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5.2.2 Microhabitat and behavioural variat i o n
Choice of shelter and the
U rchins were observed using SCUBA at five sites in southern Tasmania: Tinderbox; Ling Reef; Coningham; Alum Cliffs; and Betsy Island. Observations were made at 3 and 7 m at Tinderbox on fou r occasions, at 3 and 1 3 m at Li ng Reef on two occasions, and on one occasion at Coning ham (3 m), Alum Cliffs (5 m) and Betsy Island (7 and 1 0 m). As observations were made underwater, only dermis colours were distinguished. An area was searched thoroug h ly and all urchins were scored for dermis colour, whether they were hidde n under a rock or visible on the upper surface of a rock and whether they exhibited the covering response or not. U rchins were considered to be 'covered' if they were holding at least one piece of alga, shell or pebble on their aboral surface.
Log linear models analysis, which is a method of analysing multidi mensional contingency tables, was used to analyse the data. A series of increasin gly complex mode ls is deve loped; the first model assumes that all the variables are independent. A measure of the deviance of the observed from the expected frequencies is calculated and compared with the x2 distribution. If this model gives a reasonable fit to the data the analysis proceeds no further and the data are said to show random variation. If not, associations between variables are added, one at a time, until a reasonable fit to the data is obtained.
The sampling distribution of ihe deviance approximates the x 2 distribution if; (a) the sample size exceeds 40;
(b) the sample size is at least five times the number of cells
(c) the proportion of expected frequencies with values < 1 does not exceed 1 0%;
(d) no expected frequency is zero (an observed value of zero can have a non zero expected value (Fien berg, 1 970).
Three variables each with two categories were used; MOR, dermis colour or morph (R = red, W = white); HAB, microhabitat (H = hidden, V = easily visible);
COV, covering response (C = covered, U = uncove red). Thus the maxi mum
number of cells was 23 = 8. If all the variables are independent the model is
represented by
MOR + HAB + COV
The more complex models are built up one step at a ti me. For example, if morph and habitat are related then the model becomes
However, this is usually represented as MOR * HAB + COV.
Two other models have one dependent relationship; morph with covering response (MOR * COV + HAB) and habitat with covering response (MOR + HAB *
COV). More complex models would involve two or three dependent relationships between variables (e.g. MOR * HAB + MOR * COV; MOR * HAB * COV).
The models are compared both with the data and with the preceding model, i.e. differences between models which differ by the addition of one term only. The goodness of fit of the models is assessed as shown below;
p > 0.1 the model is an adequate description of the data 0.1 > p > 0.01
p < 0.01
weak evidence that the model is inadequate
the model is not an adequate description of the data
p > 0 . 1 the extra term does not improve the fit of the model
0.1 > p > 0.01
p < 0.01
weak evidence that the extra term is useful the extra term improves the fit of the model
The degrees of freedom, when comparing a model with the data, are given by (n • r), where n = total number of cells and r = number of i ndependent restrictions
imposed by the model. When comparing two models the degrees of freedom are given by the difference in the degrees of freedom associated with the two mode Is considered separately. The analyses were performed using the statistical package 'Genstat'.
Only the sites at Tinderbox '(3 m), Coningham and Betsy Island (1 0 m) had boulders present and could be analysed using the log-linear models method because at the other sites the substratum was flat rock or sand so there was no choice of microhabitat. At these sites G-tests of i ndependence were used to determine whether the morphs differed in their covering response. The G-tests were calculated using B I OI-TAT which applies the Yates correction where ne.cessary.
Nearest
On 1 2 vii 88 two divers observed urchins at Tinderbox (3 m) to determine
whether those of a particular dermis colour (red or white) were randomly associated with urchins of both colours. The alternative hypothesis was that urchins were more likely to occur next to an individual of the same colour. If this was found to be the case then it would seem likely that urchins of the same dermis colour were choosing similar microhabitats, even if the nature of the microhabitat was not obvious to the observers.
