Reproduction: sexual
 
Sea anemones reproduce both sexually (considered for several genera of west-coast anemones in this section) and asexully, and some species do both.  ASEXUAL reproduction is considered in another part of the ODYSSEY .

Sexes are mostly separate in sea anemones and their relatives.  Gonads develop on the internal mesenteries and gametes are released via the mouth during summer or throughout the year, depending upon species and area.  Development leads to a free-swimming larva called a planula that may feed or not feed depending upon species. 

NOTE  lit. “wandering” G.

photograph of a 3-d planula larva of a sea anemone Anthopleura elegantissima
3-day planula larva of anemone Anthopleura elegantissima (100
x 150 um). The mouth
of the larva is at the
left end.
Schwarz et al. 2002 Mar Biol 140: 471
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  Anthopleura spp.
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Research study 1
 
photograph of a cluster of sea anemones Anthopleura elegantissima
Sea anemone Anthopleura elegantissima 1X

In sea anemones the gonads grow on the internal mesenteries. A study on Anthopleura elegantissima in north San Francisco Bay, California shows that gonads build in size through spring and summer, and spawning occurs in late summer/early autumn.  Gonad indices peak coincidentally with high surface seawater temperatures. graphs of gonadal indices of sea anemones Anthopleura elegantissima and seawater temperature seasonallyEggs are released in brown mucousy masses, while spermatozoa are released in milky-white masses.  Interestingly, the author comments on finding aggregations of different sexes of A. elegantissima and makes reference to the possibility that they may be clones produced asexually through binary fission from a single individual.  Further, the author notes that when individual anemones from different aggregations are mixed together in the laboratory, they eventually separate. This seems to be one of the earliest references describing this phenomenon in A. elegantissimaFord 1964 Pac Sci 18: 138.

NOTE  gonad index is calculated as the ratio of gonad volume to live mass of animal x 100

NOTE  this topic is considered in detail elsewhere in the ODYSSEY: LEARN ABOUT SEA ANEMONES & RELATIVES: REPRODUCTION: ASEXUAL

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Research study 2
 

A similar reproductive pattern is exhibited by Anthopleura elegantissima in Bodega Bay, California.  Here, monitoring of oocyte diameter in the gonads on the mesenteries reveals size increase during spring/summer, with spawning in late summer coincidental with peak seawater temperatures.  Spermatozoa remain viable in the gonads for up to 4mo.  There is some evidence that males begin spawning earlier than females, and perhaps this stimulates the females to spawn.  In 3yr of study the author notes finding only one individual with both male and female gonads.  Jennison1979 Can J Zool 57: 403.

 

NOTE  a study on this species from Morrow Bay, California shows that levels of tissue lipid fluctuate in a manner roughly paralleling the reproductive cycle.  Lipids are important energy-storage substances in many marine invertebrates, and their levels commonly cycle in synchrony with reproduction.  Jennison 1979 J Exp Mar Biol Ecol 39: 211.

graph schematic showing oocyte-diameter cycles in different seasons for sea anemones Anthopleura elegantissima in Bodega Bay, California
 
 

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Research study 3

 

Studies in Washington show that gonads in 2 species Anthopleura xanthogrammica and A. elegantissima are produced through late winter, spring, and summer, with spawning in late summer and autumn.  The graphs show that the pattern of percentage-volume change of gonads in A. elegantissima is similar whether on the open coast (Tatoosh Island) or inland (Lopez Island), and whether individuals are located high in the intertidal region, or low.  Spawning is epidemic in a population and is initiated by male sperm release.  Sebens 1981 J Exp Mar Biol Ecol 54: 225.

NOTE  the gonads in sea anemones grow on eithe side of internal mesentaries.  The author fixes and sections sample gonads, then estimates percentage volume of each gonad as the volume of a cylinder

Gonad indices for Anthopleura elegantissima at
Tatoosh Island and Lopez Island, Washington over a
3-y period. Data for A. xanthogrammica (not shown)
are similar. "Low" and "high" refer to intertidal levels

graphs of gonad indices for sea anemones Anthopleura xanthogrammica at different tidal heights at Tatoosh and Lopez Island, Washington photograph of a great green anemone Anthopleura xanthogrammica in a tidepool
Anthopleura xanthogrammica in a tidepool 0.5X
 
