins S. franciscanus
photograph of green sea urchin Strongylocentrotus droebachiensis showing relatively short spine-lengths

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Research study 3


The first report of juvenile sea urchins sheltering under the spine canopies of conspecific adults seems to have been part of a fisheries study in Point Loma, California.  The researchers were attempting to determine whether commercial fisheries for red urchins Strongylocentrotus franciscanus affects their recruitment, and their research question was whether juveniles are recruited into various microhabitats or into specific nursery grounds.  Their collections of 437 juvenile (<2cm diameter) S. purpuratus and 343 juvenile S. franciscanus reveal that 41 and 92%, respectively, of juveniles of each species drawing of juvenile red urchins Strongylocentrotus franciscanus sheltering under the spine canopy of a conspecificadultshelter under spine canopies.  Strongylocentrotus purpuratus juveniles shelter under adults of the 2 species more or less evenly, but 81% of S. franciscanus juveniles shelter under conspecific adults, while only 11% shelter under S. purpuratus adults. Other habitats occupied by juveniles are kelp holdfasts, rocks, and coralline algae.  The results indicate that S. purpuratus juveniles1 are much more flexible in their recruitment behaviour than S. franciscanus juveniles.  To test the hypothesis that settlement/survival of juvenile S. franciscanus depends upon the presence of conspecific adults, the authors conduct a small-scale “urchin-harvest” experiment.  They remove all commercial-sized2 urchins (leaving smaller S. franciscanus and all S. purpuratus) from experimental plots and leave nearby control plots untouched.  Counts several months later show that the number of juvenile S. franciscanus on the “fished” plots is significantly lower than on the control plots.  The authors’ view is that juveniles benefit from access to kelp food snared and anchored by the adult, in addition to protection.  In support of this they point to the many other animals such as snails, shrimps, crabs, sea stars, and fishes3 also found under spine canopies of urchins and also presumably benefiting from protection.  Tegner & Dayton 1977 Science 196: 324. Photos below courtesy Kevin Lee, Fullerton, California diverkevin.

NOTE1  also, in Torch Bay, Alaska small (<1cm diameter) S. droebachiensis are observed to shelter under the spine canopies of adult S. franciscanus. Duggins 1981 Oecologia 48: 157 

NOTE2  9.5-13cm test diameter 

NOTE3  there are several examples of tropical fishes sheltering in the spines of sea urchins, and one temperate example is known from work at the Wrigley Marine Science Center on Santa Catalina Island. In this case the fish, the blue-banded goby Lythrypnus dalli, not only finds protection within and under the spine canopies of long-spined sea-urchins Centrostephanus coronatus but, in the absence of the urchins, the fishes do poorly. The researchers provide several types of experimental data to show that recruitment, abundance, and survival of L. dalli is causally linked to the presence of the sea urchins. Hartney & Grorud 2002 Oecologia 131: 506.

photograph of blue-banded goby Lythrypnus dalli courtesy Kevin Lee, Fullerton, California Blue-banded goby Lythrypnus dalli 2X photograph of sea urchin Centrostephanus coronatus courtesy Kevin Lee, Fullerton, California
sea urchin Centrostephanus coronatus 0.5X
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Research study 4

photograph of 2 juvenile red urchins Strongylocentrotus franciscanus sheltering under the spine canopy of a conspecific adult

Counts of juvenile Strongylocentrotus franciscanus in areas of British Columbia disclose that about one-third of all juveniles are sheltering under the spine canopies of adults, one-third are outside of the canopy cover but close to the adults, and one-third are apart from the adults.  The question arises as to whether the sheltering is a result of juveniles migrating under the adults after they settle and metamorphose, or a result of differential mortality of juveniles outside of the spine canopies.  Breen et al. 1985 J Exper Mar Biol Ecol 92: 45.

