Reproduction
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  Asexual
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Two types of asexual reproduction are found in sea anemones.  These are longitudinal fission and pedal laceration. A third type, budding, is mentioned here with reference to zoanthids, a group related to sea anemones.

NOTE  budding is also found in hydrozoans (see LEARN ABOUT HYDROIDS: REPRODUCTION & DEVELOPMENT). A fourth type of asexual reproduction, transverse fission (strobilation) is found in jellyfish polyps (see LEARN ABOUT JELLYFISHES:REPRODUCTION & DEVELOPMENT). All 4 types result in the formation of genetically identical clones.

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Longitudinal fission

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photograph of clones of aggregating anemones Anthopleura elegantissima growing on a rockA clue to the origin of aggregations of Anthopleura elegantissima was provided by early observations that the members of a given aggregation have identical colour pattern and sex. Ford 1964 Pac Sci 18: 138; Francis 1973 Biol Bull 144: 64.

 

 

A clone or clones of Anthopleura elegantissima growing
amongst a sponge on a rocky shoreline 0.05X

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Although Anthopleura elegantissima is commonly seen in its aggregated or clonal form, another form also exists, larger in size and solitary.  Both types sexually produce planula larvae, but the solitary form does not propagate asexually by fission as does the clonal form.  Whereas the clonal form occurs more often on the tops of boulders exposed to surf, the solitary form inhabits more protected areas, is usually lower on the shore, and may be partly buried in sand.  The solitary form is reported to reach sizes of up to 20cm basal diameter.  Although the issue remains controversial, the latest detailed genetic study of the 2 forms now suggests that they are different species. Francis 1979 Am Zool 19: 669; Sebens 1983 Ecol Monogr 53: 405; Smith & Potts 1987 Mar Biol 94: 537; McFadden et al. 1997 Mar Biol 128: 12.

NOTE  for a discussion of whether the high shore was invaded by a solitary ancestor of A. elegantissima that later evolved clonality, or whether it was a clonal ancestor that invaded the habitat of the solitary form, see Geller & Walton 2001 Evolution 55: 1781

photograph of several anemones Anthopleura elegantissima, possibly of the larger, solitary typephotograph of sea anemone Anthopleura sola with a clone of aggregating anemones A. elegantissima courtesy J.S. PearseNOTE the larger solitary species, common on rocky shores in central and southern California, is now formally designated as Anthopleura sola n. sp.  As noted above, it differs from the sibling species A. elegantissma mainly in its larger size and absence of asexual reproduction. The authors recommend “sunburst anemone” as its common name.  Their paper includes useful comparative descriptions of all 4 species of Anthopleura.  Pearse & Francis 2000 Proc Biol Soc Wash 113: 596. The photograph on the Left, courtesy J.S. Pearse, shows a single A. sola surrounded by a presumed clone of A. elegantissima

Several large Anthopleura in an aquarium tank...possibly the larger solitary A.
sola ? (note the beer bottle for scale)

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  drawings of an anemone Anthopleura elegantissima undergoing longitudinal fissionAggregations of the intertidal sea-anemone Anthopleura elegantissima are formed through asexual division (longitudinal fission) of individuals.  To divide, the 2 halves of an individual appear simply to crawl away from one another and, in time, to split in two.  The offspring are genetically identical clones, and these often exist on the shore in large aggregations separated from one another by distinct interclonal boundaries.  Fission takes between 1-8wk, and occurs generally once per year during autumn and winter when food is scarce.  Clonal aggregations may take years to become established, but can persist for several decades. Sebens 1980 Biol Bull 158: 370; Sebens 1983 Pac Sci 37: 121; Ferrell 2005 Oecologia 142: 184.
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photograph of an aggregating anemone Anthopleura elegantissima undergoing longitudinal fission
The fission leaves a scar of connective tissue that can be recognised for 2-6mo and thus provides an historical record of the occurrence of the process in a population.  Additionally, frequency of division in a population is positively correlated with a seasonal decrease of basal diameter.  In San Juan Island, Washington, for example, monitoring of these parameters shows that asexual division is least common during spring and early summer when the anemones are rapidly growing, and most common from late summer through the winter, when they are growing only slowly.  Sebens 1982 Ecology 63: 434.


