Habitat & population biology

  This topic is divided into 2 sections, the first dealing with SEA ANEMONES; the second, with CORALLIMORPHARIANS
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Sea anemones
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Research study 1
  Studies in San Juan Islands, Washington illustrate contrasting examples of habitat specialisation in 2 anemone species Metridium farcimen and Anthopleura xanthogrammicagraph showing drag forces on 2 anemone species with increasing velocity of currentIndividuals of the first species live subtidally in calm areas, are tall, reach into the current for food, and thus are maximally exposed to mainstream current velocities.  Individuals of the second species live intertidally or in shallow subtidal regions where they are exposed to strong wave action.  However, they are short in stature and are effectively hidden from mainstream current velocities.  Adaptations of M. farcimen include a thinner, flexible body wall with flexural stiffness being lowest in the upper body column.  It bends in strong currents. In comparison, Anthopleura xanthogrammica is built low to the ground, and has body-wall thickness and flexural stiffness 2 times and 10-100 times, respectively, greater that of M. farcimen.  Additionally, the middle layer of mesoglea in the 2 species differs in its mechanical properties.  In Metridium, it remains rigid over short periods of stress (as during muscular movements), but is highly extensible when stressed over long periods (as from sustained force of water currents).  In contrast, Anthopleura's mesoglea remains relatively rigid even after being stressed for several hours and has greater elastic recoil.  The studies illustrate the different adaptive strategies employed by sea anemones in different habitats.  Koehl 1977 J Exp Biol 69: 87; Koehl 1977 J Exp Biol 69: 107; Koehl 1977 J Exp Biol 69: 127.
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photograph of sea anemones Metridium farimen on the hull of a sunken ship taken from a video CLICK HERE to see a video of anemones on a sunken ship, mostly the larger species Metridium farcimen. Intentional sinking of ships, such as has been done in British Columbia, provides excellent habitat for growth of anemones and other sessile/sedentary invertebrates, and thus rich viewing spots for SCUBA divers.

NOTE the video replays automatically

Research study 2

graph showing effect on % cover of macrophytes of complete removal of sea anemones Anthopleura elegantissima from an area of San Nicolas Island, Californiaphotograph showing 3 anemones Anthopleura elegantissima inundated with sandWhat role played by Anthopleura elegantissima in the dynamics of a rocky intertidal community periodically disturbed by sand inundation? This is examined at San Nicolas Island, California by removing all A. elegantissima in selected fenced-off experimental areas. In this area, as elsewhere, these anemones may grow so densely that they form moisture-retaining mats that occlude colonisation by other organisms. Conversely, the mats may permit less air-tolerant species to colonise and flourish.  Sand burial of parts of the habitat occurs seasonally, but with summer being generally free from sand inundation and winter seeing the worst of it.  The authors include fence controls and unmanipulated controls in the study.

Within 6mo following removal of the anemones the % cover of macrophytes increases significantly (see graph), mainly owing to enhanced growth of several species of opportunistic green algae such as Cladophora columbiana and Chaetomorpha linum. Additionally, the cover of several species of coralline algae decreases through bleaching.  Numbers of grazing molluscs, including Chlorostoma funebralis, littorines, chitons, and limpets increase during the same period, then abruptly decrease. The most dramatic alteration in community structure is the colonisation and subsequent domination of space by tube-building polychaetes Phragmatopoma californica.  By 20mo following the removal of A. elegantissima this polychaete achieves a cover density greater than 75%. By 2yr following their removal, A. elegantissima has regained original densities, but not in areas that have become dominated by P. californica.  Thus, at higher intertidal levels of distribution where thermal and desiccatory stresses are maximal, A. elegantissima facilitates colonisation and subsequent survival of coralline algae, while at the same time inhibiting recruitment of opportunistic macroalgae.  At lower intertidal levels where physical conditions are less stressful, P. californica rapidly colonises and retains newly available space, preventing A. elegantissima from re-colonising and forming its typical space-dominating colonies.  In mid-intertidal regions, greater sand deposition prevents Phragmatopoma from forming persistent colonies, and Anthopleura is again able to become dominant.  The authors conclude that the importance of Anthopleura in the dynamics of community interactions in the study area of San Nicolas Island depends primarily on the compensatory mortality of its superior competitor P. californica caused by stresses of sand inundation and other physical disturbances. Taylor & Littler 1982 Ecology 63: 135; see also Littler et al 1983 Mar Ecol Progr Ser 11: 129 for a more detailed description of the San Nicolas study area.