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Research study 5

As part of a study on recruitment of red urchins Strongylocentrotus franciscanus in areas of southern British Columbia, fisheries scientists from the Pacific Biological Station, Nanaimo describe sheltering by juveniles under the spine canopies of adults. Adults tend to provide shelter at test diameters greater than 40mm, with ones of 120-140mm doing most of the sheltering.  Juveniles up to 30mm test diameter are small enough to shelter directly under the tests of adults, but as they grow larger (up to 60mm) they tend to move to positions beneath the spine canopies (see photographs).  Sloan et al. 1987 Fisheries Res 5: 55.  Photographs courtesy the authors.

photographs of sheltering by red urchins Strongylocentrotus franciscanus courtesy Sloan et al. 1987 Fisheries Res 5: 55

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Research study 6


photograph of a juvenile red urchins Strongylocentrotus franciscanus sheltering under the spine canopy of a conspecific adultThe question as to how the juveniles get under the adults is addressed in experiments in California in which settlement and metamorphosis of larval red and purple urchins to different types of habitats is monitored in the laboratory.  For purple urchins Strongylocentrotus purpuratus the percentage of larvae metamorphosing on or near adults (4%) is less than on bare rock (50%) or on rock covered in coralline algae (59%).  Apparently, coralline algae or organic films on them are favoured foods for them as juveniles. For red urchins S. franciscanus histogram showing % metamorphosis of sea urchins Strongylocentrotus onto different substratathe percentage of larvae metamorphosing on or near adults (31%) is less than on rocks on which adults have been sitting (63%) or on rocks distant from adult urchin populations (90%). Thus, there appears to be no preferential settlement of larvae of either species to adults.  As to why the larvae of S. franciscanus settle less on rocks previously occupied by adults than on rocks collected some distance from the adults, the authors suggest that potential bacterial/diatom films may have been scraped away by spine abrasion by the adults sitting there. The authors conclude that the higher densities of juveniles around adults must therefore arise from them being killed out of protective shelter of the adults (after widespread settlement of larvae), or by them migrating to the adults shortly after settlement. Cameron & Schroeter 1980 Mar Ecol Progr Ser 2: 243.

NOTE settling larvae of many or most marine invertebrates are attracted to surfaces bearing bacterial and other films. These films may be eaten by the larvae after metamorphosis or otherwise be indicative of healthy environments.  For example, in the present study the authors show in lab experiments that rocks without films (boiled) are avoided by the settling larvae (0% metamorphosis on cleaned rocks vs. 70% on filmed rocks)

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Protection from predators seems to be the likely function of the sheltering behaviour.  This should be easy to test in experiments, but what other hypotheses may also lend themselves to testing? Identify the good ones from this list then CLICK HERE to see explanations.

The juveniles are seeking shade from light. 

The juveniles are sheltering from waves and currents. 

The juveniles are able to exploit kelp foods that are weighed down and held by the adults. 

The juveniles are expendable and provide a first line of defense against predators, thus increasing survival of the large “breeder” individuals of the population. 

Adults and juveniles are competing for space. 

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Research study 7

Let’s look at the idea mentioned earlier of sheltering being favourable for the juvenile’s nutrition.  Sea urchins are known to be messy eaters, so could the juveniles be obtaining nutritional benefit from algal particulates or even dissolved1 nutrients produced from the feeding activities of the closely-sited adults?   This idea is tested at the Bamfield Marine Sciences Centre, British Columbia in a comparison of growth histogram showing growth of juvenile urchins Strongylocentrotus feeding on different types of food in juveniles of sheltering (Strongylocentrotus franciscanus: 1.4mm diameter) versus non-sheltering (S. droebachiensis: 2.6mm) sea urchins over 2- and 1-mo study periods, respectively.

First, juveniles of both species actually grow more slowly in the presence of adults than in their absence (data not shown here).  The graph on the Right shows that juveniles of both species grow well on Macrocystis iintegrifolia, the same kelp species being eaten by the adults.  As to whether the juveniles can utilise particulate2 kelp as food, the answer is not as well as they can utilise whole kelp (see "ground kelp" in graph). Finally, juveniles of S. franciscanus can utilise to some extent dissolved organic matter3 (see "kelp exudate" in graph: green urchins not tested).  The authors conclude from these and other experiments that sheltering juveniles are out-competed by the larger adults for algae, and do not grow well on particulate algae or on dissolved matter produced from the adult’s feeding activities.  This leaves protection from predators and waves/currents as the most likely function for sheltering.  Nishizaki & Ackerman 2004 Mar Ecol Progr Ser 268: 93.

