Predators & defenses
 

photograph of a red urchin Strongylocentrotus franciscans dead from disease or other causesMany different types of animals prey on adult sea urchins, including birds, sea otters, fishes, crabs, lobsters, octopuses, and sea stars.  Eggs and larvae of sea urchins are eaten by many types of carnivorous zooplankters and by suspension-feeding invertebrates including hydroids, sea anemones, and mussels, just to name a few.  

This section considers predators of west-coast sea urchins, while defenses are dealt with in 3 other sections: i) HIDING/SHELTERING/COVERING, 2) SPINES, and 3) PEDICELLARIAE.

 

The demise of this red urchin Strongylocentrotus
franciscanus
may have begun with fungal or other infection
around the spines in the upper-right part of the test 0.4X

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  Predators
  This section on predators is divided into zooplankters, sea anemones, sea stars, lobsters, fishes, birds, and sea otters.
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Zooplankters

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Research study 1
 

While eggs of echinoderms are rich in organic matter and make a good meal for planktonic larval carnivores (including fishes), because it is early in the season when the sea urchins spawn, many such predators are not yet on the scene and survival of the larvae is favoured. Starr et al. 1990 Science 247 (4946): 1071. Drawings of carnivorous decapod larvae from Hardy 1956 The Open Sea Its Natural History: Part I, The World of Plankton Collins, London.

NOTE  an echinoderm egg has more than 5 times the organic matter of a diatom of the same size.  Strathmann & Vedder 1977 Mar Biol 39: 305.

drawings of several carnivorous decapod larvae courtesy Hardy 1956 The Open Sea Its Natural History: Part I, The World of Plankton Collins, London
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Research study 2
 

Nonetheless, there are many other permanent members of the zooplankton that prey on the larvae, including medusae, ctenophores, chaetognaths, filter-feeding fishes and, later in the season, zoea larvae of crabs.  Zoeae are fast-moving voracious predators, and readily eat the early developmental stages of sea urchins and sand dollars.  Laboratory studies at the Bamfield Marine Sciences Centre, British Columbia on predatory activity of zoeae of shore crabs Hemigrapsus nudus, however, show that the 4-, 6-, and 8-armed larval stages of Strongylocentrotus franciscanus and S. purpuratus are consumed much less readily.  Part of this may relate to protection conferred by the arm spines in the later-stage larvae, but more important than this may be the ability of the plutei, especially 8-armed ones, to reverse swimming direction abruptly.  This behaviour carries the larvae out of the capture sphere of the zoea.  The authors suggest that defensive chemicals are not involved because it is unlikely that the plutei would contain them, as it is known that the earlier stages do not.  Rumrill & Chia 1985 p.333 In, Echinodermata (Keegan & O’Connor, eds.).  AA Balkema, Rotterdam.

NOTE a crab zoea is featured centrally in the set of drawings in Researh Study 1 above

histograms showing predation rates of zoeae of shore crabs Hemigrapsus nudus on pluteus larvae of sea urchins photograph of a shore crab Hemigrapsus nudus courtesy Iain McGaw, U Nevada
Shore crab Hemigrapsus nudus 1X Photo courtesy Iain McGaw, U Nevada
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Sea anemones

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Research study 1
 
photograph of a red urchin Strongylocentrotus franciscanus being eaten by a sea anemone Urticina sp.

Sea anemones are known to eat sea urchins, but it is not as common a source of mortality as some of the others featured in this section. An injured urchin would likely be more susceptible to being eaten by a sea anemone than an uninjured one.

 

 

Sea anemone Urticina sp. about to consume a
red urchin Strongylocentrotus franciscanus 0.4X

 

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Sea stars

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Research study 1
 

Sunflower stars  Pycnopodia helianthoides are known predators of sea urchins, especially of juveniles that can be swallowed whole.  In paired feeding-choice experiments at Bamfield Marine Sciences Centre, British Columbia juveniles of red urchins Strongylocentrotus franciscanus are selected 100% over juveniles of green urchins S. droebachiensis. Nishizaki & Ackerman 2007 Mar Biol 151: 135.

