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Description:
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We measured algal consumption, test growth, jaw growth, change in whole body wet
weight and gonad weight of urchins fed one of five experimental diets over an 89-day
period in a controlled laboratory setting. Urchins used in the experiment were
collected on October 2010 from a shallow (~4 m depth) boulder reef (34° 24.9 N 119°
49.8 W) located offshore of the University of California, Santa Barbara. To minimize
inherent variation in growth potential, consumption potential, and initial gonad
weight, urchins were chosen to be relatively uniform in size (horizontal test
diameter 33.5 ± 0.4 mm, mean ± SE) and collected from a denuded urchin barren where
their gonad weight was predicted to be uniformly low. Upon collection, urchins were
transported to the laboratory in insulated containers, placed in aquaria with
running seawater and starved for one week prior to the start of the experiment.
Blotted dry urchins (placed with the aboral end facing down on paper towels for 5
minutes) were weighed to the nearest 0.01 gram and their horizontal test diameter
was measured to the nearest 0.1 mm using Vernier calipers. To measure jaw growth,
each urchin was injected with the fluorescent marker tetracycline following Ebert
(1982). 1.0 g of tetracycline was mixed with 100 mL seawater and 0.2 mL of the
resulting solution was injected into each urchin through the peristomal membrane
with a hypodermic needle. Tetracycline binds to actively calcifying tissues,
effectively labeling jaw material present at the start of the experiment. Jaw
material calcified after tetracycline administration was therefore unlabeled.
Each urchin was assigned to one of 35 labeled plastic containers (32 x 19 x 11 cm)
supplied with flow-through seawater. This setup allowed us to keep track of
individuals without the use of external tags. Seawater temperatures ranged from 11.6
to 16.3 °C during the experiment and matched ambient conditions. Urchins were fed
one of five macroalgal diets: a monospecific diet of either Macrocystis pyrifera,
Pterygophora californica, Rhodymenia californica, or Chondracanthus corymbiferus
(all species hereafter referred to by genus), or an equal mixture of all four
species (hereafter referred to as the mixed diet) with n = 7 urchins per diet type.
Algae were introduced into the tanks during nine periods ranging in length from 4 to
8 days in which all experimental urchins were fed a known amount of algae (either
approximately 34 g of one species, or in the mixed diet treatment approximately 10 g
of each of the four species). During the 89-day experiment, urchins were exposed to
algae for approximately 54 days. Rhodymenia was absent from the monospecific
Rhodymenia treatment and from the mixed diet treatment for one of the feeding
periods (14% of the total exposure time) due to its lack of availability in the
field. To study algal consumption, algal wet weight (after removing excess water
with a spinning colander) was measured at the beginning and end of each feeding
period. Consumption was calculated as wet weight (g) of algae consumed per urchin
per hour using the total amount of hours urchins were exposed to algae (exposure
time). We used consumption rate rather than amount consumed to standardize for
different exposure times in the Rhodymenia treatment. To evaluate the nutritional
content of the algae, tissue samples of each species of algae were collected at
three time points during the experiment and analyzed for carbon and nitrogen content
(% dry weight). Samples were weighed wet (after removing excess water with a
spinning colander), placed in a drying oven at 60°C until dry, ground to a fine
powder, and stored in a desiccator until analyzed by the UCSB Marine Science
Institute Analytical Laboratory using the Dumas combustion method (duplicate samples
from each species at each time point were tested).
At the end of the experiment the
horizontal test diameter and wet weight (measured to the nearest 0.01 g in blotted
dry urchins) of each urchin was measured. The change in test diameter and change in
wet weight of each individual over the experiment was calculated by subtracting the
initial value measured at the beginning of the experiment from that measured at the
end of the experiment. Gonads were removed from each urchin upon dissection, placed
in an oven at 60°C until dry and weighed to the nearest 0.01g. Final gonad dry
weight was used as a measure of gonad growth because initial gonad weight was
presumed to be nil as all individuals used in the experiment were similar size and
collected from a barren. We verified this assumption by taking eight urchins (test
diameter 34.3 ± 1.0 mm (mean ± SE)) from the original collection site in the middle
of the experiment and measuring the mass of their gonads following the same
procedure described above (see Results).
Jaw growth was measured using half-pyramids
of the aristotle’s lantern following Ebert (1982). Half pyramids were removed from
each urchin and soaked in a 5% sodium hypochlorite solution for 24 hours. For one
half-pyramid per urchin, the total length from the oral tip to the flat shoulder at
the aboral end (see Ebert (1980) for pictures of points of measurement) was measured
to the nearest 0.01 mm using a dissecting microscope equipped with an ultraviolet
lamp and an ocular micrometer. Fluorescence from labeled tetracycline was observed
from the oral tip to part way up the length of the jaw, indicating that this
material had been present at the start of the experiment. Jaw growth was measured as
the length of the non-fluorescent “band” extending from the top of the fluorescent
area to the flat shoulder at the aboral end.
Differences among treatments were
analyzed separately for each response variable (consumption rate (g of algae
consumed⋅ h-1 averaged over the experiment), change in test diameter, change in jaw
length, change in whole body wet weight, and final gonad dry weight) using one-way
ANOVA and a post-hoc Tukey test to compare means (statistical significance was
determined at the p less than 0.05 level). Diet selectivity was studied by examining
the rate at which individual species of algae were consumed in mixture treatments
with container (individual urchin enclosure) included as a random effect (Gelman and
Hill 2006) as consumption rates of individual species of algae in a single container
were not independent. A post-hoc comparison of algal species effects was then
performed with False Discovery Rate corrected p-values (Benjamini and Hochberg
2000). All statistical models were fit using R version 2.15-3 (R Development Core
Team 2012) with the nlme package for mixed models (Pinhero et al. 2012) and the
multcomp library for post-hoc analyses (Hothorn et al 2008).
See paper for full citations.
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