Contributions to Zoology, 85 (3) – 2016Michelangelo S. Moerland; Chad M. Scott; Bert W. Hoeksema: Prey selection of corallivorous muricids at Koh Tao (Gulf of Thailand) four years after a major coral bleaching event
Results

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Distribution of muricids

The total of muricid individuals found within the 31 quadrats was 1249, consisting of 1145 Drupella rugosa, 60 D. margariticola and 40 Morula spinosa snails. Two quadrats were excluded from prey choice analyses due to minor uncertainties in the data and three large recruitment aggregations were left out of the mean site density calculation to prevent over-estimation. D. rugosa snails were found in every 10-m2 belt quadrat in which their number varied from one to 168 (Table 2). The average density (±SE) for D. rugosa calculated from all site densities was 3.17±0.69 indiv. m-2 in shallow areas and 3.45±1.54 indiv. m-2 in slightly deeper areas. D. margariticola occurred in relatively small numbers (shallow: 0.21±0.07 indiv. m-2, deep: 0.06±0.03 indiv. m-2) and M. spinosa occurred in even lower densities (shallow: 0.17±0.06 indiv. m-2; deep: 0.12±0.05 indiv. m-2) and generally did not seem as restricted to coral reef habitat as Drupella. All species were rare at pinnacles, with 0-2 individuals observed per dive. Related corallivores, e.g. other species of Drupella and Coralliophilinae, were not recorded at Koh Tao within and outside this study.

Although numbers of all three muricid species generally decreased with depth and were hardly seen below 8 m depth, no significant differences were found between medians of the shallow and slightly deeper quadrats (D. rugosa: W = 11, p = 0.147; D. margariticola: W = 20, p = 0.147; M. spinosa: W = 16.5, p = 0.460). Linear regressions based on the average depth of quadrats also showed slight negative trends that were insignificant (D. rugosa: p = 0.358; D. margariticola: p = 0.148; M. spinosa: p = 0.161). However, the distribution of juveniles and adults varied significantly over depth for D. rugosa (X2 = 2610.57, p < 0.0001, Fig. 3) and M. spinosa (X2 = 2.96, p < 0.0001), but not for D. margariticola (X2 = 49.50, p = 0.085). Juveniles were more abundant in shallow quadrats whereas adults were more abundant in the deeper quadrats.

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Table 2. Density (indiv. m-2) of Drupella rugosa, D. margariticola and Morula spinosa in transects at nine sites (Fig. 1) and two depth ranges: dark = 2-5 m, white = 5-8 m. Densities are the mean of 1-4 transects. Large aggregations of recruits have been omitted.

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Fig. 3. Mean density (+SE) of Drupella rugosa per size class in shallow (3-5m) and deep (5-8m) transects.

All species and their age groups were most abundant on stony coral than on any other substrate (Fig. 4). The total number of individuals on living coral was highest for D rugosa (95%) and D. margariticola (82%), which otherwise occurred on coral rubble. Relatively fewer M. spinosa snails were found on live coral substrate (65%), and they were also encountered on rock, rubble and sand. Substrate occupation differed between juveniles and adults only in D. rugosa, for which juveniles were found proportionally more on live coral (D. rugosa: X2 = Inf., p < 0.0001; D. margariticola: X2 = 1.28, p = 0.258; M. spinosa: X2 = 0.04, p = 0.842). D. rugosa were predominantly found on larger coral heads (> 10 cm diameter) (Fig. 5).

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Fig. 4. Total numbers of snail individuals per size class found on stony coral (HC) and other kinds of substrate for A) Drupella rugosa, B) D. margariticola and C) Morula spinosa.

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Fig. 5. Total number of coral colonies occupied by Drupella rugosa per colony size class based on maximum diameter (cm). Only one of the 235 occupied colonies fell within the 1-10 cm colony size range.

A significant positive correlation was found for Drupella density and healthy stony coral cover (r = 0.118, Adj. R2 = 0.199, p < 0.01). When the numerical transect variables were analysed in a generalized linear model to explain D. rugosa distribution, the simplest significant model under a quasi-poisson link function consisted of Acropora density (p < 0.01), depth (p < 0.01), and rubble (p < 0.05). The proportion of the variance that could be explained with this model is 0.56, which is quite low.

Some D. rugosa aggregations consisted of high numbers of recruits or juveniles and sometimes a few adults on the coral colony margin. These were all found on digitate, tabular and bushy Acropora colonies. All were found on different sites between 2 and 4 m depth and contained up to ~100 individuals. Three large aggregations of recruits (55, 56 and 96 individuals in size) were found within quadrats, which is three aggregations per 310 m2, or roughly one per 100 m2.