Earliest ciliary swimming effects vertical transport of planktonic embryos in turbulence and shear flow.

Eggs released by broadcast-spawning marine invertebrates are often negatively buoyant. Blastulae and gastrulae of these species are commonly motile, with passive stability that leads to upward swimming in still water. The earliest occurrence of swimming in developing embryos of diverse invertebrates may therefore permit vertical migration in nature. I used turbulent and laminar shear flows to investigate: (1) the speed and direction of transport of non-motile and newly swimming stages of the echinoids Dendraster excentricus and Strongylocentrotus purpuratus in turbulence, and (2) the limit of stable vertical orientation in swimming blastulae of D. excentricus. Swimming contributed significantly to the rate of upward transport of D. excentricus in turbulence experiments where the kinetic energy dissipation rate (ε) was ∼10(-2) cm(2) s(-3). However, swimming significantly reduced the rate of upward transport of S. purpuratus blastulae in turbulence, suggesting that passively stable swimmers of this species were turned from the vertical, crossed flow-lines, and migrated into downwelling. Observations of swimming in laminar shear indicate that D. excentricus swimming blastulae maintain a vertical orientation until shear approaches 0.26 s(-1), equivalent to sub-microscale shear in turbulence where ε is ∼10(-3) cm(2) s(-3). Swimming speeds of D. excentricus showed an unexpected dependence on shear, indicating that greater shear (within limits) can enhance speed of ciliary swimming. In D. excentricus, swimming by newly hatched blastulae should support upward migration in turbulence characteristic of coastal surface waters, whereas species differences in passive stability and swimming responses to shear may lead to differences in vertical transport and subsequent dispersal.

Abrupt change in food environment induces cloning in plutei of Dendraster excentricus.

Asexual reproduction, or cloning, of planktonic echinoderm larvae has been observed in the laboratory and in nature, but little is known about its ecology. Here we examine the effects of algal food density and of a change in food density on the incidence of cloning in larvae of the sand dollar Dendraster excentricus. Results indicate that a change in food concentration can induce cloning in plutei. Cultures transferred from a low to a high algal ration at the time when primary larvae were developing the third (posterodorsal) pair of larval arms showed decreased postoral arm length, unusual morphologies, and increased larval density in culture. These dense cultures of smaller plutei were produced within 48 h of the food pulse. The result is consistent with the occurrence of a burst of cloning, possibly through anterior autotomization. A second feeding experiment demonstrated that anterior autotomization does occur in 4- to 6-arm plutei. Rather than constituting a developmental rarity, cloning may happen early and often in D. excentricus cohorts when environmental conditions favor rapid growth.

Swimming performance in early development and the "other" consequences of egg size for ciliated planktonic larvae.

The evolutionary significance of egg size in marine invertebrates is commonly perceived in energetic terms. Embryonic size should also have direct effects upon the forces that govern swimming, a behavior common to early larval development in the plankton. If swimming is ecologically important, early larvae may need to perform to a certain "standard", or threshold of speed and/or stability. The existence of performance standards in early development could therefore act to constrain the evolution of egg size and the evolution of development. Here we present the key parameters that characterize the upward swimming speed of ciliated spheroidal larvae moving at very low Reynolds numbers. The dependence of maximum supported mass upon larval size, and the independence of neutral-weight swimming speed from size, lead to hypotheses about scaling of swimming speed with size. Experimental studies with thirteen broadcast-spawning planktotrophs demonstrate that free-living embryonic swimmers in all of these species conform to a strong negative scaling of density with size that offsets increases in mass with increasing size. This trend suggests that swimming ability is broadly under selection in early development. In experimental studies and in a hydrodynamic model of larval swimming, the performance of trochophore larvae provides support for our hypothesized scaling relationships, and also for the concept of a standard in swimming speed. Echinoid blastulae, however, show relationships between speed and size that are not predicted by our scaling arguments. Results for echinoids suggest that differences in ciliary tip speed, or possibly in spatial density of cilia over the blastula's surface, result in significant differences in species' performance. Strong phyletic differences in the initial patterning and growth of structures used for swimming thus appear to cause significant differences in the relationship of swimming ability with embryo size.

Impact of supply-side ecology on consumer-mediated coexistence: evidence from a meta-analysis.

Studies of marine nearshore hard substrates have demonstrated that consumers and abiotic disturbances can remove biomass, clearing space for species that are competitively subordinate and subsequently increasing diversity. However, studies often examine the impact of these space-opening forces on diversity in isolation from other potentially interacting factors. In marine systems, space can be closed by recruitment decoupled from local populations. Therefore, we investigated how recruitment influences the impacts of consumers on diversity with a meta-analysis of 27 experiments of community development involving sessile species on marine hard substrates. These studies allowed quantification of recruitment rates, consumer pressure, and species richness of primary space occupants. This meta-analysis demonstrated that consumers generally increase diversity at high levels of recruitment but decrease diversity at low levels of recruitment. Therefore, species diversity of sessile species is controlled by the interaction between forces that open (predation and herbivory) and close (recruitment) space.