photograph of sea anemone Metridium senile undergoing pedal laceration

Sea anemones may appear to be peaceful and sedentary, but in some species there is considerable aggression for space, both interspecific and intraspecific - the latter notably between different clones.  The following account considers aggression in several west-coast sea-anemone species.

NOTE  asexual fission and pedal laceration both lead to genetically identical clones that may form clonal aggregations



Pedal laceration in Metridium senile produces another
clone-mate within the aggregation. The "blob" is
undergoing major tissue transformations that will lead in
time to a new individual, genetically identical to its "parent" 0.2X

  black dot
  Anthopleura elegantissima
  black dotResearch study 1
As noted above, all “progeny” from asexual fission in Anthopleura elegantissima are genetically identical clones.  An experiment with A. elegantissima at Pacific Grove, California led to the discovery of clonally related aggression in the species.  The author removed a number of anemones from each of two clonal aggregations and arrayed them in a seawater tray, intermixing individuals from the two clones in alternate order.  After 2wk the anemones re-arranged themselves into 2 discrete groups based on the 2 original clonal aggregations, and contact was once again only between clonemates.  Further laboratory experiments show that juxtaposed clones will create anemone-free zones or strips between them, just as in field populations.  Francis 1973 Biol Bull 144: 64. drawings showing clonal segregation in sea anemones photograph showing clonal segregation of sea anemones Anthopleura elegantissiima in the field
Field population of Anthopleura elegantissima segregated into apparent clonal aggregations with anemone-free zones between them 0.1X

black dotResearch study 2


drawing showing anonism between non-clonemates of Anthopleura elegantissima
The separation between adjacent clones in Anthopleura elegantissima is maintained by agonistic (aggressive) interactions between members of the different clones.  On contact, opposing interclonal “attack” polyps develop aggressive tentacles, or acrorhagi, just below the line of feeding tentacles.  The acrorhagi are packed with dense batteries of potent nematocysts and are used to sting opposing attack polyps.  On aggressive contact either or both opposing polyps rear up, exposing the now-swollen acrorhagi, and then slowly bend over and sting the opponent.  Often the outer skin of the acrorhagi will detach and remain fixed for a time to the other polyp.  The toxin in the acrorhagal nematocysts is so potent that the affected area of the stung polyp will undergo necrosis and rot off.  The affected polyp generally recovers, however, and later may itself be aggressively dominant in a later encounter.  The final result is that the polyps withdraw to “standoff” distance, thus photograph of a sea anemone Metridium sp. collapsed and protruding its acontiacreating the interclonal boundary.  For reasons of “self-self” recognition, clone-mates never fight.  Francis 1973 Biol Bull 144: 73.

NOTE also called "warrior" polyps: special aggressor polyps that live at the boundaries of clonal aggregations

NOTE  lit. “topmost” “grapes or berries” G. acrorhagi occur in a circlet just below the crown of feeding tentacles, can be inflated in size, and are usually are never longer than a feeding tentacle.  They possess large, potent nematocysts.  On application to an aggressor, the distal part of the acrorhagi may peel off and remain attached to the skin and cause necrosis or.  In west-coast anemones acrorhagi are present in Anthopleura elegantissima, A. xanthogrammica, and A. artemesia. In addition to acrorhagi there are 2 other types of “aggressive tentacles” in cnidarians: 1) catch or fighting tentacles, which are the primary or innermost circlet of tentacles around the mouth.  They are thick and blunt at rest, but can be extended many times further than the regular feeding tentacles.  They contain highly toxic nematocysts that on discharge can attach the tentacle so firmly that its tip will break off and cause localised necrosis. Among common occuring west-coast actiniarians, catch tentacles are present only in Metridium senile. In this species they are used in interclonal disputes.  2) a second type is known as sweeper tentacles. These occur only in certain corals and are similar in form and function to catch tentacles.

A third kind of structure used in feeding and defense, the acontium, is found in some sea-anemone species.  The acontia arise from the mesenterial filaments on the free edges of incomplete mesenteries and generally function in quelling the activities of captured prey after ingestion.  In some species of anemones (e.g., Metridium spp. and corals), they may be extruded from pores in the body wall and act in defense.  The acontia contain potent nematocysts. Williams 1991 Hydrobiologia 216/217: 539.

Collapsed Metridium sp. with acontia protruding from the mouth 0.25X

3-D drawings of non-clonemate sea anemones interacting aggressively taken from a video

CLICK HERE to view a 3-D animation of aggression in A. elegantissima. You will see initial contact, rearing up of the dominant individual, inflation of acrorhagi, stinging and applying of acrorhagal cap, and withdrawing of the recessive individual.