An aggregating anemone Anthopleura
elegantissima
undergoing longitudinal fission 1X

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Further studies of fission in Anthopleura elegantissima in San Juan Islands and Tatoosh Island, Washington show that rates are greatest during autumn and winter, averaging about 0.2 divisions per clonal individual per year at all study sites.  Only the larger individuals in a clone divide.  Clones in more favourable habitats produce larger individuals and these clones therefore have greater reproductive output. In comparison, clones in marginal habitats such as the high intertidal region are composed of small individuals, with the lowest rates of asexual division and lowest rates of reproductive output (see accompanying graph). Note in the graph that eproductive output is significantly higher in older, larger individuals of A. elegantissima.  Based on rates of division and rates of disappearance from monitored clones, the author estimates lifespans of 2-3yr for individual anemones, but suggests that some probably live much longer. There are no extimates on the ages of clonal populations. Sebens 1983 Ecol Monogr 53: 405.

NOTE in comparison, individuals of A. xanthogrammica are thought to live for several decades, perhaps to an age of 100yr or more

graph showing gonadal volume in relation to size in sea anemones Anthopleura elegantissima
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histogram showing locomotory rates of polyps of the corallimorpharian Corynactis californica in the laboratoryAfter the polyps of Anthopleura or Corynactis divide, they move slowly away from the colony centre to provide room for further growth.  Laboratory measurements of polyp movement in Corynactis californica at the Bodega Marine Laboratory, California indicate a mean rate of 5mm . mo-1, much slower than in actiniarian polyps such as Anthopleura.  Corallimorpharians apparently lack the basilar muscles present in actiniarians, so perhaps some of the movement is by “tidal” photograph of a corallimorpharian Corynactis californica in the process of dividing by fissionflow from increase in mass in the central part of the colony.  The authors also report fission rates in the laboratory (at Santa Cruz, California) of once every 2mo, a rate that is intermediate between actiniarians and scleractinians.  Chadwick & Adams 1991 Hydrobiologia 216/217: 263.

The corallimorpharian Corynactis californica
reproduces asexually by longitudinal fission to
form large, single-sex, and often colour-distinct,
clonal aggregations. The photograph shows an
individual in the process of dividing 2X

 

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What advantage is gained by Anthopleura elegantissima forming clonal aggregations? Think about the answers provided then CLICK HERE to see explanations.

Reduces water loss in intertidal aggregations. 

Situates the acrorhagal-rich warrior polyps on the periphery where they can better defend against attack from predatory snails.

Allows for rapid/effective space utilisation. 

Enables cooperative prey capture. 

Maximises reproductive effort. 

NOTE all polyps of A. elegantissima possess special aggressive tentacles known as acrorhagi located around the upper column, just below the line of feeding tentacles. These are densely packed with large, potent nematocysts. The role of acrorhagi is considered elsewhere: AGGRESSION.

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Pedal laceration

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Research study 1
 
Some sea-anemone species, such as Metridium senile reproduce asexually by leaving bits of themselves around to grow into fully-functional individuals.  The offspring are genetic clones of the adult.  Sometimes, it seems as if the basal disc (pedal area) simply gets snagged on something, or possibly a portion of it may be released through contraction of pedal musculature. photograph showing pedal laceration in a plumose anemone Metridium senile
Pedal laceration in the anemone Metridium senile. The lacerated bit will grow into a (tiny) new individual. The brown polyps are those of the zoanthid Epizoanthus scotinus 0.3X
photograph of a plumose anemone possibly in the process of asexual division
Two Metridium senile anemones riding piggy-back? It may be an instance of pedal laceration producing a clonal individual which has yet to break free from its "parent" 0.5X
  photograph of a clonal aggregation of plumose anemones Metridium senileMost or all of this aggregation of plumose anemones Metridium senile are genetically identical clonemates, produced by pedal laceration. The orange polyp may be a colour variant of M. senile, or a juvenile of the giant plumose anemone Metridium farcimen ) 0.2X photograph of a plumose anemone Metridium senile with 3 newly produced clonal offspring
3 or more juveniles-to-be produced from pedal laceration of a plumose anemone. The specimen featured is likely a colour variant of the short plumose anemone Metridium senile, as the larger species M. farcimen is not reported to reproduce asexually 0.5X
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Research study 2
 

