Field and laboratory work

Field data collection was carried out during April 2015 in LA and August 2015 in PB reefs. Because J. pustulata normally dwells the underside of coral colonies when inactive (daytime, Figure 2), loose colonies from the reef framework were surveyed, using SCUBA diving (twice a day). All individuals were counted and measured (shell length in mm) in order to determine the size structure of the species in each reef. In addition, 15 individuals from each reef (and an identical number of coral fragments) were collected and used for the coral consumption experiment performed under controlled conditions in the Gorgona NNP laboratories.

Figure 2 Jenneria pustulata feeding on a coral colony. Photography by Juan Felipe Lázarus. 

Each pair, composed of one specimen of J. pustulata and one coral fragment (n =15 pairs) were individually introduced into 16 L capacity aquaria over a period of 24 h. The coral fragment assigned to each snail was obtained from the colony where the snail was originally found and captured. During the experiment, the aquaria were continuously monitored, and water was replenished and oxygenated (with ventilation motors). The injuries caused to the fragments by the snails (Figure 3) were photographed using a Canon D30 camera. Using the free software ImageJ 64 (^{Abramoff et al., 2004}), the area (mm2) of the injuries was calculated from the photographs.

Figure 3 Pocillopora sp. fragment before (A) and after (B) corallivory by Jenneria pustulata during 24 h. 

Statistical analysis

After confirming the assumptions of the parametric tests, snails size was compared between reefs (2 levels fixed) using one-way analysis of variance. The size structure for each reef was compared using an Anderson-Darling test for two samples (^{Engmann et al., 2011}). In addition, a one- way ANOVA was used to evaluate coral tissue consumption between reefs. Finally, an ANCOVA was performed on coral tissue consumption with reef as the explanatory variable and shell size as the covariate. All the statistical analyses were carried out using the free software R (R Development Core Team, 2014) and following the statistical assumptions of ^{Zar (Zar, 1999)}.

Size and size structure

A total of 201 individuals of J. pustulata were examined and measured, with 74 (36.8%) collected from LA and 127 (63.2%) from PB. The mean (±DE) shell length was slightly higher for specimens from PB (20.5±3.40 mm) than for those from LA (19.6±4.71 mm) (Figure 4). Although the previous values appear similar, statistical analysis showed differences (F (0,05(2)1;199) =11.24; P < 0.001). In addition, PB snails’ size ranged from 16.5 to 25.0 mm, whereas in LA the size variation was wider and ranged from 14.0 to 24.0 mm.

Figure 4 Mean size (box: SD, whiskers: minimum and maximum) of Jenneria pustulata in La Azufrada and Playa Blanca reefs. 

The size distribution showed that most (60.8%) of the snails from LA ranged between 17 and 21 mm long; this size distribution differed from the Normal distribution (A=0.92; P=0.019) (Figure 5A). The same analysis for the snails of PB showed that most (78.4%) clumped between 18 and 22 mm; this size distribution also differed significantly from the Normal distribution (A=1.44; P<0.001) (Figure 5B). Finally, a comparison of the size structure between both reefs showed that they depicted different structures (A=5.48; P=0.002).

Figure 5 Size distribution of Jenneria pustulata in La Azufrada (A) and Playa Blanca (B) reefs, Gorgona Island. 

Consumption of coral tissue

The corallivory experiment with LA snails had a success rate of 44.4%; that is, of the initial 18 snails, only 8 consumed tissue. However, among the PB snails, 66.7% (10 of 15) consumed coral tissue. A positive relationship between snails’ size and the amount of coral tissue consumed in LA (R2=0.81) and PB (R2=0.53) was observed (Figure 6). The average consumption of coral tissue (mm2day-1) per snail ±DE was 105.9±77.01 in LA and 130.5±105.50 in PB. The consumption range varied between (31.1 - 242.2) and (21.0 - 298.0) mm2day-1 for LA and PB respectively.

Figure 6 Relation between size (shell length in mm) and coral tissue consumed (mm2) per day by Jenneria pustulata in La Azufrada (P=0.002; circles) and Playa Blanca (P=0.01; triangles) reefs. 

Without considering the size of the snails, the average consumption of coral tissue was higher in PB than in LA (130.5 vs. 105.9); however, the ANCOVA showed that snail size has a very significant effect (P), with a snail of a given size consuming more coral tissue in LA than in PB (Figures 6 and 7).