NOTE the video replays automatically


black dotResearch study 3

Anemones at the front line of contact of a clonal aggregation are smaller and specialised for combat (so-called warrior polyps), while those within the clone are larger and specialised for reproduction (reproductive polyps).  In one area in California, warrior polyps of a clonal aggregation of Anthopleura elegantissima have much smaller gonads, but almost 3 times the number of acrorhagi, as compared with more centrally located reproductive polyps.  Even with the size difference, staged combats between warrior polyps of one clone and reproductive polyps of an adjacent clone show that warrior polyps still win about 50% of the matches.  Inter-clonal dominance seems to be largely determined by relative allocation of resources to the acrorhagi.  Ayre & Grosberg 1996 Anim Behav 51: 1233. photograph of adjacent clones of sea anemones Anthopleura elegantissima in the field Assuming that these 2 aggregations of Anthopleura elegantissima represent different clones, then warrior polyps would be sited on the clonal boundaries (purple and yellow), while reproductive polyps would be located centrally 0.1X
  black dotResearch study 4
Other observations show that even though members of adjacent clones of Anthopleura elegantissima may be more or less equal in terms of size and acrorhagal allocation per polyp, some clones tend more to attack while others tend more to retaliate.  If this is true, what explains the long-term stability (up to several years) of inter-clonal borders?  Habituation in confronting warrior polyps is one possibility (i.e., they fight less), although this is not evident in laboratory experiments where polyps from adjacent clones always seem to fight.  Combative equality is another possibility; however, as noted, clones are known to vary in level of aggressiveness and a “superior” polyp will mostly dominate in pair-wise (or even "triple-wise") contests. Francis 1976 Biol Bull 150: 361; Ayre & Grosberg 1995 The Amer Nat 146: 427; drawing adapted from Ayre & Grosberg 1996 Anim Behav 51: 1233. drawing of outcome of a single superior warrior polyp interacting with 3 recessive warrior polyps
  black dotResearch study 5
photograph of clones of aggregating anemones Anthopleura elegantissima on a wave-exposed shore

Agonistic interactions between different clone-mates of Anthopleura elegantissima can be used to examine the spatial extent of clonal aggregations in the field.  For example, in San Juan Island, Washington, clones may be discontinuous and distributed over areas of up to 100m2.  Interestingly, clonal sizes are greater in wave-protected areas than in wave-exposed areas, and greater in protected microhabitats (crevices or concave-shaped areas) than in exposed microhabitats (rock protrusions or convex-shaped areas).  In other words, there are more separate clones per unit area in exposed than in protected locations.  The author suggests that this results from single clones conforming to discontinuities in the available substratum.  Sebens 1982 Ecology 63: 434.



Clones of Anthopleura elegantissima in wave-exposed area 0.05X

  black dotResearch study 6

We are familiar with the role that mucus plays in anemonefish/anemone symbioses in the Indo-Pacific region.  To answer the question as to whether chemical properties of an aggregating anemone’s mucus is similarly involved in the recognition process in its aggression, we can ask another question. Does the mucus of Anthopleura elegantissima cause nematocysts to fire in other anemones?  This can be tested by touching a glass rod coated in A. elegantissima mucus to the tentacles of different anemone species and observing whether or not the nematocysts discharge.  Ertman & Davenport 1981 Biol Bull 161: 366.

NOTE  mucus is produced by most animals, including humans and soft-bodied invertebrates.  It is a combination of carbohydrate (mucopolysaccharides) and protein.  Contact with foreign mucus (e.g., that from a human) would normally elicit nematocyst discharge in a sea anemone

NOTE  Corynactis californica is not a sea anemone; rather, it is a close relative, a corallimorpharian.  Its inclusion in the experiment is for comparative interest

table listing % nematocysts of Anthopleura elegantissima discharging into conspecific and heterospecific anemones and corallimorpharians


  black dot

What do these results tell us about the mucus? Think about the answers provided, then CLICK HERE to see explanations.

Mucus does not act as a recognition cue between these cnidarians. 

Mucus is not involved in the aggressive response between non-clonemate Anthopleura elegantissima

Mucus is generally an unreliable tool to test for potential aggressive responses among anemones. 

  black dotResearch study 6

histogram comparing heat-shock protein levels in outside and inside polyps of cnidarians Anthopleura elegantissima and Corynactis californicaAre there chemical manifestations of stress in cnidarians resulting from aggressive competitions for space?  This is examined in intertdial-inhabiting anemones Anthopleura elegantissima and subtidal-dwelling corallimorpharians Corynactis californica at Bodega Marine Laboratory, California.  The researchers use levels of HSP70 protein expression in the tentacles of the polyps as a measure of stress. 

