As a sea star, is it better to broadcast spawn or to brood?  This question is addressed for populations of Pisaster ochraceus, a spawner, and Leptasterias hexactis, a brooder, at Friday Harbor Laboratories, Washington.  The species differ 100-fold in size (600 vs. 6g live mass, respectively).  As an evolutionary strategy, spawning permits rapid colonisation of new habitats, reduces local competition for space, avoids local concentrations of predators, and allows long-distance dispersal – especially important for a sedentary or sessile species.  An open-water life for a larva, however, is risky and the trade-offs are poor survival and poor recruitment.  An average-sized (600g) female Pisaster releases 40 million eggs during springtime spawning, of which 99.99% will die before, during, or shortly after settlement.  For this reason, only about 6% of the San Juan Islands populations of P. ochraceus are comprised of “immatures” (<70g).  

In comparison, as an evolutionary strategy, brooding leads to a much higher proportion of offspring surviving to recruitment age.  In the San Juan Island populations of L. hexactis, this translates to an average of 31% of “immatures” being present.  Based on these recruitment percentages and the number of eggs produced by each species, Leptasterias, therefore, has 270,000 times better chance of surviving to recruitment age than does Pisaster.  The author notes that a broadcast-spawning strategy, with its inherent risks of survival, can only work for a large species that produces large numbers of eggs.  For example, based on mean number of eggs produced by a single “large-species” female P. ochraceus and their percentage survival as noted above, a similarly broadcast-spawning small species L. hexactis would have to reproduce annually for 1600 years to produce 2 mature offspring.  The author notes that this is not a viable evolutionary strategy. Menge 1975 Mar Biol 31: 87.

Sea stars are not the only invertebrate group in which brooding species are smaller in size than spawning species.  The phenomenon occurs in chitons, bivalves, ophiuroids, slipper limpets, and other taxa. The previous Research Study 1 on sea stars convinces us that broadcast spawning is a viable evolutionary strategy only for large species that produce large numbers of eggs, but what selective forces in evolution lead to small sizes being associated with brooding?   This question has been addressed by researchers but with no definitive answers.  Two sample ideas from a long list involve the possibility that the adults (perhaps also as juveniles), being small, are themselves dispersed in waves and currents.  So, one idea is: 1) if small adults are themselves dispersed then they would derive less advantage from dispersal via larvae, and selection has been for brooding with its high survival success in locally favourable areas. 

Another idea: 2) relates to the fast generation time of small adults and their ability, via brooding, to exploit a locally favourable area quickly and effectively over just a few generations.  When conditions deteriorate, they can then exploit their small size and escape as juveniles or adults via current-dispersal to new areas.   The authors of a review paper on the subject, in which a number of hypotheses are presented and discussed, conclude that since none of the hypotheses fit all cases of small adult sizes and brooding, then the trend may result from selection for other, perhaps unrelated life-history traits, in the different invertebrate groups.  See the excellent review by Strathmann & Strathmann 1982 Am Nat 119: 91 from which these ideas are taken.