About the Cover
Cover
The green sea urchin Strongylocentrotus droebachiensis has a circumboreal distribution in cold-temperate and arctic waters. This herbivore plays a pivotal role in determining the structure of rocky subtidal communities throughout its range. Dense aggregations of urchins (or feeding fronts) form along the margins of kelp beds and destructively graze all upright seaweeds, creating urchin barrens of much lower productivity, habitat complexity, and biodiversity. In this photograph, urchins in a front are grazing laminarian kelps (Laminaria digitata and L. longicruris) at 10-m depth near Halifax, Nova Scotia, Canada. The formation of these fronts results in the emergence of three distinct zones on the rocky seabed, each with its characteristic subpopulation of sea urchins. Fronts are bands 2- to 3-m wide composed of large adult urchins with a high reproductive effort (gonadic index) at maximal densities (hundreds per square meter). Kelp beds in advance of these fronts harbor low densities of urchins that are generally smaller and have an intermediate reproductive effort. Barrens in the wake of feeding fronts have intermediate densities of relatively small urchins with a low reproductive effort. The differences in density and subpopulation characteristics influence the fertilization success of these free-spawning invertebrates (releasing gametes into the water column for external fertilization) in each zone.
On pages 195-205 of this issue, Lauzon-Guay and Scheibling present a spatial fertilization model for Strongylocentrotus droebachiensis. By incorporating population characteristics such as urchin density, size, and gonadic index into their model, they show that fertilization success is much greater than estimated from previous field experiments and theoretical studies. Their results suggest that sea urchins can achieve high fertilization rates, even at relatively low density (as in kelp beds) or with low reproductive effort (as in barrens), when eggs are allowed to disperse over realistic times and distances to encounter sperm from multiple males located upstream and downstream. These results challenge the conventional hypothesis that sperm is limiting, and consequently fertilization rates are low, except in very dense populations of free spawners.
Credits: Photo, Robert E. Scheibling (Dalhousie University); cover layout, Beth Liles (Marine Biological Laboratory).
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Copyright © 2007 by the Marine Biological Laboratory.