Results show that during intraspecific interclonal interactions the outside or warrior polyps of A. elegantissima express 4-fold more HSP70 than the inside polyps.  After prolonged (48h) interspecific interactions with C. californica the expression of HSP70 by A. elegantissima polyps becomes nearly 20 times greater that at the start.  In comparison, when different clones of C. californica interact, HSP70 expression is similar in the outside photograph of corallimorpharian Corynactis californicaand inside polyps (see histogram).  Because of the comparatively high constant expression of HSP70 in the polyps of C. californica, the authors speculate that clones of C. californica are generally “prepared for war”.  In this unique study the authors propose that stress proteins can be used to quantify different types of aggression among sessile marine invertebrates.  Rossi & Snyder 2001 Biol Bull 201: 385.

NOTE  the expression of Heat Shock Proteins (HSPs) is commonly used to assess stress in animals resulting from physical factors such as temperature, UV light, and so on.  HSP70 is actually a complex of these proteins, but for convenience is expressed collectively as “HSP70” by the authors.  More on HSPs can be found at LEARN ABOUT ABALONES & RELATIVES: PHYSIOLOGICAL ECOLOGY:HEAT-SHOCK PROTEINS and LEARN ABOUT MUSSEL: LIFE IN THE INTERTIDAL ZONE: HEAT-SHOCK PROTEINS

At least 3 clones of Corynactis californica on and
around a rock scallop Crassadoma gigantea 0.6X

  black dot
  Anthopleura xanthogrammica
  black dotResearch study 1
drawing showing aggression between individuals of sea anemones Anthopleura xanthogrammica drawing showing aggression and acrorhagal display between individuals of sea anemones Anthopleura xanthogrammica Interestingly, another species of west-coast anemone Anthopleura xanthogrammica, not known particularly for its aggressiveness, may also display acrorhagi on contact with members of the same or other species.  Current thinking is that the intraspecies acrorhagal inflation may be a way that closely sited individuals use to assess (and control) the positions of their neighbours. The result is that individuals move apart and thereby utilise their habitat more effectively.  Drawings adapted from photos in Bigger 1980 Biol Bull 159: 117; see also Sebens 1984 Biol Bull 166: 457.
  black dotResearch study 2

The common absence of such aggression between closely packed individuals suggests that if they can’t move apart, they habituate to one another, like good neighbours.  Sebens 1984 Biol Bull 166: 457.





Several Anthopleura xanthogrammica at high tide. Although
there is no evident display of acrorhagal-type aggression, each
individual seems to be keeping its tentacles to itself. The dark
blob in the lower centre of the photo is a piece of kelp 0.15X
photograph of a cluster of great green anemones Anthopleura xanthogrammica
  black dot
  Metridium senile
  black dotResearch study 1
photograph showing aggression between 2 plumose anemones Metridium senile
This meeting between a pair of Metridium senile seems amicable enough, but note the presence of fighting tentacles on the right-hand individual (longer white structures at the 2-o'clock position) 0.5X

Plumose anemones Metridium senile reproduce asexually by pedal laceration and the resulting clonal aggregations may be aggressive to one another, just as described above for aggregating anemones Anthopleura elegantissima. There are no acrorhagi in Metridium senile; rather, long fighting tentacles extend from around the mouth, reaching out 4-5 times the diameter of the central disc. These tentacles are used only in aggressive interactions, not in feeding.  In fact, they are absent in naïve individuals that have never engaged in fighting.  However, on contact with an aggressor, they develop within 9wk from feeding tentacles sited closest to the mouth.  The feeding tentacle enlarges and the nematocyst complement changes from the types normally present in feeding tentacles to much more potent types characteristic of "catch tentacles".  On wharf pilings in Monterey Harbor, California clones may be separated from neighbouring clones by anemone-free zones.  It is the individuals along the borders of these zones that have fighting tentacles.  Purcell 1977 Biol Bull 153: 355.

NOTE originally, these long tentacles were thought to be involved in catching food, and were termed "catch" tentacles.  Another more appropriate name, based on their use in aggression, is “fighting” tentacles, and this is the name used here.  Purcell & Kitting 1982 Biol Bull 162: 345

  black dotResearch study 2

On contact with non-clonemates, the tips may break off from the fighting tentacles and adhere, causing localised tissue necrosis.  Contact typically results in back-and-forth movements of the adversaries, taking days, until one or both individuals move away.  The author notes that interclonal aggression in Metridium senile is mediated by sex. Non-clonemates become aggressive only against the same sex. Thus, males of one clone fight males of another clone, and females fight females. When the tentacles are exhibited in mixed-sex confrontations, rather than being for fighting, they are photograph showing aggression between plumose anemones Metridium senileactually mutual probings that may provide information on reproductive status. In such a scenario, the to-ing and fro-ing of the protagonists may be a means for reproductively mature individuals to draw closer together for most effective release of gametes. The idea is supported by the fact that M. senile does not spawn synchronously throughout a colony, and such pair-wise spawnings would greatly increase the probability of fertilisation. This idea could also explain the intermingling of clones described below. Kaplan 1983 Biol Bull 165: 416.