NOTE  12-13mm test diameter

photograph of a red urchin being approached by a sunflower star Pycnopodia helianthoides
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Research study 2
 

Rose stars Crossaster papposus will eat sea urchins whole.  After a day or two the spines and photograph of a leather star Dermasterias imbricataother hard bits of the prey are egested from the mouth. photograph of a rose star Crossaster papposus

In Pt. Loma, California observations of 437 leather stars Dermasterias imbricata disclose that 47% are eating purple urchins S. purpuratus.  Mostly, the leather stars catch and digest the sea urchins while the latter are in their protective holes; presumably, in the open, the leather stars would be too slow to catch the faster moving urchins.

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Research study 3
 
map of Torch and Glacier Bays, Alaska

In Torch Bay, Alaska the sunflower star Pycnopodia helianthoides is the only significant invertebrate1predator of sea urchins.  However, not all west-coast species will run from its presence.  If a few dismembered arms of a sunflower star are enclosed in a cloth bag and placed within a mixed assemblage2 of 3 species of Strongylocentrotus in the field, only S. droebachiensis and S. purpuratus will run away. Strongylocentrotus franciscanus stays put3.  This species is not only larger than the other two and can reach size refuge from the predator, but its spines are relatively longer, and this feature may confer extra protection. Duggins 1981 Oecologia 48: 157.

NOTE1  in other areas of Alaska where sea otters are present, sea-urchin populations are decimated

NOTE2  in this area all 3 species are subtidal and reach their maximum densities in the sublittoral fringe, about 2-3m below MLLW

NOTE3  the results shown in the graph represent 3 replicate experiments.  Light-shaded bars in each pair represent time-0 densities of urchins, while dark-shaded bars represent densities 1-h later. The CONTROL experiment employs an empty cloth bag

histograms showing escape responses of sea urchins to the presence of arm-parts of sunflower stars Pycnopodia helianthoides
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Research study 4
 
As noted above, in Torch Bay, Alaska 3 species of sea urchins histograms showing preferred size of sea-urchin prey for sunflower stars Pycnopodia helianthoidesare eaten by sunflower stars Pycnopodia helianthoides. Contact with 2 of them, Strongylocentrotus droebachiensis and S. purpuratus elicits strong escape responses, but contact with the third, S. franciscanus, causes only small individuals to run away.  Note in the graph on the far Right that small individuals of S. franciscanus will move a further distance when stimulated by Pycnopodia than will larger individuals. However, once S. franciscanus reaches a test diameter >10cm they enter size refuge. Note in the histograms on the near Right that the preferred size of sea-urchin prey for Pycnopodia is 6-8cm test diameter (bottom histogram), suggesting that red urchins larger than this have reached size refuge. The author estimates that it takes about 28h for Pycnopodia  to capture, eat, and digest an urchin in this preferred size range. Duggins 1983 Ecology 64: 1610. graph showing effect of size on distance moved away from threatening sunflower stars by sea urchins Strongylocentrotus franciscanus
 
photograph of a confrontation between a red urchin Strongylocentrotus franciscanus and a sunflower star Pycnopodia helianthoides
Sunflower star Pycnopodia helianthoides approaches a red urchin Strongylocentrotus franciscanus 0.15X
photograph of a confrontation between a red urchin Strongylocentrotus franciscanus and a sunflower star Pycnopodia helianthoides
Red sea urchin crawls over a quiescent sunflower star 0.15X
photograph of a confrontation between a red urchin Strongylocentrotus franciscanus and a sunflower star Pycnopodia helianthoides
Sea star Orthasterias koehleri tangles with a red urchin 0.2X
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Lobsters