1 31
Urchins were chosen haphazardly and their dermis colour and that of their nearest neighbour were recorded for 1 05 pairs of urchins. lf an urchin did not have a neigh bour within 50 em it was not used in the analysis. A G-test for independence was calculated.
5.2.3 Reproductive cycle and investment
Urchins were retained from the repeated samples taken at Tinderbox (7
samples) and Ling Reef (5 samples) d uri ng the study. The urchins were placed i n aerated drums overnight before dissecti on. Measurements taken included live body weight and wet weight of the gonads, the dermis co lour and sex (where possible) of each urchin were recorded and if mature eggs or sperm were present the urchin was noted as 'ripe'.
The proportions of ripe urchins of each morph in each sample were used to determi ne whether the morphs matured at different times. To determine whether reproductive investment varied between morphs, go nadosomatic ratios (gonad weight I body weight) were calculated, but were found to be stro ngly correlated with body weight for s ome samples. Therefore the data were log-transformed to improve the homogeneity of variances and regressions of log body weight against log g o n ad weight were calculated and compared between morphs for each sample by analysis of covariance (ANCOVA). Cochran's C test was used to determi n e whether the assumpti on of homogeneity of variance was met by the transformed data for each sampre. Statistics were calculated using the statistical
package S PSSX.
5.3.4 Tube feet strength experiment
Twenty-eight urchins were col lected from an area of about 1 0 0 m2 at Ti �derbox o n
2
i i 88 and maintained i n flow-thro u g h aquaria at ambient temperature for five days. The method of Sharp and Gray(1 962)
was modified as the urch i n s refused to attach themselves firmly to either a large rock or the underside of a piece of clear perspex. When placed on the gravel-covered bottom of an aquarium the urchi n s either remained where they were placed or moved slowly until they reached one of the sides. When they were placed with their oral surface against a g lass side of the aquarium they attached themselves firmly and in some cases began to climb up the side.Urchins were chosen haphazardly and placed in a bag of nylon monofilament, placed against the side of the aquarium and allowed 5 minutes to attach
themselves. The bag did not impede the movement of the tube feet but allowed the attachment of a spring balance. After the u rchin had attached itself, a force was applied through the spring balance which was gradually increased, over about 20 seconds, to the maximum pull of 3 kg. The force which was needed to remove the urchin was recorded but if it had not released its hold at 3 kg, the time for which it could resist this force was noted. The urchin was then placed on the gravel floor of the aquarium and allowed to rest for at least 5 minutes. The experiment was then repeated. The test diameter of each urchin was measured using Vernier calipers.
Urchins which had resisted a 3 kg pull for varying lengths of time were scored as 3 kg for statistical tests, giving a conservative estimate as they would undoubtedly h ave resisted greater forces. Regressions of test diameter against Pull 1 and {Pull 1 - Pull 2) for each dermis colour were calculated to determine whether strength of tube feet varied with the size of the urchin. Unpaired t-tests were used to determine w�ether the mean force required to remove red and white dermis urchins differed. Paired t-tests were used to determine whether the forces required to remove urchins decreased for Pull 2 for each morph.
5.3 RESULTS
5.3.1 Morphometries and meristics
Means, standard errors and ·sample sizes for morpho metric variables at each site are given in Appendix 4a-d. The data are given for both red and white dermis urchins and for the pooled sample, except for Fortescue Bay where o n ly white dermis urchins occur.
There were differences between red and white dermis urchins from Tinderbox in the maximum width of the ambu lacrum {ANCOVA - for slopes, df1.55, F = 1 5.53, p
�
0.001 ) , the number o f plates in the ambulacrum {ANCOVA - for slopes, df1,55, F = 8 . 1 6 , p < 0 .0 1 ) and the number of plates i n the interambulacrum {ANCOVA - forslopes, df1,55, F = 8.03, p < 0.01 ) ; the maximum width of the ambulacrum inc.reases
more rapidly with size for red dermis urchins whereas the number of plates in the am bulacrum and i nterambulacrum i ncrease less rapidly with size for red urchins (Figure 5.1 ). No significant differences were found between dermis colours at Ling
Reef for any of the variables.