NOTE the authors present data for several species of plants and animals, not shown graphically here


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Research study 3


A 2-yr investigation of population ecology of Anthopleura spp. in San Juan and Tatoosh Islands, Washington shows that density is the greatest influence on individual growth.  Areas with experimentally reduced densities and control areas with lowest densities show the greatest individual growth rates. The author suggests that exploitative competition by adults for food, and direct interference competition by tentacle crowns of adults blocking capture of food by smaller individuals, are the mechanisms responsible. During the course of the study the author notes a complete absence of predation on large specimens of A. xanthogrammica.

Estimates of natural life spans of Anthopleura spp. are not easily come by.  Division rates and rates of disappearance from natural clones indicate that individual A. elegantissima have an expected mean lifetime of 2-3yr.  However, the author suggests that potential lifetime of both species could measure in the decades.  Sebens 1982 Ecology 63: 434; Sebens 1983 Ecol Monogr 53: 405.

NOTE  density in the “Experimental removal” site is 3 individuals . m-2, while density in the “Study site” site is 20 individuals . m-2 The author presents data for two other non-manipulated sites with densities of 7 and 51 individuals . m-2, respectively, and with results following the same pattern as shown here

graphs showing effects of density on growth of anemones Anthopleura xanthogrammica
Annual growth increments of individual Anthopleura xanthogrammica over a 1-yr period. The diagonal line in each graph is the zero-growth isocline; thus, points above this line indicate growth, while points below indicate shrinkage. Overall, the pattern of results indicate that anemones in lower-density plots have greater growth
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Research study 4

Other studies by the same author in Tatoosh Island, Washington show that young Anthopleura xanthogrammica (less than about 3cm diameter) find refuge in mussel beds until they reach a size large enough to move to lower shore positions.  One of the factors influencing this migration appears to be aggressive encounters with other anemones in the mussel bed.  Sebens 1981 Science 213: 785.


A bed of sea mussels Mytilus californianus harbour juvenile Anthopleura
until they reach a size where they are able to fend for
themselves in lower-level tidepools and surge channels 0.15X

photograph of a bed of mussels Mytilus californianus harbouring juvenile sea anemones Anthopleura xanthogrammica

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Research study 5


photograph of a wall of anemones Anthopleura xanthogrammica courtesy Dave Cowles, Walla Walla University, WashingtonCasual observation of populations of great green anemones Anthopleura xanthogrammica over time generally leads to the conclusion that not much is going on, as changes are slow and slight, and rarely if ever are juveniles seen.  This is confirmed by researchers at Oregon State University in a study of population characteristics of A. xanthogrammica on the Oregon coast.  After 2yr of collecting data the authors find the following:

    1. 1) no significant population fluctuations
    2. 2) adults exhibit little movement
    3. 3) no recruitment or mortality
    4. 4) experimental removal of adults neither promotes larval settlement nor do nearby adults move into the freed-up area

The authors summarise that A. xanthogrammica is a long-lived, slow-growing species with limited, low, or sporadic recruitment, and with highly stable intertidal adult populations.  A major reason for the study was to determine if west-coast populations of A. xanthogrammica could sustain a major harvesting programme.  The reason for this was that two polypeptides, Anthopleurin-A and -B had been isolated from the species and found to be clinically useful cardiotonic agents with fewer undesirable side-effects as compared with cardiac glycosides in use at the time; hence, the prospects of a harvesting programme for the species.  On the basis of their results, the authors conclude that harvesting of A. xanthogrammica on a continual basis would not be possible.  Batchelder & Gonor 1981 Est Coastal Shelf Sci 13: 235.