NOTE1  seawater is rich in dissolved organic matter, including amino acids, fatty acids, and other nutritive substances, and many marine invertebrates can take up this matter directly across their skin.  Much research has been done on the subject of DOM (dissolved organic matter).  Here, the question is whether juveniles could somehow take up and utilize the DOM created by the feeding activities of the adults

NOTE2  created by grinding whole kelp in a food blender

NOTE3  this is actually seawater syringed up from beneath feeding adults and includes some feces.  The treatment involves keeping juveniles in containers with the “exudates” swirling about them

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Research study 8

In a later paper the authors of the above study on juvenile nutrition test jthe notion of protection from predators and water currents with red urchins Strongylocentrotus franciscanus (a sheltering species) and green urchins S. droebachiensis (more of a non-sheltering species) at the Bamfield Marine Sciences Centre, British Columbia.  The graph on the upper Left shows that when exposed to the urchin-eating sea star Pycnopodia helianthoides, 90% of solitary juvenile S. franciscanus are eaten versus 5% of sheltering juveniles. For S. droebachiensis the values are 85 and 75%, respectively. 

histograms showing comparative mortality of sea urchins Strongylocentrotus with and without adults being presentMoreover, in the presence of the predator, more (44%) of juvenile S. franciscanus shelter with adults than in the absence of histograms showing effect of presence of predators on extent of sheltering in juvenile sea urchins Strongylocentrotus spp. the predator (13%). In comparison, sheltering in S. droebachiensis is infrequent and does not seem to be related to predation risk (7% vs. 5% for high and low risk, respectively). 

Finally, with respect to water flow, currents around adult urchins are reduced by >60% (data not shown here), and this is reflected in the proportion of juvenile red urchins sheltering (52% vs. 13% for high and low flow rates, respectively). As histograms showing effect of current-flow rate on extent of sheltering in juvenile sea urchins Strongylocentrotus spp.expected, juvenile green urchins appear to be indifferent to water flow (5% vs. 4% in high and low flow rates, respectively). 

Field observations tend to support the notion that juvenile S. franciscanus shelter with adults more in areas of high current flows.  Sheltering tends to be species-specific, with more S. franciscanus juveniles sheltering under adult conspecifics than under adult green urchins, and vice versa for the relatively few S. droebachiensis juveniles that do shelter.  The authors note that their study provides the first experimental evidence that sheltering protects juvenile red urchins from both predators and water motion.  Nishizaki & Ackerman 2007 Mar Biol 151: 135.

NOTE  surface-area-to-volume ratios would lead to small urchins being swept away in currents more readily than large urchins

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Research study 9

In areas around the Bamfield Marine Sciences Centre, British Columbia, sheltering by juvenile red sea-urchins Strongylocentrotus franciscanus appears to be in response to a pheromone released by the adults in response to the presence of predatory sea stars, most notably, the sunflower star, Pycnopodia helianthoides.  Laboratory studies show that the juveniles are otherwise unresponsive to sea stars held in laboratory flow tanks upstream of them or to adults similarly held upstream.  Adult urchins themselves are also unresponsive to the chemical.  The authors suggest that the behaviour confers a selective advantage for juveniles, allowing them to balance risk of predation versus potential competition with the adults. Ackerman 2005 Limnol Oceanogr 50: 354.

Sunflower star Pycnopodia helianthoides tangled
in the spines of a red urchin S. franciscanus 0.4X

photograph of a sunflower star Pycnopoida helianthoides tangled in the spines of a red urchin Strongylocentrotus franciscanus
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Research study 10

histrograms showing comparative survival of red and purple urchins in the presence of adult conspecific when preyed upon by fishes and spiny lobstersIn Point Loma, California sea urchins Strongylocentrotus franciscanus have 2 main predators, sheepshead fishes Semicossyphus pulcher and spiny lobsters Panulirus interruptus.  The predators have most success with intermediate-sized urchins, 50-80mm test diameter, because smaller individuals (< 40mm test diameter) gain protection by sheltering under the spine canopies of larger urchins (> 90mm test diameter), which are themselves in partial size-refuge from the predators.  The partial size-refuges are effective enough that collections of red urchins in the area, especially from 18m depth (just at the boundary of the kelp forest), show bimodality of size distributions (top set of data in histogram). Similar frequency histograms for collections of purple urchins S. purpuratus in the same area are consistently unimodal (bottom set of data). Purple urchins are short-spined, and juveniles do not shelter, so no one size-class enjoys protection from predators.   Tegner & Dayton 1981 Mar Ecol Progr Ser 5: 255.

