Predators & Defenses
photograph of a sea star Pycnopoida helianthoides withdrawing from contact with the tentacles of a burrowing anemone Pachycerianthus fimbriatus

Predators of sea anemones include nudibranchs, sea stars, and fishes. The principal line of defense for anemones is their nematocysts. Little is known about the effect of nematocyst toxins on larger invertebrates, but those with sensitive skin, such as sea stars, tend to shy away from contact. Thicker-skinned forms such as crabs and other crustaceans seem generally to be resistant.


Juvenile sunflower star Pycnopodia helianthoides
avoids being stung by a burrowing anemone
Pachycerianthus fimbriatus

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  Predators are considered in this section, and DEFENSES in another.
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Opisthobranchs & other snails

  Predators such as opisthobranchs & other snails, sea stars, and fishes on sea anemones are considered in this section. A short additional section on predators of red tree-corals Primnoa pacifica is included as Research Study 6 below.
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Research study 1
photograph of an aeolid nudibranch Aeolidia papillosa with a possible prey anemone Metridium senile
Aeolidia papillosa with a small plumose anemone Metridium senile 1X

The nudibranch Aeolidia papillosa is found on both Pacific and Atlantic coasts where it eats different types of sea anemones.  Nematocysts seem not to be effective in defense of aggregating anemones Anthopleura elegantissima against Aeolidia, as the tentacles tend to withdraw and the acrorhagi, so important in inter-clonal aggression, are not employed.  A much more aggressive defense, however, is employed by plumose anemones Metridium senile against AeolidiaMetridium responds by extending its acontia from its mouth and through pores in the body column.  The filaments are endowed with highly potent nematocysts, are ciiated, and can crawl about on the anemone’s surface.  Contact with the filaments may cause the nudibranch to withdraw or, if it becomes entangled and stung, may cause it to lose cerata.  In preference tests, Metridium senile is least preferred as food by Aeolidia over other species of anemone, perhaps for reason of its defenses.  Edmunds et al. 1976 J Mar Biol Assn UK 56: 65; Waters 1972/73 Veliger 15: 174.

NOTE  more about Aeolidia and its sea-anemone prey can be found in LEARN ABOUT NUDIBRANCHS: FOODS & FEEDING

NOTE  acontial nematocysts are also much larger than tentacle nematocysts. Acontia will be considered in more detail in the section on DEFENSES


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Research study 2


photograph of a close view of the aeolid nudibranch Aeolidia papillosa about to eat an anemone Metridium senile

Laboratory observations on Aeolidia papillosa preying on Anthopleura elegantissima in San Juan Islands, Washington show that a single, 2.5cm long individual can consume 3 anemones per week over a 4-wk period.  At this rate, such an individual could consume about 80 anemones per year (assuming a 6-mo feeding time) and the author of the study suggests that predator density would not have to be great to have a significant effect on the anemone population.  For example, at a division rate of 17% measured in populations in San Juan and Tatoosh Islands, Washington densities of Aeolidia would need be only about one per 470 anemones to consume all newly cloned offspring.  Yet, the author notes that for some reason the real densities of the nudibranch predator are far below that figure in sites being monitored (Aeolidia is found only rarely on wave-exposed coasts). Sebens 1983 Ecol Monogr 53: 405.



A nudibranch Aeolidia papillosa about to eat a
sea anemone Metridium senile in the lab 5X

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Research study 3

Studies in Plymouth, England using resident Aeolidia papillosa and imported west-coast anemone prey Anthopleura elegantissima reveal the following behaviour: the nudibranch attacks, gets stung, retreats, attacks again, and begins to feed. Following this, the anemone pulls in its tentacles, inflates, rises up, and begins to crawl away.  The crawling is slow (2-4 cm. h-1), and involves about 6-8min between successive pedal waves. Eventually the sea anemone detaches and floats away.  The locomotory movements themselves may cause some of the column inflation.  Edmunds et al. 1976 J Mar Biol Assoc UK 56: 65.


drawings illustrating attack by an aeolid nudibranch Aeolidia papillosa on a sea anemone with times of different phases

