Variation in egg spawning
among subpopulations of sea urchins Strongylocentrotus droebachiensis: a
theoretical approach
S.K.
Meidel, R.E. Scheibling-2001
Marine Ecology Progress Series, 213 : 97-110
(from Current Contents)
Abstract :
Variation in zygote production, by determining the
initial size of the larval pool, can affect the population dynamics of
marine invertebrates with a planktonic larval stage. In this paper, we model
temporal and spatial patterns in egg spawning of the sea urchin
Strongylocentrotus droebachiensis in the shallow subtidal zone along the
Atlantic coast of Nova Scotia. In this region, population outbreaks of sea
urchins have caused major transitions from kelp beds to barrens (areas
devoid of fleshy macroalgae). We define 3 community states and associated
subpopulations of sea urchins: (1) the established kelp bed state, where sea
urchins are in low density and small; (2) the transition state, with sea
urchins in kelp beds, newly created (transitional) barrens, and grazing
fronts (dense aggregations of large sea urchins along the edges of kelp
beds); and (3) the barrens state after fronts have dispersed and
intermediate densities of moderately sized sea urchins remain in
post-transitional barrens. Using data from the literature and unpublished
sources, we parameterize mathematical models to predict egg spawning for
each subpopulation, both on an areal basis and for the entire coast. The
predicted number of eggs spawned per m(2) is 1 order of magnitude higher in
grazing fronts (7.1 x 10(7)) than in transitional and post-transitional
barrens (5.8 x 10(6) and 4.4 x 10(6), respectively), and 4 to 6 times higher
in barrens than in established kelp beds (1.0 x 10(6)). Differences among
subpopulations in the number of eggs spawned are directly related to
differences in adult density, female gonad output, and body size. The total
number of eggs spawned (all subpopulations combined) on a coastal scale
increases similar to6-fold from the established kelp bed state (5.6 x
10(14)) to the late transition state (3.1 x 10(15)) and then drops slightly
in the barrens state (2.4 x 10(15)). During most of the transition state,
sea urchins in barrens spawn the greatest number of eggs. Based on published
values, we estimate that fertilization rates are highest in grazing fronts
(62 %), intermediate in transitional and post-transitional barrens (36 and
43 %,respectively) and lowest in kelp beds (15 %), resulting in up to 2
orders of magnitude differences in the number of zygotes produced in these
subpopulations 14.4 x 10(7), 2.1 x 10(6), 1.9 x 10(6), and 1.5 x 10(5)).
Total zygote production (all subpopulations combined) on a coastal scale
increases similar to 16-fold from the established kelp bed state (8.1 x
10(13)) to the late transition state (1.3 x 10(15)) and then drops slightly
in the barrens state (1.1 x 10(15)). During most of the transition state,
sea urchins in barrens contribute the greatest number of zygotes to the
total zygote pool, with those in grazing fronts contributing up to 44 % of
all zygotes. Sensitivity analysis and evaluation of model assumptions
indicate that our predictions should be correct in a relative sense (i.e.
comparing among subpopulations), although absolute numbers may be
overestimated. A disparity between temporal patterns of estimated zygote
production and observed settlement rates of S. droebachiensis in the shallow
subtidal zone suggests that zygote production interacts with other factors
to determine settlement rates of this species.
(Univ Maine, Darling Marine Ctr, 193 Clarks Cove Rd,
Walpole, ME 04573, USA, e-mail: meidel@maine.edu)