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Research study 1
 

map of the Channel Islands, California showing study sites for investigation of effects of lobster predation on sea urchinsIn the Channel Islands of southern California spiny lobsters Panulirus interruptus are important predators of purple sea-urchins Strongylocentrotus purpuratus and, to a lesser extent, of red sea-urchins S. franciscanusphotograph of diseased sea urchin Strongylocentrotus franciscanusAnalysis of a 20-yr data-set on kelp-forest communities in the Channel Islands National Park by a researcher at the Marine Science Institute, UC, Santa Barbara shows that historically the kelp forests, urchins, and lobsters have  likely existed in trophic inter-relationship.  Lobsters subsist on sea urchins, and the sea urchins subsist on kelp.  If lobsters are fished out, the sea-urchin populations increase to the extent that they over-graze the algae and the urchins sometimes starve.  On occasion, as in 1992, densities of sea urchins reach levels high enough to promote disease, and their numbers drastically decline. The disease thus acts as a density-dependent mortality source.  The work shows how over-fishing of a top predator can indirectly favour disease transmission in a prey population.  Lafferty 2004 Ecol Applications 14: 1566.

NOTE  the disease, possibly Vibrio sp., causes dark blotches to appear on the test, accompanied by spine loss and tissue damage

 

Diseased red sea-urchin
Strongylocentrotus franciscanus

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Fishes

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Research study 1
 

effect of predatory sheephead fishes on population densities of red urchins Strongylocentrotus franciscanusmap showing location of San Nicolas Island off the coast of southern CaliforniaSea urchins Strongylocentrotus franciscanus at San Nicolas Island, California are eaten by sheephead fishes Semicossyphus pulcher.  Although sea urchins rank only 7th in relative importance in the diet of sheepheads at San Nicolas, if the fishes are removed en masse from experimental areas the sea-urchin population responds with 26% increase in numbers in a single year.  No change is noted in nearby control sites. Based on density of sheepheads (185) and number of sea urchins eaten per day (22), the author calculates that about 8000 urchins are eaten per year per hectare by the resident sheephead population.  Sea-urchin counts of 31,000 per hectare means that sheepheads consume about one-quarter of the standing crop of urchins each year.  Absence of sheephead fishes in the experimental plots allows the sea urchins to emerge from protective crevices and pockets to roam graph showing effect of presence of sheephead fishes on behaviour of red urchins Strongylocentrotus franciscanusabout more openly. Sea-urchins placed in the open in the CONTROL site either quickly move into shelters or suffer immediate attack by sheepheads.  Cowen 1983 Oecologia 58: 249.

NOTE  the fish are removed by spearing, photograph of sheephead fish Semicossyphus pulcher courtesy John Snow via mexfish.com220 initially, and 10-20 bimonthly thereafter over the 2yr duration of the experiment.  Analysis of stomach contents of these fishes indicates that brachyuran crabs are most important prey items, followed in order by barnacles, polychaetes, bivalves, mole crabs, amphipods, sea urchins, gastropods, and shrimps.

 

Sheephead. Note the strong
jaws and sharp teeth for
consuming shellfishes and urchins.

Photo courtesy John Snow mexfish.com

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Research study 2
 
West-coast sea urchins are eaten by several other species of fishes including, most notably, wolf eels Anarrhichthys ocellatus, shown here with a red urchin in its mouth (Left photo), and kelp greenlings and striped perches, shown here with a "staged" (i.e., broken by SCUBA divers) red urchin as food (Right photo) photograph of a wolf eel Anarrhychthys ocellatus eating a red urchin Strongylocentrotus franciscanus "staged" photograph of kelp greenling and striped perches eating a red urchin Strongylocentrotus franciscanus
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Birds

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Research study 1
 

photograph of a sea-urchin Strongylocentrotus sp. eaten by a birdGlaucous-winged gulls Larus glaucescens in the western Aleutian Island, Alaska eat primarily shallow-dwelling marine invertebrates.  Examination of over 2300 regurgitated pellets at 5 locations in the Islands reveals that remnants of sea-urchins Strongylocentrotus polyacanthus are present in 78% of the diets of gulls at 2 of the sites.  Large individuals are dropped from the air onto a hard surface to crack the shells, while small individuals are swallowed whole.  The author comments that at one of 3 sites where sea urchins are not well represented in the diet, sea otters are common.  This suggests that gulls and sea otters may compete indirectly for a common food resource.   Trapp 1979 Wilson Bull 91: 412.

