Reproduction & development
 

photograph of a colorful brittle star Ophiopholis aculeata crawling amongst tunicates Cnemidocarpa finmarkiensis courtesy Bill Austin, Khoyotan Marine Laboratory, Victoria, BCOphiuroids have diverse reproductive modes including planktotrophic larvae (ophiopluteus), pelagic non-feeding larvae (vitellaria), brooded embryos, and benthic encapsulated development. Photograph courtesy Bill Austin, Khoyotan Marine Laboratory, Victoria, British Columbia




Colorful brittle star Ophiopholis aculeata crawling
amongst tunicates Cnemidocarpa finmarkiensis 0.6X.

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  Basket stars
  This section on reproduction & development is divided into 3 parts: basket stars, considered here, and BRITTLE STARS and GENE FLOW dealt with elsewhere.
 

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Research study 1

 

graph showing "maturity index" of basket star Gorgonocephalus eucnemis over the course of a yearphotograph of basket star Gorgonocephalus eucnemisA 13-mo study of reproductive cycle in the basket star Gorgonocephalus eucnemis at Friday Harbor Laboratories, Washington shows that spawning time varies with geographic location.  For example, one population in the San Juan Islands may spawn Nov-Mar while another, at a different location, may spawn in Jun-Nov.  The author describes 5 stages in the reproductive cycle from post-spawning to spawning, based on size and condition of the gonads.  A tally of the number of individuals in each stage at each collection date leads to a sort of "mean reproductive index", which the author refers to as “maturity index”.  Note in the graph that maturity indices are highest in late summer, with synchronous spawning of both sexes in Oct-Dec. Sexes are separate (1:1 sex ratio), with about 3% of a sample studied being hermaphrodites.  Eggs and sperm are formed in special bursa located on the arms and then released into the seawater.   Patent 1969 Biol Bull 136: 241.

NOTE  because there is no centralised gonad mass in basket stars, the more customarily used “gonad index” for echinoderms cannot be determined. A value of 5 would indicate that all individuals in a population have maximally ripe gonads

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Research study 2
 

photograph of an alcyonacean soft coral Alcyonium sp. formerly known as Gersemia rubiformismap of collecting site in San Juan Islands, Washington for study of life cycle of basket stars Gorgonocephalus eucnemisThe life cycle of the basket star Gorgonocephalus eucnemis is quite remarkable, for after fertilisation and at a size of about 0.7mm disc diameter, the embryos are ingested by polyps of the alcyonarian Gersemia sp. and take up life in the pharynges of the polyps.  Apparently, the juveniles can be easily seen in situ with a microscope if the Gersemia polyps are back-lit.  After living in the polyps for a while and presumably feeding on food that the Gersemia polyps have captured, the juvenile basket stars crawl out onto the surface of the Gersemia colony.  At this time the juveniles can be seen with their arms inserted into the histogram of number of individuals of basket stars Gorgonocephalus eucnemis living with adults or free-living mouths of the polyps,
perhaps searching for food or just hanging on.  The 2 species breed at the same time of year, so while Gersemia is brooding its own eggs to a planula stage within its polyps, young basket stars are also present.  The author of the study, done in San Juan Islands, Washington, suggests that the Gorgonocephalus juveniles may be feeding on the eggs and embryos of the brooding Gersemia

At a size of about 3mm diameter, the young basket stars move from the Gersemia colonies and take up life on adult basket stars.  They live on the adults until reaching a diameter of 4-5cm, then crawl off and take up a solitary life (see accompanying histogram).  On the adult, the juveniles (5-20 in number) may hang out near the mouth, perhaps filching food, or cling to the bursal slits where their rough, hooked arms may cause abrasive damage.  Of 274 individuals examined in the 19-mo study, the smallest free-living individual found by the author was 2.5cm diameter.  Growth is quite fast to a diameter of 5cm, then it slows until an adult size of 7-8cm is reached.  An adult may attain a size of 9-10cm.  Adult colours are mainly shades of brown and tan, although pink, coral, white, and even bright orange-red colours are also seen.  Patent 1970 Ophelia 8: 145; Patent 1970 Mar Biol 6: 262.

NOTE  although this intermediate incubatory species has been documented in other studies on G. eucnemis, it is unclear whether it is an obligatory, or facultative, life phase.  If so, distribution of the 2 species must be identical.  The author notes the difficulty in keeping juvenile basket stars alive on their own beyond 1-2d

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