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2. Materials and methods
Sphaerechinus granularis individuals were collected in the Glenan Archipelago in 1991, 1992 and 1993 at two different seasons corresponding to two stages of their
reproductive cycle. The first group was sampled during the maturation stage, at the end of January or early February, when the period of possible winter gonad decreases had
ended Guillou and Lumingas, 1998. The second group was collected in mid July after the breeding season. These sea urchins were transferred to the Marine Laboratory of
Concarneau. In February and July 1991 only 40 urchins whose size ranged within 85 and 100 mm were kept for experimental use whereas in 1992 and 1993 the method was
improved by both selecting 80 urchins and reducing the size interval to 85–95 mm to minimise the variation in organ indices between individual urchins. The collection of
samples ended in January 1993.
The urchins were randomly placed in four glass tanks 82 3 37 3 38 cm each of them containing the same number of individuals, i.e. ten in 1991, 20 in 1992 and 1993
respectively. The tanks were filled with fresh running seawater at a temperature ranged from 7 to 98C in winter to 18–198C in summer Fig. 1 close to that found in the natural
environment. There were two replicates of each treatment, the ‘fed 1’ and ‘fed 2’ replicates were fed every 4 d on freshly collected Laminaria digitata Hudson
Lamouroux, their preferred algae Guillou and Michel, pers. obs which was supplied always in excess. Each time they were fed, the tanks were cleaned, and on the basis of
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the previous study of Guillou and Michel 1994, about 500 mg DW d ind
of Laminaria were introduced into them, but no measurement was done to control the
amount of food ingested. The ‘starved 1’ and ‘starved 2’ replicates were not fed at all. In 1992 and 1993 the experiment carried out on individuals in the gonadal growth stage
sampled in January–February ended in April as soon as spawning was observed in the natural environment where the reproductive condition of the sea urchins was monitored
monthly. This experiment was denoted ‘Maturity period’. In 1991 growth measurements were carried out until the end of June. For the urchins sampled after spawning
mid-July, the experiment was stopped at the end of October 1991 and of September in 1992 and was denoted ‘Recovery period’ Fig. 2.
In order to investigate the effects of food supply on energy allocation during these two stages of the reproductive cycle, three set of parameters were determined: i the changes
Fig. 1. Seasonal changes in seawater temperature in the tanks during the experiments.
186 M
. Guillou et al. J. Exp. Mar. Biol. Ecol. 245 2000 183 –196
Fig. 2. Schematic diagram showing the temporal changes in the gonad indices of Sphaerechinus granularis in the Glenan Archipelago with location of the maturity M and recovery R periods and indication of the list of
parameters analysed during these two stages in 1991, 1992 and 1993.
in the diameter of the tests and wet weights of the urchins over the experimental period were recorded, ii gonad-, gut- and lantern-indices were measured at the end of
experiment, and iii the percentage of organic matter OM contained in the different organs was determined at the end of experiment Fig. 2. For both the 1992-recovery
period and 1993-maturity period, the three organ indices were estimated on 30 urchins of the same size collected by diving in the sampling site, the Glenan Archipelago, before
pre and after post laboratory experiments. Before these experiments the OM percentage in the different organs of these urchins was also determined. Only growth
measurements were made during the ‘1991 maturity period’. All urchins were measured and weighed at the beginning of each experiment, then approximately every 2 weeks
over the experimental period. Horizontal test-diameter was measured along two perpendicular axes 0.5-mm accuracy. The mean diameter was used for comparisons.
22
Total body wet weight was measured 10 g accuracy once superficial water had been
drained on filter paper. At the end of experiment all the urchins were dissected, then their gonads, emptied guts, Aristotle’s lanterns and tests were dried to constant weight at
608C. The gonad-, gut- and lantern-indices were calculated as the dried organ to eviscerated test dry weight ratio multiplied by 100. The percentages of organic matter in
body wall, gonads, guts, lanterns and peristomial membranes were deduced from ash values weighted once tissues had stayed in a muffle furnace at 4508C for 4 h.
To estimate the size and weight increase over the whole experiment, the mean size and weight were regularly calculated per tank at the time of each measurement as
M . Guillou et al. J. Exp. Mar. Biol. Ecol. 245 2000 183 –196
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indicated above, then compared. There were four to seven measurements per experi- ment depending on the measurement periodicity. One-factor ANOVA P , 0.05 with
the LSD test was applied to compare the data, i.e. mean sizes and weights, various indices and the percentages of organic matter per components, once homogeneity of
variances had been tested. All analyses were performed with the statistical software
STATGRAPHICS
.
3. Results