NOTE  at one of the 3 sites only a relatively few pellets are collected; hence, the competition idea is made more convincing

 

 

Bird-killed purple sea-urchin
Stronglylocentrotus purpuratus
1.2X

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Research study 2
 

histogram showing dietary preferences of glaucous-wing gulls Larus glaucescens in different areas of the western Aleutian Islands, AlaskaGlaucous-wing gulls Larus glaucescens feed on many species of marine invertebrates, but the prey available to them is related to the state of the tide.  Observations on foraging by these gulls at 3 sites in the western Aleutian Islands, Alaska show that at neap tides mussels Mytilus tossulus living in the higher intertidal zone comprise most of the gulls’ diet (see histogram, spotted bars). photograph of gulls Larus glaucescens

 

At spring tides, however, these prey are abandoned in favour of sea urchins Strongylocentrotus polyacanthus, chitons Katharina tunicata, and limpets Lottia spp. that dwell in the lower intertidal zone (histogram: black and white bars). Gulls tend to select the larger individuals of a given prey species, thus maximising energy gain.  Where sea otters Enhydra lutris are abundant and sea-urchin stocks depleted, the gulls switch their preferences to even more diverse prey resources and, under the most intense sea-otter predation, to offshore fishes.  Observation of gulls preying on sea urchins reveals that urchins are swallowed whole if small, or the urchin’s test fractured by  beak-strikes or air-drops if large.  Irons et al. 1986 Ecology 67: 1460.


Glaucous-wing gulls Larus glaucescens
of mixed ages take to flight

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Research study 3
 
Remains of urchins Strongylocentrotus purpuratus at a “bird anvil” in Barkley Sound, British Columbia. Known bird predators of sea urchins in British Columbia and Washington are glaucous-wing gulls Larus glaucescens, black oystercatchers Haematopus bachmani (photo on Right), and northwestern crows Corvus courinusWooton 1997 Ecol Monogr 67: 45. photograph of bird-killed urchins Strongylocentrotus purpuratus at a "bird anvil" in Barkley Sound, British Columbia photograph of a black oystercatcher Haematopus bachmani
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Research study 4
 

histogram showing incidence of prey-stealing between western gulls foraging on purple sea urchinsPurple sea-urchins Strongylocentrotus purpuratus in the Palos Verdes area of southern California are intensely preyed upon by western gulls Larus occidentalis.  A 2yr study by researchers from California State University, Long Beach show that the gulls’ predation  accords with optimal foraging theory; that is, most yield is gained for least effort.  Results show that prey selection is largely based on size/age of the gull, whether predatory behaviour involves pecking or dropping, and the group size of the foragers.  With respect to the latter, for example, sea urchins selected by gulls when foraging in larger groups are smaller than when foraging in smaller groups or singly, perhaps because of the greater risk of kleptoparasitism, or stealing of prey by conspecifics (see histogram).  Overall, the gulls strongly prefer sea urchins over snail or sea-star prey and, depending upon time of year, their depredations can significantly impact the size and vitality of local sea-urchin populations.  As evidence for this, based on average population size of gulls in the study area and their daily energy requirements, the authors calculate that over 145,000 S. purpuratus could be consumed annually by gulls out of an estimated base population of 400,000 urchins.  Snellen et al. 2007 Can J Zool 85: 221.