 

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Research study 4

 
photograph of aggregating anemones Anthopleura elegantissima at high tide

Further studies in Tatoosh Island and San Juan Islands, Washington suggest that both Anthopleura elegantissima and A. xanthogrammica preferentially settle in mussel beds, and that settlement may be quite irregular from year to year.  One massive settlement of A. elegantissima at Tatoosh Island in late 1972/early 1973 could be followed for several years afterwards. During the several-year study on Tatoosh Island, numbers of A. elegantissima in the mussel beds more than doubled, mostly owing to a wholesale wave of asexual fission in the cohort in July 1975.  During the 5-y study the author reports no settlement of A. xanthogrammica in the Tatoosh Island area, although juveniles are seen within the mussel beds.

Anthopleura elegantissima in and around an open-coast
mussel bed, photographed at high tide 0.2X

 
If areas are cleared near to and below the mussel beds in San Juan Islands, the juveniles crawl out into the clearings from the beds. Two distinct annual cohorts are visible in these data, each arising from winter settlement (November-March) and size measurements over time show strong seasonal differences in growth, with a similar pattern being present for both cohorts.  graph showing growth of anemones Anthopleura elegantissima at San Juan Island, Washington

 

 

Change in basal diameters of 2 cohorts of Anthopleura
elegantissima
in San Juan Islands, Washington.
The smallest cohort settled in Nov-Mar, 1975

 
Anthopleura xanthogrammica also recruit to mussel beds, then crawl downwards into tidepools and surge channels when they reach a larger size.  The author suggests that the migration of juveniles of both species from the mussel beds is likely driven by the need for light for their photosynthesing symbionts.  Sebens 1982 J Exp Mar Biol  Ecol 59: 103. photograph of green anemones Anthopleura xanthogrammica in a surge channel

 

 

 

Anthopleura xanthogrammica in a surge channel
with ochre stars Pisaster ochraceus 0.1X

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Research study 5
 

As described in a study in San Juan Islands, Washington, the planula larvae of both Anthopleura elegantissima and A. xanthogrammica catch food from the plankton on a trailing thread of mucus.  The thread extends from the mouth and catches particles that are later drawn in with the mucus.  The planula is free-living for several weeks, then settles to the sea bottom and metamorphoses into a polyp.  Siebert 1974 Can J Zool 52: 1383.

 

NOTE  food in the study food consists of plastic micro-spheres and unicellular algae. Although both types of particle are pulled into the mouth on the mucous line, the anemones subsequently reject them, so feeding is intimated but not actually demonstrated in the larvae of these species in the study

photograph of an early planula larva of the sea anemone Anthopleura xanthogrammica showing capture of food particles (here, plastic microspheres) in a trailing mucous strand ourtesy Siebert 1974 Can J Zool 52: 1383Early planula of Anthopleura xanthogrammica feeding on a mixture of plastic microspheres and unicellular algae. Food particles are caught on a trailing thread of mucus (Above) and later ingested (Right) 150X photograph of an early planula larva of the sea anemone Anthopleura xanthogrammica showing ingestion of food particles (here, plastic microspheres) caught in a trailing mucous strand ourtesy Siebert 1974 Can J Zool 52: 1383
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Research study 6
 
photograph of a planula larva of a sea anemone

A description of the nervous/sensory system of the planula larva of Anthopleura elegantissima indicates a complexity comparable to that found in some adult sea anemones and hydroids.  At the front end of the larva is an apical sensory tuft closely associated with an underlying apical sensory “organ” and an endodermal nerve plexus.  Another set of ectodermal sensory cells are positioned at the oral end of the larva.  The authors suggest that recognition of edible particles is likely to be by these ectodermal sensory cells.  As the planula swims along the sea bottom near the end of its free-swimming life the apical tuft sways from side to side and is directed downwards suggesting a tactile sensory purpose possibly associated with settling.  Chia & Koss 1979 J Morph 160: 275.

