Reproduction & development
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  Brittle stars
  The section on reproduction & development is divided into 3 parts: brittle stars, considered here, and BASKET STARS and GENE FLOW dealt with elsewhere.
 
Research study 1
 

photograph of ophiopluteus larva of a brittle star courtesy Tim Rawlings, Cape Breton Uphotograph of a juvenile britttle star Ophiopholis aculeata courtesy Balser1998 Biol Bull 194: 187The most common life cycle of brittle stars in temperate latitudes includes spawning of eggs and sperm into the open water, with development leading through a 4-armed ophiopluteus larva to an 8-armed settling stage.  After a few weeks feeding on phytoplankton the larva settles to the sea bottom.  Prior to this the larva has commenced metamorphosis and comes to a form resembling a small sea star (see photograph on Right).  Two of the 4 large larval arms have been resorbed, leaving the juvenile attached to the remaining 2, as shown in the photo.  The juvenile is relatively large and heavy at this time, and settlement may be simply passive sinking owing to increased mass.  Balser 1998 Biol Bull 194: 187. Photo of ophiopluteus larva courtesy Tim Rawlings, Cape Breton University, Nova Scotia.


NOTE
  lit. “serpent” G. combined with “mantlet, breastwork, parapet” L.

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Research study 2
 

graph of gonadal indices of brittle stars Ophiothrix spiculata, Ophiopteris papillosa, and Amphiodia occidentalis over timeA collection of 5 species of ophiuroids in the Monterey region of California reveals 3 that broadcast spawn leading to planktotrophic ophioplutei larvae (Ophiothrix spiculata, Ophiopteris papillosa, and Amphiodia occidentalis), and 2 that brood their young (Amphipholis (Axiognathus) squamata and Ophioplocus esmarki).  The first 3 spawn in winter and spring (see graphs on Left). 

Amphipholis squamata is viviparous and broods throughout the year while Ophioplocus esmarki is ovoviviparous and broods during late spring/summer (see graph lower Right).  Brood number in the larger-sized Ophioplocus is roughly200 times greater than in the smaller-sized Amphipholis.  graphs showing brood sizes of brittle stars Amphipholis squamata and Ophioplocus esmarki

 

 

 


Egg sizes are roughly the same for the broadcast spawners and for Amphipholis (around 100µm diameter), but are 3 times larger in Ophioplocus (330µm diameter).  Rumrill & Pearse 1985 p. 633 In, Echinodermata (Keegan & O’Connor, Eds) AA Balkema, Rotterdam. Photographs of O. spiculata and A. occidentalis courtesy Peter Bryant, UC Irvine; all other photos courtesy Bill Austin, Khoyotan Mar Laboratory, Victoria.

NOTE producing living young instead of releasing eggs

NOTE producing living young from eggs that hatch within the body

 

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Research study 3

 

photograph of brittle star Ophiopholis aculeata courtesy Dave Cowles, Walla Walla U, WAphotographs of cloning larvae of brittle star Ophiopholis aculeata courtesay Balser 1998 Biol Bull 194: 187
Studies at Friday Harbor Laboratories, Washington on the brittle star Ophiopholis aculeata disclose a unique cloning behaviour in the larva, which has the potential to increase the distributional potential of the species without additional reproductive input.  In this species metamorphosis is mostly complete by the time of settlement and, at the sea bottom, the tiny juvenile remains attached to 2 of the long larval arms.  Within a short time the juvenile releases its attachment to the arms, which float away (see Research Study 1 above).  However, after a 5-d period (under laboratory conditions at 10OC), a new larva regenerates from residual tissues at the point where the arms meet on the original larval arms and swims off for a further stay in the plankton (see photo series on Right). The asexually produced secondary larval clone closely resembles the primary larva.  It is also planktonic until settlement.  At this time, the arms may be released to initiate a new tertiary larval generation.  The author suggests that the cloning process could continue as long as environmental conditions can support a planktonic existence. Balser 1998 Biol Bull 194: 187. Photograph of O. aculeata courtesy Dave Cowles, Walla Walla University, Washington www.wallawalla.edu.

NOTE  although cloning is well known in brittle stars and sea stars, its occurrence in other echinoderm classes has only been recently documented.  A recent study shows that it occurs spontaneously in larvae of sea cucumbers Parastichopus californicus (e.g., 12% of doliolaria larvae), sand dollars Dendraster excentricus (4%), and sea urchins Strongylocentrotus franciscanus (5%). The diversity of mechanisms by which cloning occurs suggests that it may have evolved independently in these various groups or has diverged widely from an ancestral mode. Eaves & Palmer 2003 Nature 425: 146.

Early pentacula larva of sea cucumber
Parastichopus californicus
with clone attached

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What advantages are there in Ophiopholis producing successive generations of larval clones? Four in the following list of 6 are advantages; 2 are not. Try to identify these 2 and then CLICK HERE to see explanations.

It saves an individual the energy costs of producing gametes for sexual reproduction. 

It increases the geographic range of the species. 

It leads to increased fecundity. 

It increases the power of selection for a given genotype. 

It increases the chance of a larva finding a suitable spot to settle. 

It increases larval survival. 

NOTE  the quiz is predicated on the assumption that the clones go on to reproduce sexually.  This has not yet been shown to happen, and the possibility exists that the clones are sterile

 

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Research study 4

 

photograph of 45h gastrula of brittle star Amphiodia occidentalis photograph of 65h gastrula of brittle star Amphiodia occidentalis The intertidal- and subtidal-inhabiting brittle star Amphiodia occidentalis has a highly modified and unusual pattern of development.  In culture at Friday Harbor Laboratories, Washington the fertilised eggs are negatively buoyant and gastrulate after 30h (at 8-10OC; see photograph above Left). By 46h of age the gastrulae begin to show crystalline deposits that presumably later develop into elements of the skeleton (see photograph above Right). Hatching occurs at about 70h post-fertilisation and the embryo develops into a flattened bilaterally symmetrical disc lying immotile on the bottom of the culture vessel.

photograph of 115h post-gastrula of brittle star Amphiodia occidentalis By 115h the central and radial skeletal plates are visible and the embryo now exhibits pentamerous radial symmetry (see photograph on lower Left). At about this time buds of the podia (tube feet) become visible. A mouth primordium appears about 7d after fertilisation, and by 12d the mouth can open and close (see photograph on lower Right). By 8d podial photograph of 65h gastrula of brittle star Amphiodia occidentalismovements are evident.  By 19d the juveniles are feeding and crawling by use of the single distal pairs of podia on each ray.  Note that during development there is no indication of ciliated bands, larval arms, or other features typical of an ophiopluteus larva, nor of ciliated bands of a vitellaria. Because developmental stages identical to those described here are collected in plankton tows at Sunset Bay, Oregon, the author is confident that the pattern of development seen in the laboratory are the same as in the field.  Emlet 2006 Invert Biol 125: 154.

NOTE  obtained from adult specimens collected intertidally at Orcas Island, San Juan Archipelago, Washington in spring and early summer.  These animals spawn while in the collecting buckets, but the author comments that many subsequent attempts to induce spawning in this species have failed

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