All forms of asexual reproduction in invertebrates rely on good powers of regeneration – a characteristic found in many primitive invertebrates. 

 

This large Urticina lofotensis ate a basket of snails intended to be
used in a classroom experiment at the University of British Columbia. 
The only way to retrieve the snails was to cut the anemone open
lengthwise with a scalpel.  The snails were fine, as was the anemone
after just a single day of healing.  No attempt was made to stitch the
wound, but the edges healed perfectly with no visible scars. The
specimen lived forseveral more years before being returned to the sea 0.6X

photograph of a sea anemone Urticina lofotensis that has eaten a plastic container full of snails
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Research study 3
 

In habitats characterised by disturbance, such as shifting substrata or wave action, Metridium senile increases its rate of pedal laceration. Sea anemones growing on mussels also show significantly increased replication by pedal laceration over ones growing on dead mussels. Bucklin 1987 J Exper Mar Biol Ecol 110: 225, 41; Anthony & Svane 1995 MEPS 124: 171. Photograph courtesy Joseph Dougherty and calphotos.

NOTE  actually mussel shells experimentally cleaned of all living tissues, both inside and out

 











Mussels Mytilus californianus covered with
sponges and clonal growths of corallimorpharians
Corynactis californica 0.1X

photograph of mussels Mytilus californianus covered in clonal growths of corallimorpharians Corynactis californicus courtesy of Joseph Dougherty and calphotos
 
Research study 4
 

drawings showing regeneration in an asexually lacerated fragment of the sea anemone Metridium senilegraph showing growth of fed and unfed sea anemones Metridium exilisA more detailed laboratory study by the same author as in the foregoing Research Study on 3 species of Metridium collected in Bodega Bay, California provides comparative information on maximum size attained and asexual reproductive proficiency.  Metridium exilis is a small intertidal species (typically 0.2cm2 pedal disc area) that reproduces asexually by binary fission (longitudinal fission).  Ad libitum feeding on adult brine shimps increases the rate of asexual reproduction, but has little effect on maximum size (1.2cm2 as shown in the accompanying graph). The author notes that sexual reproduction in M. exilis may be quite rare

Metridium senile is a larger species (to 45cm2), lives from mid-intertidal to shallow subtidal regions in Bodega Bay, and reproduces both sexually and asexually, the latter by fragmentation (=pedal laceration).  Experimentally inflicted lacerations indicate that regenerative capability is excellent.  All lacerated fragments of column and pedal disc grow to tentacled state within 3wk (seawater temperature not given). 

Finally, a larger (>1m in height) exclusively subtidal species (presumed to be M. farcimen), does not regenerate well from experimentally excised fragments (only 53% total regeneration after 19wk).  This species is not known to reproduce asexually and no pedal laceration occurs in the laboratory.  Bucklin 1987 J Exp Mar Biol Ecol 110: 41.

NOTE  examination of 356 individuals collected monthly over 2.5yr reveals no evidence of gonadal development.  Thus, intertidal populations likely consist of a few clones

NOTE  the process is described in older literature referred to by the author and goes as follows: the ends of the torn fragment curl together, fuse, and the piece increases in height.  Tentacle buds appear, lengthen, and a mouth forms.  This, combined with internal development of gastrovascular cavity, pharynx, and mesenteries eventually leads to an individual that is indistinguishable from a sexually produced one (see drawings on Right)

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