A large range in shell sizes was observed within Coralliophila spp. Females were on average larger than males, which is consistent with the fact that C. galea is a protandrous hermaphrodite (Baums et al., 2003a; Johnston and Miller, 2006). The same pattern was observed within C. caribaea, suggesting a similar life history, which has been suggested to be a synapomorphous trait among the Coralliophilinae (Richter and Luque, 2002, 2004). In addition, host-associated size structuring existed within both C. galea and C. caribaea populations. Differences in shell length related to host species have been observed before within C. galea (Hayes, 1990a; Bruckner et al., 1997; Baums et al., 2003a; Johnston and Miller, 2006). Ecotypes with distinctive shell lengths have also been observed in their Mediterranean congener C. meyendorffii, where small snails are associated with scleractinian hosts and large snails are associated with sea anemones (Oliverio and Mariottini, 2001b;Kružić et al., 2013). The results of the present study confirm and expand this pattern of host-associated size structuring in C. galea, and show that it also exists in C. caribaea.
Migration between host species with age could induce host-associated size structuring. In the Indo-Pacific corallivorous snail Drupella cornus (Röding, 1798), prey preference seems to change as snails age (Black and Johnson, 1994; McClanahan, 1997; Schoepf et al., 2010; Moerland et al., 2016). Age-dependent host preference would result in a clear host-associated size structuring as seen within Coralliophila spp. However, no evidence exists to support that C. galea or C. caribaea migrate between host species. As both male and female snails were found on all but one host species (for which more than one specimen was collected) of C. galea, it seems unlikely that size-related migration between specific host species also occurs in C. galea, i.e., that juvenile snails (which would all be males) are associated with different host species than adults.
Another potential mechanism behind host-associated size structuring can result from size-dependent susceptibility to predation (Johnston and Miller, 2006). Selective predation on larger snails, which on some host colonies are more exposed than smaller snails, would result in host-associated size structuring. Wells and Lalli (1977) hypothesized further that brooding females of C. galea are, compared to C. caribaea, more vulnerable to predation because of the placement of egg capsules in the mantle cavity. Predation on larger snails would therefore decrease mean shell size and skew the male to female ratio. Variation in male to female ratios, as observed in C. galea, would be expected in case of size-specific predation. However, information on predation on Coralliophila spp. is limited, resulting from laboratory experiments or anecdotal observations (Goldberg 1971; Wells and Lalli, 1977; Baums et al., 2003a; Sharp and Delgado, 2015).
Besides a large range in shell size, high intraspecific variation in shell shape was found in both C. galea and C. caribaea. Despite this high intraspecific variation, no distinct ecotypes based on shell shape could be identified within either species, though weak host-associated differences in mean shell shape and allometric patterns existed in both C. galea and C. caribaea. Differences in shell shape were subtle with strong overlap among snails associated with different host species. Tested factors explained little of the observed variation in the models of shell shape suggesting the presence of factors not considered here.
Differences in growth rate could also explain host-dependent size structuring as well as the intraspecific variation in shell shape and allometry (Kemp and Bertness, 1984; Boulding and Hay, 1993; Chiu et al., 2002; Urdy et al., 2010a, 2010b). While growth rate has not been measured in the present study, the strong correlation between average male and female size separated by host species (assuming sex-change occurs at the same relative age) is consistent with the idea that growth rates differ among snails associated with different host species, confirming previous studies (Baums et al., 2003b; Johnston and Miller, 2006). Such differences in growth rate may result from, for example, differences in nutritional quality of host tissue (Szmant et al., 1990), anti-predatory mechanisms of host species (Barnes, 1970; Brauer et al., 1970; Moore and Huxley, 1976; Glynn and Krupp, 1986; Pawlik et al., 1987; Harvell et al., 1988; Harvell and Fenical, 1989; Van Alstyne and Paul, 1992; Pawlik, 1993; O’Neal and Pawlik, 2002; Gochfeld, 2004; Lages et al., 2010), predation pressure on snails (Fraser and Gilliam, 1992; Connell, 1998; Nakaoka, 2000) or intraspecific competition (Williamson et al., 1976; Cameron and Carter, 1979), among other factors. Baums et al. (2003b) suggested that differences in environment or nutrition (and by extension, host species) played a role in the growth rate of C. galea, Johnston and Miller (2006) suggested a role for nutritional quality and secondary metabolites, as well as intraspecific competition in the population structure of C. galea. Shell morphology has also been related to vulnerability to predation (Ebling et al., 1964; Kitching et al., 1966; Vermeij, 1974, 1993; Cotton et al., 2004). Variation of these factors across the range of hosts species, with vastly different colony shapes, may therefore result in host-dependent variation in shell size, shape and allometry as observed in the present study. However, little is known about the extent to which these factors play a role in corallivores in general or in Coralliophila spp. specifically. The mechanisms behind the observed patterns in shell size and shape remain therefore largely unknown.