Sequence of attack by Aeolidia and movement away by the prey. Note the
pedal wave commencing at 90sec and lasting for approximately 280sec.
A second pedal wave is just beginning at 370sec. Arrows show
direction of movementof the anemone's body column

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Research study 4
photograph of close view of predatory nudibrach Aeolidia papillosa eating an anemone Metridium senile In the Gulf of Maine a common prey anemone of Aeolidia papillosa is Metridium senile.  The anemone defends itself by extrusion of acontia through pores in its body wall.  These entangle with the cerata of Aeolidia resulting sometimes in death.  Laboratory studies where Aeolidia are offered large groups of mixed sizes of Metridium as prey show that preference by Aeolidia is principally for smaller-sized anemones.  Acontial nematocyst defenses in small anemones are much less potent than in larger anemones.  Small anemones are attacked and eaten in their entirety.  Large anemones are attacked and lose some tissue mass to the predator, but usually survive.  The author suggests that this size-related predation by Aeolidia skews the distribution of natural populations of M. senile to the large end of the size range.  Harris 1986 Veliger 29: 38.

Close view of Aeolidia papillosa eating a small Metridium senile (anemone is
on the upper Left). Note the presence of acontial threads from the anemone,
some of which have attached themselves to the expanded pharynx of the
nudibranch (the bulbous white structure pressed onto the anemone) 8X

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Research study 5

two specimens of opisthobranchs Pleurobranchaea california mating at a depth of 265m courtesy John Butler, Southwest Fisheries Science Center, California and NOAAThe notaspidian Pleurobranchaea californica inhabits deep water off the west coast from Oregon to San Diego and may feed on sea anemones.  One specimen collected off the Farallon Islands, California of 35cm in length is noted to have eaten a medium-sized Anthopleura elegantissima in the laboratory in 10min.  Chivers 1967 Proc Calif Acad Sci (Ser 4) 32: 515.


Two specimens of Pleurobranchaea californica mating at a depth of 265m.
The animals are positioned with R sides opposing. The filamentous structures
are the large ctenidia, projecting outwards from the R sides ("side-gilled slugs")
and the peni can be seen between the partners 0.25X
Photograph, from a ROV,
courtesy John Butler, Southwest Fisheries Science Center and NOAA

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Research study 6

Researchers at Auke Bay Laboratories, Alaska describe deep-water “thickets1” of gorgonians Primnoa pacifica2 in areas of the Gulf of Alaska that are unique in their density and size, and in the habitat they provide for other organisms, not least of which are their own predators. These last include nudibranchs Tritonia diomedea, more commonly known for eating sea pens Ptilosarcus gurneyi in shallow-water habitats, and at least 2 other taxa of predators, snails Akoya platinum3, and seastars Hippasteria phrygiana (see photographs below). Stone et al. 2015 ICES J Mar Sci 72 (3): 900. Photographs courtesy the authors.

NOTE1 observations of thickets at 70-255m depth are made using a submersible

NOTE2 also known as “red tree corals”, these alcyonacean gorgonians are not corals and should not be confusedly referred to as “corals”. Like in similar areas in other parts of Alaska and British Columbia inhabited by Primnoa pacifica, these populations in the Gulf of Alaska are threatened by fishing gear-induced trauma that not only damage or destroy them outright (the authors note that up to 30% of colonies in some areas show such damage), but may make them more susceptible to invertebrate predators. The authors join like-minded colleagues elsewhere in Alaska and British Columbia who are advocating for Marine Protected Area status for this unique species

NOTE3 no information on this species seems to be readily available online, other than they are in the Family Calliostomatidae

photograph of gorgonian Primnoa pacifica showing damage from fishing gear photograph of gorgonian Primnoa pacifica being preyed upon by the nudibranch Tritonia diomedea photograph of gorgonian Primnoa pacifica being preyed upon by calliostomatid snails Akoya platinum photograph of gorgonian Primnoa pacifica being preyed upon by a spiny seastar Hippaasteria phrygiana
Damage to a colony of Primnoa pacifica from long-line fishing gear at 193m depth A nudibranch Tritonia diomedea creates visible damage to a colony at 208m Note the bare stalks created by these calliostomatid snails Akoya platinum 224m Extensive damage from grazing by a spiny seastar Hippasteria phrygiana 207m
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Sea stars

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Research study 1
  photograph of leather star feeding on sea anemones Metridium senile and hydroidsLeather stars Dermasterias imbricata primarily eat sea anemones, corallimorpharians, holothurians, and sea pens, although the precise make-up of their diet depends on geographical location.