NOTE although it is generally thought that clones in M. senile arise from pedal laceration, this author describes the occurrence of longitudinal fission. Other authors note, however, that while partial fission does occur in this species, it never results in complete separation of individuals

Note the inter-clonal spacing in this photo of Metridium senile and the
presence of "fighting" tentacles in several individuals of the 2 clones.
This implies inter-individual communication between the clone-mates of
the larger right-hand clone regarding an attack from the left-hand clone,
or perhaps several individuals of the right-hand clone have been
simultaneously stimulated.. Note also the white-coloured individual
in the 5-o'clock position that could not be a clonemate of the others,
but that seems also to be displaying fighting tentacles 0.8X

  black dotResearch study 3
Studies in Monterey Harbor, California show that aggregations of Metridium senile may consist of a single clone, or often may contain 2 or more intermingled clones.  Laboratory tests show that different clones vary in the intensity of their aggression, suggesting that the mildly aggressive clones may be the ones that intermingle in the field.  Confounding this idea, however, is the finding that clones that don’t intermingle are not always the most aggressive ones and, thus, aggression may not be involved in their segregation.  Moreover, intermingled clones that have been peaceful for a time may suddenly renew their fighting.  The answer, according to the authors, seems to lie in one clone becoming habituated to contact with another, and that loss of this habituation occasionally occurs, leading to renewal of fighting between previously peaceful intermingled clones.  Purcell & Kitting 1982 Biol Bull 162: 345. photograph showing possible clonal aggregation of plumose anemones Metridium senile
  black dot

The aggression described here for Metridium is similar to that described earlier for aggregating anemones Anthopleura elegantissima.  However, an interesting twist is that non-clonemate Metridium senile appear mostly to fight against same-sex individuals.  Confusing, isn’t it?  However, think about it a bit, and then answer this question: what advantage is conferred by the behaviour? CLICK HERE to see explanations for the answers provided.

Clonal aggregations would remain separated from one another.  

Clonal aggregations would mingle. 

Only males would mingle between adjacent clones. 

Only females would mingle between adjacent clones. 

Male and female clones would mingle.

  black dot
  Corynactis californica
  black dotResearch study 1

Clonal polyps of strawberry anemones Corynactis californica are highly aggressive towards various sea-anemone species, but not toward their own clonemates or even to non-clonemates.  In fact, in the field differently coloured clones often intermingle, with no polyp-free zones separating them as would be the case, for example, in clones of aggregating anemones.

NOTE C. californica is a corallimorpharian, only distantly related to sea anemones

photograph of clonal aggregation of corallimorpharian polyps Corynactis californica
  black dotResearch study 2
photograph of clonal aggregations of the corallimorpharian Corynactis californica

Aggression in Corynactis californica takes the form of initial contact between the polyps, then extrusion of mesenterial filaments from the mouth or through breaks in the body wall of Corynactis towards the other non-Corynactis polyp.  The polyp attacked may then rise up and retaliate, or move away, or detach from the substratum. 

NOTE just as in anemones, the space within a corallimorpharian polyp is the gut cavity, where digestion occurs.  It is subdivided by vertical mesenteries, which hang down into the space like curtains.  Extending from the bases of the curtains are numerous strands known as mesenterial filaments or acontia, each endowed with especially potent nematocysts.  The filaments can be extruded from the mouth or, in some species, from breaks in the body wall

A blood star Henricia leviuscula extends an arm into a field of
polyps. It is not known the extent to which, or even
if, Corynactis is aggressive to non-anemone species 0.2X
  black dotResearch study 3

In confrontations between Corynactis (Left images) and either cup corals Balanophyllia elegans or aggregating anemones Anthopleura elegantissima, Corynactis generally wins (top 2 scenarios). 

However, when the species attacked is a plumose anemone Metridium senile, the outcome is generally a draw (bottom scenario).  Chadwick 1987 Biol Bull 173: 110

photo schematic showing outcomes of interactions between the corallimorpharian Corynactis californica and several other cnidarians
  black dotResearch study 4

The extent of aggressiveness of Corynacti can be estimated from the proportion of individuals that extrude mesenterial filaments when they contact other species.  The data shown are the % of Corynactis individuals that extrude filaments on contact with a cerain species.  Data from Chadwick 1987 Biol Bull 173: 110. 

NOTE Epizoanthus scotinus is a zoanthidian

photograph of the zoanthidian Epizoanthus scotinus
The zoanthidian Epizoanthus scotinus 1X

% of Corynactis extruding filaments:

Corynactis clonemates  0
non-clonemates   0
Anthopleura elegantissima  98 
Metridium senile  89
Epiactis prolifera 100

Balanophyllia elegans  6
Epizoanthus scotinus   14

puncture Corynactis with a glass needle   5
touch with a clean glass rod  2


  black dot