Planula larva of a sea anemone, but without an
apical tuft of cilia. The anterior end is to the
Left; the oral end to the Right 120X

 
Research study 7
 

graph showing change in lipid content of tissues of sea anemones Anthopleura elegantissima in Morrow Bay, California over a 2.5yr periodA 2.5-yr study on lipid levels in sea anemones Anthopleura elegantissima in Morrow Bay, California reveals fluctuations that roughly parallel the reproductive cycle.  The same 6 clones (3 male and 3 female) are sampled throughout the study, thus reducing genetic variability.  The data show that lipid contents are independent of sex and directly proportional to size (see accompanying graph). Note that each spawning time (Aug-Sep) is associated with a drop in percentage lipid, suggesting that the major lipid component is likely involved in gamete production.  Jennison 1979 J Exp Mar Biol Ecol 39: 211.

NOTE  lipid contents are determined by mass loss after extraction of dried samples for 48h in a 5:1 mixture of ether:chloroform in a Soxhlet apparatus.  Because it is not possible to separate the gonads from the other body parts, the method does not distinguish between gonadal and somatic lipids

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  Corynactis californica
 
Research study 1
 

photograph of 2 corallimorpharians Corynactis californicaAn 18mo study of the corallimorpharian Corynactis californica in the Hopkins Marine Refuge, Monterey, California shows that female clones develop eggs from Aug-Dec.  Male clones develop synchronously with the females.  Spawning is in late Nov-early Dec, coincidental with rising seawater temperatures and likely cued by them.  Fertilisation is external and development is to a free-swimming planktonic planula larva (140µm in diameter).  The larvae are not reared to metamorphosis in the laboratory.  The authors comment that their study is the first to be published on sexual reproduction in C. californicaHolts & Beauchamp 1993 Mar Biol 116: 129.

NOTE  the authors note that C. california is the only west-coast species of corallimorpharian

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  Cribrinopsis fernaldi
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Research study 1
 

Male Cribrinopsis fernaldi in San Juan Islands, Washington release sperm in springtime.  The sperm swim or are drawn into the mouths of the females and fertilise the eggs, some of which are still in the gonads, while others are floating freely in the gastrovascular cavity, in the hollow tentacles, and in the swellings around the upper outer surface of the body column.  Development in laboratory culture (12oC) proceeds within the gastrovascular cavity, through gastrulation (3d) to swimming planula larva (10d), and then to release of the swimming larvae via the mouth (15d).  The authors note that the embryonic development of Cribrinopsis is similar to that of other anemones, e.g., Urticina crassicornis.  The main difference is in the brooding behaviour of Cribrinopsis, which lasts for two or more weeks.  The authors note the lecithotrophic nature of the eggs and suggest that the function of brooding is protective rather than nutritive. Siebert & Spaulding 1976 Biol Bull 150: 128.

NOTE at 20d in laboratory culture the planulae of Cribrinopsis fernaldi begin to settle in response to the presence of polychaetes Phyllochaetopterus sp., which are added to the culture vessels for this purpose

photograph of sea anemones Cribrinopsis fernaldi
Two Cribrinopsis fernaldi 0.25X
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  Epiactis prolifera
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Research study 1
 

Some west-coast anemone species brood their young, either outside on the body column, or inside on the mesenteries.  The commonest of such species in northern Washington and British Columbia is Epiactis prolifera, which broods externally. The species is unusual in that the offspring start out life as females, then become hermaphroditic.  As a female, E. prolifera may be parthenogenetic, producing all-female offspring without the necessity of its eggs being fertilised or, rarely, be cross-fertile (presumably being fertilised by release of sperm from another hermaphrodite individual). Later, as a hermaphrodite, an individual routinely fertilises itself.  Bucklin et al. 1984 Mar Biol 84: 175; Edmands & Potts 1997 Mar Biol 127: 485.

photograph of a brooding anemone Epiactis prolifera  with a few dozen offspring on the body columnNOTE 3 other Epiactis species exist in British Columbia whose reproduction varies from external brooding with separate sexes (E. lisbethae), to internal brooding with separate sexes or with hermaphroditism (E. ritteri and E. fernaldi).  Apparently, less than 20 brooding species of sea anemone photograph of a brooding anemone Epiactis lizbethae  with a few large juveniles still clinging on the body columnexist out of about 800 species worldwide.  Dunn 1975 Biol Bull 148: 199; Fautin & Chia 1986 Can J Zool 64: 1665; Edmands & Potts1997 Mar Biol 127: 485.