NOTE  since the Oregon study Anthopleurin-A has been synthesised, and the amino-acid sequence of Anthopleurin-B has been determined, so perhaps there is no longer need, or at least a lesser need, for natural sources of the substances


Judging by the evenly spaced distribution of green
anemones Anthopleura xanthogrammica on this
intertidal wall in the Olympic Peninsula, there may
be little space available for new recruits. Intermixed
in the community are ochre stars Pisaster ochraceus.
Photogaph courtesy Dave Cowles, Walla Walla University,
Washington rosario.wallawalla.edu


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Research study 6


Field studies on sea anemones often require that individuals be marked for identification.  Regular plastic or wire tags tend to work themselves out in a short time, and likely irritate the animal as the tags wiggle about in waves.  A non-invasive method of marking is described whereby a vital stain, neutral red, is applied as a paste to outer epithelium of the anemones in situ.  Numbers and other marks created in this way last for 12mo in the lab, and 9-12mo in the field (at Tatoosh Island, Washington).  Sebens 1976 J Fish Res Bd Can 33: 1407.


Top view of an (unmarked) Urticina sp. 0.33X

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Research study 1

graph showing effect of macroalgae on % cover of corallimorpharians Corynactis californica in the absence of sea urchinsIn a field study in Santa Barbara Channel a researcher from University of California, Santa Barbara investigates the interaction between the corallimorpharian1 Corynactis californica, red and purple sea urchins (Strongylocentrotus franciscanus and S. purpuratus, respectively), and several species of turf algae. In this area, the corallimorpharian competes for space with the algae, but whether it becomes overgrown by the algae depends upon the intensity of grazing by sea urchins. To assess competitive interactions between Corynactis and macroalgae in the absence of herbivory, the author performs a series of removal experiments in replicated plots along a section of the reef at 11m depth and monitors them for 12mo. First, all sea urchins are collected and relocated to distant areas. Then, in some plots, all macroalgae are removed to test effects on cover and density of Corynactis, while in others all macroalgae are removed to test the reverse. In parallel 8d experiments, densities of sea urchins are maintained at 3 levels2: low (absent), intermediate, and high. Results show that at low grazing intensity, Corynactis competes for space with turf algae and is out-competed and sometimes overgrown by macroalgae (see graph upper Left showing the dominance of algae in the absence of herbivory by sea urchins). At intermediate grazing intensity, Corynactis can still provide refuge3 for some algae, while at high grazing intensity the sea urchins consume most algae. In this association, Corynactis is aiding survival of the algae, and hence in the jargon is known as a facilitator. Deterrence of sea urchins by Corynactis owes to their tube feet being stung4 by Corynactis’ nematocysts, which are notably powerful. However, at high levels of grazing, the sheer density of urchins appears to overwhelm the ability of Corynactis to facilitate survival of the algae. The study is remarkable in the number and diversity of experiments, none easy to accomplish using SCUBA even under the best of conditions. Levenbach 2009 Oecologia 159: 181.

NOTE1 the author refers to C. californica both as a “small anemone” and a “colonial anemone”, perhaps because its common name is “strawberry anemone”. All are technically incorrect. Anemones are in Order Actiniaria, while C. californica is in O. Calliomorpharia

NOTE2 initial urchin density in the experimental plots is about 50 . m-2 (96% S. purpuratus and 4% S. franciscanus. For the low-grazing-intensity treatment all urchins are removed;for intermediate, half are removed; and for high, original densities are maintained

NOTE3 this type of refuge is referred to as an associational refuge

NOTE4 in an earlier paper the author shows in laboratory tests that locomotion of both purple and red urchins is inhibited by contact with Corynactis tentacles. The response is less in red urchins, likely because their spines are 3 times longer than in purple urchins, and they are able to “tip-toe” along on their spines with their sensitive tube-feet mostly retracted. Levenbach 2008 Mar Ecol Progr Ser 370: 45.

The following photographs were taken in different areas along the west coast, and are presented here simply to convey the essence of the author's results:

macroalgal/Corynactis californica association at low levels of sea-urchin grazing macroalgal/Corynactis californica association at intermediate levels of sea-urchin grazing macroalgal/Corynactis californica association at high levels of sea-urchin grazing
At LOW levels of sea-urchin grazing, macroalgae dominate Corynactis californica At INTERMEDIATE levels of sea-urchin grazing, macroalgal dominance is greatly reduced At HIGH levels of sea-urchin grazing, macroalgae are eliminated and Corynactis californica dominates
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