For the Tinderbox CVA classifying urchins by dermis colour (red or white) the canonical variate was significant {Wilks' A. = 0.744, df = 4 , p < 0 .0 1 ) with red dermis
( a ) E ::J L u .0 E ro '-- 0 .c. .... X ro E Q) 0 ( b ) E :::> ' u ro - ::J .0 E ro c: en Q) +-> ro a. 0 c: 0) 0 1 .4 1 . 3 1 .2 1.1 1 0
( c£ J 5
:::> ' u ro .0 1 4 E ro L Q) .... c: (j) Q) ... ro F i g u r e 5. 1 a. 0 c: 0) 0 1 3 1 2 1 1 1 33 l og 1 i v e w e ight • Red 0 W h i t eR e g r e s s i o n s for R a n d W derm i s T i n d erbox u r c h i n s o f l o g l i v e w e i g h t a g a i n s t ( a ) l o g m ax. w i dth a m b u l a cr u m , C b ) log n o . p l a t e s i n a m b u l a cr u m , ( c ) n o . p l a t e s i n i n t e ra m b u l a c r u m .
interambulacra) and heavier tests and spines (Table 5.2). Although 71 . 1 9% of individuals were correctly classified, the Mahalanobis' distance indicated that the centroids of the two g roups were not significantly different from each other (D =
1 . 1 53, p > 0.05). The CVA's using dermis colour at Ling Reef and spine colour at
Tinderbox and Ling Reef showed no variation between morphs (Tables 5.3-5.4). When urchins were compared among all four sites (ignoring dermis and spine colours) the three canonical variates all explained a significant amount of the variation (1 st c.v. , 57. 6 1 %; 2nd c.v. , 33.04%; 3rd c.v., 9.35%; Table 5.5a). The centroids and i ndividual points for the first two canonical variates are plotted i n Figure 5.2 and the Mahalanobis' distances show that all pai rwise comparisons are significantly different (Table 5.5b). The first canonical variate separated Cowrie Point from the other three sites and the second variate separated Tinderbox and Ling Reef, with Fortescue Bay intermediate. The third variate separated Fortescue Bay from the other two sites in southern Tasmania. The major components of the fi rst variate were primarily lantern dry weight and secondarily length of the longest spine, with Cowrie Point having lower values for both. The major components of the second variate were test thickness and test height with Tinde rbox having higher values for both. The major component of the third variate was test height with Fortescue Bay having relatively flatter tests than Tinderbox and Ling Reef. Overall 77.33% of individuals were classified i nto their correct sites with 90% of Cowrie Point urchins correctly classified (Table 5.5c).
Most of the morphological v�riation therefore seemed to occur between sites rather than between morphs. To confirm this, urchins were classified i nto groups based on site and morph and reanalysed. The first three canonical variates explained a significant amount of the variation (1 st c.v. , 51 .65%; 2nd c.v., 31 .67%; 3rd c.v. , 1 1 .20%; Table 5.6a). The Mahalanobis' distances agai n showed significant differences for all pai rs of groups from different sites and no significant differences within sites (Table 5.6; Figure 5.3). This approach reduced the power of -the analysis with an average of only 56.67% of i ndividuals being correctly classified into groups (Table 5.6c). However, it can be seen that very few of the Cowrie Point urchins were classified into any other site and the largest number of misclassifications occurred between Ling Reef and Fortescue Bay.
5.3.2 Microhabitat and behavioural variat i on
The study of the interrelationships between dermis colour, 'coveri ng' response and position of the urchin showed that for two of the four samples taken at
1 3 5
Table 5.2 Resu Its of can onical variate a nalysis using
dermis colour (red, w h ite) for Tinder box urch ins. (a) Percentage variation exp la ined b y t h e first canon ical
variate, can o n ical correlation, W i l ks' la mbda and the loadings (latent vectors) for the stan d a rdised m o rphometric varia b les.
0/o