Figure 7 Pocillopora sp. tissue consumed (mm2day-1) by Jenneria pustulata in La Azufrada and Playa Blanca reefs. 

Size and size structure

The length of a gastropod is a characteristic that can be affected by internal factors, such as the animal’s stage of development and age, and by external factors, such as environmental conditions (^{Wilbur and Owen, 1964}). According to ^{Bashevkin and Pechenik (2015)}, temperature has an important effect on the growth of mollusks, with high temperatures leading to faster growth rates than low temperatures. The reefs were visited during two different seasons: LA was visited during April 2015, while PB was visited during August of the same year, and these two seasons have contrasting climatic conditions. Between January and April, the waters cool due to upwelling in the central part of the Gulf of Panama that generates a cold front from the north and influences the waters of the Colombian Tropical Eastern Pacific, whereas from May to December, the water temperatures are higher (^{Giraldo et al., 2008}). Seasonality may have been partly responsible for the observed differences in shell length (larger snails on average in August in PB [warm waters] and smaller in April in LA [cold waters]) between the snails in both reefs; however, this hypothesis should be tested via simultaneous sampling. Moreover, it has been shown that there is a directly proportional relationship between sample size and population density (^{Connor et al., 2000}), which could also be interpreted as samples with higher abundances could sample a wider range of population characteristics. In PB, more individuals were captured than in LA; therefore, the size range of PB individuals would likely be broader, which would be reflected in the larger average sizes from this reef.

Consumption of coral tissue

The ecological role of corallivores has been considered by some authors as a driver of the structural dynamics of prey populations, although this role can be affected by various factors, such as the relative abundance of the prey and the size and density of the predator in the reef (^{Bruckner et al., 1997}). Jenneria pustulata is a highly specialized predator, almost exclusively feeds on scleractinian corals and shows a marked preference for the genus Pocillopora (^{Bertsch, 1984}; ^{Glynn, 2004}; Figure 2). Considering that the two reefs are relatively similar in species composition and total size, the snails from each of these reefs were not expected to present significant variations in the amount of coral consumed in the experiments. However, according to the results of the ANCOVA, snails that were comparable in size would consume on average more coral tissue from LA than from PB. Since the snails from LA are on average smaller, they may be fulfilling the same function at LA as the large snails at PB in terms of coral consumption. Although this represents one possible explanation, whether interference competition occurs in this species should be investigated to help explain why the smaller snails from LA have the same consumption rate as the larger snails from PB. Studies on other corallivores, such as Coralliophila erosa, Drupella cornus and D. rugosa (Bruckner et al., 1997; ^{Miller, 2001}; ^{Cumming 2009}), show higher rates than those found for J. pustulata in the Gorgona reefs (Table 1). This could indicate that although this species is larger on average than C. erosa (^{Del Mónaco et al., 2008}) and of similar size to Drupella sp, it has a less significant effect on the coral ecosystem relative to the previously mentioned species. In addition, the J. pustulata radula has two pairs of lateral teeth with a rectangular base and strong hook-shaped rigid cusps (^{Simone, 2004}), and this provision may allow it to feed without causing permanent damage to the corallites, thus allowing for more rapid and less energetically costly coral recovery.

Table 1 Shell lengths (mm) and coral tissue consumed (mm2.day-1) for different mollusk species (mm2.n-1): Recorded over 12 h. * Measured in cm. 

Studies focused on the ability of branched corals of the genus Pocillopora to recover from tissue injuries (where the calcareous skeleton is not damaged) show that although this process is energetically less expensive than recovering from a lesson in the calcareous skeleton, these wounds tend to be colonized by algae 4 or 5 days after the injury appears in the tissue. Moreover, although the coral tends to recover almost completely from these wounds, this recovery involves a significant energy expenditure since the coral must not only generate new polyps to cover the injured area again but also regenerate the polyps that have been partially damaged by the corallivore (^{Titliyanov et al., 2005}; ^{Traylor- Knowles, 2016}). Therefore, although the damage caused by the feeding of J. pustulata is less than that of other corallivorous mollusks, this species may have a strong effect on the condition of the coral due to the important energy investment required for the corals to recover from the damage and to prevent the concomitant colonization of the calcareous skeleton by algae. These complications support the need for additional research to broaden our knowledge about the structure and population dynamics of this snail and corroborate the significance of its effects on the coral reefs of the Gorgona National Park.