Reproduction
 

Sexes are separate in sand dollars and, as in other echinoids, the 5 gonads are discrete and easily accessible, providing data on seasonal changes in growth, maturation, and spawning. Gametes are released during late spring into the open water where fertilisation occurs.  Development includes a succession of pluteus larval stages, which feed on phytoplankton for several weeks before the last stage settles to the sea bottom and undergoes metamorphosis. 

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  Gonad growth, spawning, & fertilisation
  Spawning & fertilisation are considered in this section, while LARVAL DEVELOPMENT & BEHAVIOUR, LARVAL FEEDING & GROWTH, and SETTLEMENT & METAMORPHOSIS are considered in other sections.
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Research study 1
 

As in most west-coast echinoids, gonads in sand dollars Dendraster excentricus increase in size in autumn/winter and spawning occurs in springtime. Not surprisingly, small-scale differences in geographic location may have considerable effect on growth and reprodutive output in different populations.  For example, note the differences in gonad indices in sand dollars from 2 populations around San Diego, California – one from a wave-exposed area near La Jolla; the other, from a protected channel at Mission Bay.  Not only are individual gonadal indices much greater in the Mission Bay population, but mean sizes of individuals are also about 50% greater in this area.  Relative gonad size is commonly used as an indicator of differences between habitats because, in the absence of energy-storage organs in echinoids, the gonads represent all residual energy available after metabolism.  The author explains the differences by the greater demands of the more wave-exposed La Jolla habitat for maintenance metabolism at the expense of growth and reproduction.  Niesen 1977 Mar Biol 42: 365.

NOTE expressed here as the ratio of the volume of gonad (ml) to total (formalin-preserved) mass X 100.  This is equivalent to approximate %, with maximum gonadal indices being about 3.5-4.0%.

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Research study 2
 

photograph of cluster of juvenile sand dollars Dendraster excentricus on a sand beachEchinoids with large eggs tend to have a higher probability of fertilisation than ones with smaller eggs, a situation thought to result simply from more collisions with sperm.  However, experiments at the Bamfield Marine Sciences Centre, British Columbia with sand dollars Dendraster excentricus show that another egg attribute, namely, jelly-coat volume, also plays a role in fertilisation success. This is especially true in circumstances where sperm numbers are limited.  Of the 2 features, however, egg volume is about 9-fold more important than jelly-coat volume.  Thus, while both attributes are important selective features in sand dollars, the authors note that the more important role of cell volume in increasing fertilisation success likely owes to the fact that fertilisation actually becomes more difficult with increasing thickness of jelly coat.  Levitan & Irvine 2001 Evolution 55: 2479.

 

Cluster of juvenile sand dollars Dendraster excentricus
at Crescent Beach, British Columbia 0.3X

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