Sea anemones & relatives
Reproduction: Asexual

Research Study 1

Fig. 1.  Typical size and colour-patterning of subtidal Urticina crassicornis from Vancouver Island, British Columbia, about three times the size of the species shown below from intertidal areas of the Barents Sea

In temperate latitudes the sea anemone Urticina crassicornis (Fig. 1) reproduces sexually by releasing gametes that fertilise externally, leading to a planula larva and to later settlement and metamorphosis. Recently, however, a group of Polish and Russian scientists believe they have identified aggregations of this species in intertidal zones of the Barents Sea that appear to originate asexually from a few large brooding females. The authors point to colour-pattern similarity among members of an aggregation, a size gradation from small to large at increasing distance from the purported large brooders, and observations of juvenile anemones both within the coelenteron (gastrovascular cavity) and appearing on the outer column of these same adults after emergence from their mouths (Figs. 2 - 3). The authors additionally point to the possibility that the monochromatic aggregations result from asexual reproduction, but how and where this might occur are not specified. These would be fascinating findings were it not for the absence of corroborative molecular or other evidence to support the assertion that the species in question is, indeed, U. crassicornis and not another species . This should be a good follow-up research project for the authors. 

Fig. 2.  Closed-up intertidal individuals of apparently the same species Urticina crassicornis 
Fig. 3.  Adult Urticina crassicornis from Barents Sea with purported cloned offspring (newly released from the mouth of the adult)
Kaliszewicz et al.   2012   Polar Biol 35: 1911.
Longitudinal fission

Research Study 1: Longitudinal fission

Fig. 1.  A clone or clones of Anthopleura elegantissima growing amongst red sponges on a rocky shoreline 

A clue to the asexual origin of aggregations of Anthopleura elegantissima was provided by early observations that the members of a given aggregation have identical colour pattern and sex (Fig. 1).

Ford   1964   Pac Sci 18: 138
Francis   1973   Biol Bull 144: 64.

Research Study 2: Longitudinal fission

Fig. 1.  Anthopleura elegantissima undergoes longitudinal fission by tearing itself asunder

Aggregations of the intertidal sea-anemone Anthopleura elegantissima are formed through asexual division (longitudinal fission) of individuals.  To divide, the two halves of an individual appear simply to crawl away from one another and, in time, to split in two (Fig. 1).  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   Sebens Pac Sci 37: 121
Ferrell   2005   Oecologia 142: 184.

Research Study 3: Longitudinal fission

Fig. 1.  Aggregating anemone Anthopleura elegantissima undergoing longitudinal fission

The fission leaves a scar of connective tissue on Anthopleura elegantissima that can be recognised for 2 - 6mo and thus provides an historical record of the occurrence of the process in a population (Fig 1).  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

Research Study 4: Longitudinal fission

Fig. 1.  Single Anthopleura sola surrounded by a presumed clone of A. elegantissima
Courtesy J.S. Pearse
Fig. 2.  Several large Anthopleura in an aquarium tank...possibly the larger, solitary A.sola ? (note the beer bottle for scale) 

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 longitudinal 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 (Fig. 1).  Although the issue remains controversial, the latest detailed genetic study of the two forms now suggests that they are different species (see Fig. 2).

NOTE 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 four species of Anthopleura.  Fig. 2 shows a single A. sola surrounded by a presumed clone of A. elegantissima

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

Francis   1979   Am Zool 19: 669
Sebens   1983   Ecol Monogr 53: 405
Smith & Potts   1987   Mar Biol 94: 537
Geller & Walton   2001   Evolution 55: 1781
McFadden et al.   1997   Mar Biol 128: 12.
Pearse & Francis   2000   Proc Biol Soc Wash 113: 596.

Research Study 5: Longitudinal fission

Fig. 1.  Gonad size relative to body size in Anthopleura elegantissima

Further studies of asexual reproduction 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.  Note in Fig. 1 that reproductive 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 estimates on the ages of clonal populations, but a long time, perhaps many decades, would be a good guess.

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

Sebens   1983   Ecol Monogr 53: 405.

Research Study 6: Longitudinal fission

Fig. 2.  Locomotory rates of polyps of the corallimorpharian Corynactis californica in the laboratory
Fig. 1.  The corallimorpharian Corynactis californica reproduces asexually by longitudinal fission to form large, single-sex, and often colour-distinct, clonal aggregations. The photograph appeares to show an individual on the Right having just divided

After the polyps of Anthopleura elegantissima or Corynactis californica (Fig. 1) divide, they move slowly away from the colony centre, thus providing 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 (Fig. 2), 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 colony-wide “tidal” flow 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 sea anemones and stony corals.