NOTE  lit. “virgin-producing birth”.  Parthenogenesis always results in female offspring

Epiactis lisbethae with
large juveniles attached 1X

Epiactis prolifera with a few dozen offspring. Note
that 2 of the juveniles have crawled off the adult 0.75X

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What advantage is it for Epiactis prolifera to be self-fertile? Consider the validity of the answers provided, then CLICK HERE to view possible explanations.

Fertilisation is a certainty. 

Maximises survival. 

Evolutionary success of the species is increased. 

Allows colonisation of available patches in the habitat. 

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Research study 2
 
In adult Epiactis prolifera the eggs and sperm mature simultaneously in gonads on the mesenteries and eggs are fertilized just prior to, or during, their release.  Only a few dozen large eggs are produced and these are carried to the mouth, probably by cilia.  On their release they tumble down the column of the anemone and, just above where the column joins the pedal disc, are caught up in folds of the epithelium.  If older juveniles are still on the column, they may catch and eat the eggs, but later spit them out unharmed. The juveniles live (and feed) for about 3mo on the parent polyp before crawling off to take up solitary life.  Dunn 1975 Biol Bull 148: 199. photograph of a brooding anemone Epiactis prolifera  with a few large offspring on the body column
Epiactis prolifera bearing several large juveniles near the base of the column 1X
photograph of an embryo of Epiactis prolifera caught up in an epithelial fold of the adult courtesy Dunn 1975 Biol Bull 148: 199
Yolky embryo of Epiactis prolifera caught up in an epithelial fold of the adult. The embryo would normally fit tightly in the fold but is shrunken because of fixation. It is about 300um in diameter
 
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Research study 3
 

Often a gentle poke of the column of a sea anemone Epiactis prolifera with a finger can send the juveniles scattering onto the substratum (this takes several moments), but later they most often return to the safety of the adult.  It is not clear whether the youngsters return to the same parts of the adult they occupied previously, or to other parts.  Studies at Bodega Marine Laboratory, California show that juveniles dislodged from the parent when smaller than 4mm basal diameter usually do not survive in the intertidal area. The author notes that at any given time during the year in this area of California, 25-50% of adults are brooding.  Dunn 1977 Mar Biol 39: 41.

NOTE colours may be quite variable in Epiactis prolifera.  Studies in Bodega Bay, California indicate 3 colour morphs within a relatively small geographical area, with colour (and size) correlating with different types of substratum.  Dunn 1977 J Nat Hist 11: 457.

Epiactis sp. with several large juveniles, at least one of which
has moved off the parent onto the rock surface 1X

photograph of a brooding anemone Epiactis sp.  with a few  offspring that have crawled the body column
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The protective advantages to the brooding strategy of Epiactis are obvious, but what are the DISADVANTAGES? Think about the validity of the answers provided, then CLICK HERE to view possible explanations.

There is less recruitment to the juvenile stage. 

There is limited genetic exchange. 

There is reduced potential for geographic colonisation. 

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The main advantage of brooding in Epiactis prolifera seems to be in the safety provided by the adult’s protective umbrella of tentacles, and there is no known transfer of nutrients from the adult to the juveniles.  This being the case, we might predict certain changes in a brooding adult’s morphology and/or behaviour that would favour survival of a brood, especially in areas where the risk of predatory attack is greater. Here are 8 possibilities, 2 of which are not so good.  Think of which these not-so-good ones might be, then CHECK the explanations provided for them.

1.  tentacles grow longer to increase protective canopy.
2.  “attack” tentacles develop to ward off predators.
3.  smaller brood-individuals shelter under larger ones.  
4.  more nematocysts are produced in the tentacles.
5.  brooding adults move closer together.
6.  more tentacles are produced for greater protection.
7.  adults crawl more readily and quickly when carrying a brood.
8.  adults accommodate brood within pharynx in response to a predatory attack.

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  Metridium
 
Research study 1
 

photograph of a (likely) clonal aggregation of sea anemones Metridium senileStudies on sexual reproduction in Metridium senile in Bodega Harbor, California shows that gonadal growth in females occurs from autumn to summer and peaks in August of each year.  Spawning in both sexes occurs in Sept-Oct.  Interestingly, while the pattern of gonadal maturation is asynchronous among 3 populations studied (one on a harbor float and 2 intertidal), spawning is synchronous.  Bucklin 1982 Can J Zool, Lond 60: 3241.