NOTE  the authors equate “yield” with energy, which traditionally has been the popular but not the only currency to use in such studies, and they discuss the limitations of this approach.  Nutritional components must factor heavily into the equation, but are rarely if ever considered perhaps owing to lack of knowledge of nutritional requirements of non-domesticated species of animals.  For some reason the authors intermix the terms “calories” and “joules” throughout their paper, even though the latter is the correct one to use when defining work energy

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Sea otters

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Research study 1
 

photograph of 2 sea otters Enhydra lutris, one of which is eating a green urchin Strongylocentrotus droebachiensisHow many sea urchins can an otter eat?  Observation of 50 sea otters Enhydra lutris feeding in a rocky area near Pacific Grove, California for 8-10h a day over 2mo disclosed the following prey eaten: 5280 red urchins Strongylocentrotus franciscanus, 301 sea mussels Mytilus californianus, and 380 red abalone Haliotis rufescens.  Thus, a minimum estimate for an answer would be 90 urchins per day, with the actual total liHkely being much more than that.  Overall, 120 prey items are consumed by each sea otter per day during the 2-mo observation period.  McLean 1962 Biol Bull 122: 95, citing unpublished data from R. Boolootian. Photograph courtesy Karl Kenyon, Seattle.

 



Sea otter Enhydra lutris eating a green
urchin Strongylocentrotus droebachiensis.

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Research study 2
 

In situ observations of sea otters hunting in kelp forests reveal that the otters do a lot of reaching around rocks and kelp holdfasts, and patting with their forepaws to locate and identify urchins, snails, and other prey.  The author suggests that vision may also be less important than touch when otters are hunting in poor-visibility areas such as mud and sand for buried clams and innkeeper worms Urechis caupoShimek 1977 Cal Fish Game 63: 120.

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Research study 3
  In the Aleutian Islands, Alaska, sea otters Enydra lutris primarily eat sea urchins Strongylocentrotus spp.  Only in waters too deep for the otters to dive do the sea urchins gain refuge from predation.  Where densities of otters are high, sea urchins tend to be rare and kelps, such as Laminaria spp. and Agarum cribosum, are allowed to form luxuriant canopies.  Estes et al. 1978 Ecology 59: 822.
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Research study 4
 

map showing sites in Alaska and Oregon where sea otters are observed feedingInterestingly, where sea otters Enhydra lutris have been in an area for several decades, such as at Amchitka Island, Alaska, they spend a much larger portion of daylight hours foraging (55%) than in areas where they are more recently established, such as at Attu Island, Alaska or at Blanco Reef, Oregon (17%).  The difference is in the relative ease of capturing sea urchins and other macroinvertebrates.  Once these are exhausted, the otters must chase after fishes and these are much harder to catch.  Estes et al. 1982 Amer Nat 120: 242.

NOTE  with the demise of sea urchins, kelps flourish and this leads to greater abundance of associated fish fauna

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Research study 5
  photograph of a sea otter Enhydra lutris in Monterey Bay, California eating some sort of shellfishSurvival of sea urchins is highly dependent upon their size in relation to that of the predator.  For example, in Carmel Bay, California where sea otters have been around for 30yr, the only surviving red and purple sea urchins are those small enough to occupy crevices in the rocks.  Once they exceed a diameter of 2-3cm, however, the otters eat them (30 per feeding bout is not uncommon for a sea otter).  Kenner 1992 Mar Biol 112: 107.
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histograms comparing sizes of sea urchins Strongylocentrotus polyacanthus eaten vs. sizes available in 2 island areas of the Aleutian islands, AlaskaA finding by researchers in Alaska is that sea otters Enhydra lutris in the Aleutian Islands rarely eat sea urchins Strongylocentrotus polyacanthus less than 1.5cm diameter (depending upon area; see graph) This size represents  individuals of about 3yr of age, of an approximate 14-16yr maximum life span.  Note in the graphs that for both areas featured, sea urchins consumed are significantly larger than ones available in the habitat.  The authors note that the likely reason for such size-selective feeding is that while the differential costs to a sea otter in finding, capturing, and consuming different-sized prey are probably negligible, the benefits of selecting large prey are not.  Nutritional and energetic benefits scale as the cube of test diameter.    Estes & Duggins 1995 Ecol Monogr 65 (1): 75.