Dermasterias imbricata wrapped around
a cable covered with sea anemones
(Metridium sp.) and hydroids 0.3X

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Research study 1

Studies in Oregon show that mosshead sculpins Clinocottus globiceps uniquely prey on sea anemones, with both Anthopleura elegantissima and Epiactis prolifera being preferred dietary species.  In some localities up to 40% of the fish’s diet is made up of sea anemones.  In that the fishes more often take bites out of the anemones rather than engulf them whole, and in that these bitemarks usually heal, the sculpin, in one point of view, could be considered as “grazing” on a renewable resource (the anemones) or, in another, to be parasitic on them. Yoshiyama et al. 1996 J Exp Mar Biol Ecol 204: 23.

NOTE apparently no other cottid species relies so heavily on sea anemones as food

A sculpin rests near some anemones Epiactus sp. 0.75X

photograph of a sculpin Clinocottus sp. resting near some anemones Epiactus sp.
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Research study 2
  There is no hesitation by mosshead sculpins Clinocottus globiceps to touch or bite the nematocyst-bearing tentacles of its prey, but this is apparently not a process of adaptation as seen in anemonefishes to a new host anemone, that chemically modifies the fish’s mucus to prevent the new host’s nematocysts from discharging.  In the case of the sculpins, it is not known whether the anemone’s nematocysts do not discharge or, if they do discharge, whether the fish is tolerant or resistant to the toxin.  Yoshiyama et al. 1996 J Mar Biol Assn UK 76: 793.
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Research study 3

As noted in the foregoing Research Study, nematocysts of Anthopleura elegantissima are no obstacle to tentacle-eating depredation by mosshead sculpins Clinocottus globiceps.  Interestingly, the sculpins prefer tentacles that contain symbionts and, of the 2 types of symbionts, they prefer zooxanthellae over zoochlorellae.  In experiments at Shannon Point Marine Center, Washington, 26 sculpins consume 629 tentacles from anemones in table of preference data of sculpins for different types of sea anemones Anthopleura elegantissimaa 1.25:0.83:0.77 ratio of zooxanthellate, zoochlorellate, and asymbiotic anemones, respectively. Later analyses of feces from the fishes shows that zooxanthellae are completely digested, while zoochlorellae pass through the guts of the fishes undigested.  The sculpins find symbiont-containing tentacles generally to be more tasty (or perhaps more visually attractive) and, by the greater proportion eaten, seem to find zooxanthellate tentacles to be more nutritious.  The authors remark that the greater preference of the sculpins for zooxanthellate anemones suggests that zoochlorellae confer a previously unknown associational benefit to the host, that of protection from being eaten by sculpins.  However, they don’t really, because for maximal avoidance of being eaten, one could argue that it is actually better for an anemone to be without any symbionts at all.  Augustine & Muller-Parker 1998 Limnol & Oceanog 43: 711.

NOTE each fish is allowed to feed on 7 anemones over a 6-h period (3 zooxanthellate, 3 zoochlorellate, and 1 asymbiont).  The reason for the imbalanced ratio is that it is harder to find asymbiotic (white-coloured) anemones than the other two types.  The "expected" values are the number of tentacles expected to be consumed if there is no selection by the fishes (based on the ratio of anemone-types presented to each fish: 3:3:1). Each fish on average eats 4.3 zooxanthellate tentacles, 2.8 zoochlorellate tentacles, and 2.7 asymbiontic tentacles, or about 10 in total, over a 6-h experimental feeding period

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