NOTE   a seasonal cycle is not so clear in males





Aggregation of anemones Metriium
senile
, likely clones 0.4X

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  Stomphia didemon
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Research study 1
 
photograph of a swimming anemone Stomphia didemon attached to the substratum

Laboratory studies at Friday Harbor Laboratories, Washington suggest that the swimming anemone Stomphia didemon spawns in April-May.  The eggs, released from the mouth, are 750-800µm in diameter, orange in colour, and filled with yolk.  The eggs float at the surface for the duration of development.  A blastula forms at 3d of age, a swimming planula at 5-7d, and settlement occurs at 8d (at 8-12oC).  The larva swims to the sea bottom with the apical end down and attaches.  A basal disc forms and attachment is complete at 10d.  By 13d of age tentacles appear, and by 90d the juvenile anemone has reached 1mm in diameter across the oral disc.  Siebert 1973 Pac Sci 27: 363.

 

 

Swimming anemone Stomphia didemon 0.3X

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  Urticina spp.
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Research study 1
 

In subtidal areas in Carmel Bay, California gonads in the sea anemone Urticina lofotensis mature during autumn, and spawning of gametes begins in early winter and extends through the year. The graph shows how gonad index (GI) changes with season.  Wedi & Dunn 1983 Biol Bull 165: 458.

NOTE  GI is calculated as the ratio of mass of gonad divided by total body mass including gonad  multiplied by 100 to give %

graph  of gonadal indices of male and female Urticina lofotensis in Carmel Bay, California photograph of a sea anemone Urticina lofotensis
Sea anemone Urticina lofotensis 0.2X
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Research study 2
 
Studies on laboratory-reared planula larvae of Urticina crassicornis show that the larvae will swim for as long as 18d without food and then settle onto pieces of green alga Ulva, possibly in response to bacterial films on the algae.  After swimming and crawling about on the alga for a few moments, the larva settles on its hind (aboral) end, becomes squat in shape, and begins to develop a mouth. Stricker 1985 J Morph 186: 237. schematic drawing showing a planula larva of the sea anemone Urticina crassicornis swimming over and settling on a piece of green alga Ulva
Schematic drawings showing planula larva swimming over surface of green alga Ulva with its aboral end foremost. Shortly after settlement a mouth, or stomodaeum, breaks through at the oral (upper) end. The cross-section shows the juvenile at 6h post-settlement, with the mouth forming, and the inner, endodermal, cells still packed with yolk 110X
photograph of a sea anemone Urticina crassicornis
Sexes are separate in the sea anemone Urticina crassicornis, as in most anemone species 0.3X
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Research study 3
 

Studies on development and early growth of the sea anemone Urticina crassicornis in San Juan Island, Washington indicate the following stages (12oC):

Spawning occurs in April-June and involves strings of mucus containing gametes being extruded from the mouth.  The eggs first aggregate, sink, separate, then float to the surface.  The eggs are yellow-tan in colour and about 600µm in diameter.  The sperm become suspended in the water column soon after their release. The photos below show post-fertilisation stages. Chia & Spaulding 1972 Biol Bull 142: 206.

NOTE juveniles 14mo old are 15mm diameter with 45-50 tentacles.  The authors note that size is no sure indication of age, as growth is dependent upon amount of food eaten. 

 

Day 1 cleavage to 16-cell stagephotograph of a 16-cell stage of the sea anemone Urticina crassicornis courtesy Chia & Spaulding 1972 Biol Bull 142: 206:

Day 3 gastrulation
Day 8 planula is 530x750µm:
photograph of a Day8 stage of the sea anemone Urticina crassicornis courtesy Chia & Spaulding 1972 Biol Bull 142: 206Day 10 the planula explores, and temporarily sticks to, the substratum


Day 11 the planula settles and metamorphoses. The juvenile is 0.6mm diameter and begins to develop a mouth (protruding on the Right in the photograph) and tentacle buds:photograph of a Day11 settling and metamorphosing stage of the sea anemone Urticina crassicornis Chia & Spaulding 1972 Biol Bull 142: 206
Day 17 the juvenile has 4 tentacles.
Day 21 juveniles have 8 tentacles and commence feeding:

photograph of Day21 juveniles of the sea anemone Urticina crassicornis Chia & Spaulding 1972 Biol Bull 142: 206

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