NOTE  the authors show data for 4 study sites, of which one set for each species is presented here

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Research study 11

photograph of purple sea urchins Strongylocentrotus purpuratusIn intertidal areas of erodable sandstone on Vancouver Island, British Columbia, burrow densities of Strongylocentrotus purpuratus may reach 150 . m-2, of which about half may be occupied at any given time. Burrow occupation by this species may be for protection against waves or predators, or both. 



A lone red urchin Strongylocentrotus franciscanus
inhabits a shallow burrow-shelter along with
many sheltering purple urchins S. purpuratus

photograph of intertidal area showing purple urchins Strongylocentrotus purpuratus sheltering in depressions
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Research study 12
  photograph of purple sea urchin Strongylocentrotus purpuratus with spine-coverings of debrisIn more wave-exposed intertidal locations, especially on harder substrates that do not permit the excavation of burrows, purple urchins Strongylocentrotus purpuratus are often completely covered with debris, sometimes including still-living snails and hermit crabs. Studies by researchers from University College, Cork in British Columbia show that about 25-50% of purple sea urchins even in burrows also hold bits of rock, shell, algae, and other material close to their bodies using their tube feet.  Verling et al. 2004 Mar Ecol 25: 191.
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Covering is common among sea urchins the world over, and seems to be more prevalent in intertidal locations than in subtidal ones.  What possibilities exist for its function? Consider the answers below, then CLICK HERE for explanations.  Ideas from Verling et al. 2004 Mar Ecol 25: 191; Douglas 1976 Pac Sci 30: 83.

Minimises desiccation. 

Provides visual or tactile camouflage from predators.

Provides a mechanical shielding against predators. 

Reduces displacement by waves or battering in storms? 

Reduces exposure to potentially harmful UV irradiation. 

Enables food to be stockpiled for later consumption. 

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Research study 13

map of study sites for sheltering investigation in red urchinsgraphs showing sizes of red urchins being sheltered and doing the shelteringThe potential recruitment-enhancement effect of sheltering by juvenile red urchins Strongylocentrotus franciscanus by is further investigated in a study at the Pacific Biological Station, Nanaimo, British Columbia.  The authors use 15yr of survey data collected from 3 areas of British Columbia to assess the role of adult-population density on recruitment success.  Results show that sheltering decreases significantly with increasing size of the juvenile sheltering of juveniles by adult red urchins Strongylocentrotus franciscanusand, interestingly, with decreasing density of adults.  With respect to the latter, one would think it would be the other way round.  Conversely, the probability of an adult providing shelter to a juvenile increases with size of adult and greater density of juveniles.  Note in the sample graphs for the Tofino site that the probability of a juvenile being protected falls off with size until, at about 40mm test diameter, it is essentially zero.  Conversely, the probability of an adult providing protection to a juvenile increases exponentially after a test size of about 90mm.  Even so, at this and the other 2 sites, sheltering frequency remains generally less than 15%.  Overall, the authors conclude that adult densities have a positive effect on recruitment. They also note that theirs is the first study to describe probabilities of sheltering in relation to individual size, and juvenile and adult densities. Zhang et al. 2011 Fisheries Res 109: 276.

Three juvenile red urchins Strongylocentrotus
being sheltered by 2 adults 0.7X

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Research study 14

photograph of shore crab Pachygrapsus crassipes sheltering under sea anemoneA research group primarily from Villanova University in Pennsylvania but working at the Universidad Autonoma de Baja California, Ensenada confirm in laboratory experiments that shore crabs Pachygrapsus crassipes readily eat juvenile purple urchins Strongylocentrotus purpuratus. To test the efficacy of spine-canopy protection conferred by adults, the researchers first check that juvenile sea urchins will seek shelter under the spine canopies of adults when introduced into test arenas. They do, and quickly. However, in subsequent experiments using a range of sizes of P. crassipes, sheltering under spine canopies of adults is found to offer non-significant protection from predation. Overall lack of statistical significance owes to "poor" performance of larger crabs. Smaller-sized crabs can reach under the adult canopy, but larger ones seem to have more difficulty. Clemente et al. 2013 Mar Biol 160: 579. Photograph courtesy Rick Anderson, California and Divebums.


Whether it knows it or not, this striped crab
Pachygrapsus crassipes
is sheltering under
the protective "canopy" of a sea anemone 1.5X

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