Chadwick & Adams   1991   Hydrobiologia 216/217: 263.

Test Your Understanding

What advantage is gained by Anthopleura elegantissima forming clonal aggregations?  [Click each option to see commentary]
 

Research Study 7: Longitudinal fission

Fig. 1.  Intertidal height of Anthopleura elegantissima relates to symbiont status
Courtesy the authors
Fig. 2.  Effects of light level and symbiont status on frequency of fission of anemones Anthopleura elegantissima

A publication by researchers at Shannon Point Marine Laboratory, Washington shows that the reproductive strategy of sea anemones Anthopleura elegantissima is influenced by the particular species that it hosts as a symbiont and the degree of irradiance that it experiences (Fig. 1).  Thus, individuals hosting the dinoflagellate Symbiodinium muscatinei and exposed to high irradiance and summer temperature conditions, tend to clone more frequently (Fig. 2), while individuals hosting the chlorophyte Elliptochloris marina incline more to gonadal growth.  Note in Fig. 2 that anemones hosting S. muscatinei, which photosynthesises poorly under low irradiance, are less fit even than aposymbiotic anemones.  Since S. muscatinei-hosting anemones live higher in the intertidal zone (see Fig. 1) and must endure more stressful conditions than lower-dwelling E. marina-hosting ones, the authors consider the strategy as one of offsetting stress-related costs without compromising the species’ competitive spatial dominance in the upper intertidal zone.

NOTE  the researchers expose the sea anemones to three levels of light: high (85%), medium (43%), and low (2%), for periods of up to 11mo, then measure how much growth, size of gonads, and frequency of cloning by fission

NOTE  lacking symbionts altogether.  Some of these types of anemones can be seen in Fig. 1

Bingham et al.   2014   Proc Roy Soc B 281: 1.
Pedal laceration

Research Study 1: Pedal laceration

Fig. 1.  Most 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 

Some sea-anemone species, such as Metridium senile reproduce asexually (Fig. 1) by leaving bits of themselves around to grow into fully functional individuals (Figs. 1 - 3).  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 is released through contraction of pedal musculature. 

Fig. 2.  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 
Fig. 3.  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"
Fig. 4.  Several juveniles 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 known to reproduce asexually
   

Research Study 2: Pedal laceration

Fig. 1.  Sea anemone, Urticina lofotensis,  with a plastic basket partially protruding from its mouth

All forms of asexual reproduction in invertebrates rely on good powers of regeneration, a characteristic found in many primitive invertebrates.  The Urticina lofotensis shown in Fig 1 ate a basket of snails floating above it in a shallow aquarium, 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 overnight with no visible scars. The specimen lived for several more years before being returned to the sea.

   

Research Study 3: Pedal laceration

Fig. 1.  Sea mussels Mytilus californianus covered with sponges and clonal growths of corallimorpharians Corynactis californica
Courtesy Joseph Dougherty & CalPhotos

In habitats characterised by disturbance, such as shifting substrata or wave action, Metridium senile increases its rate of pedal laceration. Sea anemones growing on live mussels also show significantly increased replication by pedal laceration over ones growing on dead mussels.  In Barkley Sound, B.C. corallimorpharians Corynactis californica similarly do well on a substratum of living mussels (Fig. 1)

NOTE  actually mussel shells Mytilus californianus that were experimentally cleaned of all living tissues, both inside and out

Bucklin et al.   1987   J Exper Mar Biol Ecol 110: 41, 225
Anthony & Svane   1995   MEPS 124: 171.

Research Study 4: Pedal laceration

Fig. 1.  Regeneration of asexually lacerated fragment of basal disc of anemone Metridium senile in the laboratory
Fig. 2.  Effect of food intake on regeneration in Metridium exilis

A more detailed laboratory study by the same authors as in the foregoing Research Study on three 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 only a small effect on maximum size (1.2cm2 as shown in Fig. 1). 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; Fig. 1).  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), is not known to reproduce asexually and does not regenerate well from experimentally excised fragments (only 53% total regeneration after 19wk).  No pedal laceration occurs in the laboratory. 

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

Bucklin et al.   1987   J Exp Mar Biol Ecol 110: 41.