NOTE  determined from sizes of jaw ossicles obtained from otter scat

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Research study 6
 

A surprising observation by a group of researchers in California is that sea otters Enhydra lutris have strong individual preferences for certain prey.  Data are gathered during 2 time-periods (1983-85 and 1985-90) on the same 10 adult females in the Monterey Peninsula area of California.  Total number dives observed are 17,641 and 7973, respectively, for the 2 periods, and total number of prey items captured exceeds 4600.  A major finding in the data is that while 8 prey taxa are selected overall, sea urchins are preferred by at least 1 female, abalones by 5 (but with 4 different prey types ranking second in frequency of selection), turban snails by 2, and mussels by 1.  Only a single individual appeared to show no particular preference.  A second finding is that these individual preferences are remarkably consistent over the 8yr period of study.  Also of interest is that pups of these 10 females are found to mimic the feeding habits of their mothers, suggesting a matrilineal transmission of dietary preference.  In explanation, the researchers suggest that although sea otters seem to be adept, quick-learners, it may be beyond the learning and performance capacity of any single individual to master the radically different sensory and motor skills necessary for efficient utilisation of more than one or a few different prey types.  Estes et al. 2003 J Anim Ecol 72 (1): 144.

NOTE  surprising only in the sense that no-one seems to have noticed or commented on it before for sea otters, although it is well known for other related predators, including European otters

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Research study 7
 

graph showing frequency of occurrence of prey items in the diets of sea otters Enhydra lutris at San Nicolas Island, CaliforniaStudies on sea otters Enhydra lutris in San Nicolas Island, California reveal that 13 invertebrate prey items make up most of the diet.  Red and purple urchins together comprise 50% of the diet, with the next favoured items being kelp crabs Pugettia sp. at 13%, crabs Cancer spp. at 5%, and turban shells Lithopoma undosa at 4%.  Bentall 2005 Msc Thesis Univ Calif Santa Cruz, 80pp.

 

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Research study 8
 

map showing distribution of sea otters Enhydra lutris kenyoni around the Olympic Peninsula, WashingtonObservation of sea otters Enhydra lutris kenyoni feeding at open-ocean and protected straits around the Olympic Peninsula, Washington (see map) by scientists from Denmark and California provides abundant information on depredation of sea-urchin and other benthic invertebrates.  Some of these data in summary form:

Period of study: 6yr
Number of individual dives by otters: 13,847
Percentage successful prey-capture dives: 77%
Dive durations (mean): 55sec
Surface interval (mean): 45sec (range: 1sec – 32min)
Major prey categories (by proportion of total number eaten):
  sea urchins: S. franciscanus & S. purpuratus…27%
  bivalves: mostly littleneck clams Protothaca staminea…50%

Outer-coast diets favour bivalves, while Strait of Juan de Fuca diets favour sea urchins.  In total, the authors identify 19 prey species, of which turban snails Chlorostoma spp. (hard to crunch?) and gumboot chitons Cryptochiton stelleri (leathery texture?) may be the most unusual.  Now, sea otters are fast eaters, but it is hard to imagine an otter gaining entry into and consuming a large sea urchin, butter clam, Dungeness crab, sea star, or gumboot chiton in just a few dozen seconds as listed above.  Unfortunately, the authors do not list specific portions consumed for any of these large prey or whether tools are used, such as a belly rocks for pounding on, nor do they list actual duration of surface intervals spent feeding on the various prey.  However, they do note that the shortest surface-interval time involves purple urchins (a relatively small sea-urchin species) at 36sec.  One guesses that the otters preferentially eat the energy- and nutrititive-rich gonads first and discard most of the remainder.  Laidre & Jameson 2006 J Mammal 87 (4): 799.

NOTE  the authors remark that sea otters were re-introduced into Washington in 1969-70 after their extermination by fur-hunters in late 1800s through early 1900s

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