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.

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. Eggs 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. elegantissima.  Ford 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

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.

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

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

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. Photographs courtesy the author.

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

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 120X

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 (12^oC) 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

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

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 8-12^oC).  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

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 %

Day 1 cleavage to 16-cell stage:

Day 3 gastrulation
Day 8 planula is 530x750µm:
Day 10 the planula explores, and temporarily sticks to, the substratum