Relationship between fish density and habitat features
The density of O. trinitatis observed in the present study concurs with densities reported for this species and for the related O. atlanticus on tropical rocky and coral reefs (e.g. Nursall 1981, Mendes 2007). The available literature suggests a high variability in the density of the latter species among different Caribbean sites, ranging from 0.29 ind. m–2 at Punta de Betín (Rylander and Koster 1982) to 2.2 ind. m–2 at Barbados (Labelle and Nursall 1992). This somewhat high variability may be attributed to stochastic fluctuations in recruitment (Sale 1978, Sale and Douglas 1984), but also to geographical variations in the availability of space and suitable substrate (Labelle and Nursall 1992). In the present study, local variation in fish density (i.e. among sites separated by few kilometres) was observed.
Local differences in the density of O. trinitatis among the four study sites may be attributed to differences in habitat characteristics, which eventually influenced territory size (see below). Although rugosity measures were similar across all sites, this factor had a significant positive effect on the distribution of this blenny, given that individuals were always observed near to, or associated with, small caves and crevices. In fact, presence of crevices seems to be a central habitat requisite for this species, and tight correlations between blennies and crevices are well documented (e.g. Rylander and Koster 1982, Bath 1990, Labelle and Nursall 1992, Mendes 2006, 2007). As complementary evidence, during two years of field experience in the study area, O. trinitatis was never observed at low-complexity unconsolidated sites, and was very rare even at the rocky reef-sand ecotone. A similar correlation was previously observed in a small shallow reef (Medeiros et al. 2010a), supporting the prominent habitat selectivity of this species.
Contrary to initial expectations, benthic composition was a somewhat poor determinant of O. trinitatis density (see PCA results), and though substrate diversity was significantly higher at sites with high fish density, only indirect assumptions on the influence of single benthic components are possible. For example, samples with a high contribution from turf and coralline algae showed higher fish densities, whereas samples with a high contribution from live coral and sand showed lower densities. Presence of turf algae inside the territories of these blennies is likely to be related to their feeding preferences (see below), but since this item is highly abundant at all sites (a common feature of south Atlantic reefs; e.g. Ferreira et al. 2004, Floeter et al. 2005, Medeiros et al. 2010a), it seems not to be a limiting factor on the abundance of O. trinitatis. Instead, as mentioned above, presence of crevices is particularly more important. An investigation on the required surface area of turf algae to supply each individual blenny is, however, necessary and should elucidate this (but see Nursall [1981], who suggested that the territory should include more resources than the minimum required for survival). Nonetheless, optimal sites seem to be those located in shallow depths (Rangel and Mendes 2009) with a complex rocky terrace covered by a sufficient percentage of turf cover, with a low vertical profile (low macroalgae cover), and low quantities of coral and sand. In fact, hiding places and high-quality food seem to be common prerequisites for all small territorial herbivores (e.g. Low 1971, Ebersole 1977, Labelle and Nursall 1992, Letourneur 1992, Haley and Müller 2002, Medeiros et al. 2010b), given the disadvantageous net costs of defending low-quality territories (see reviews by Dill 1978 and Ebersole 1980).
Feeding behaviour
Randall (1996) reported that turf algae were the only preferred benthic item of O. trinitatis and filamentous algae were the primary food item of O. atlanticus. Given the high abundance of turf algae at all study sites (covering a mean of 39.4% of the benthic surface), food is unlikely to be a limiting factor for this species. Roberts (1987), studying Pacific blennies and damselfishes, also suggested that the presence of algae outside territories was evidence that food was not a limiting factor for these fishes. Our observations suggest that turf algae are also highly abundant in unoccupied interstitial spaces outside the territories of O. trinitatis and at the sand-reef ecotone, further supporting this hypothesis. On the other hand, crevice availability seems to be more limited. In fact, blennies were observed associated with substrate of high structural complexity with somewhat low turf cover, but never in turf-rich sites lacking crevices.
Confirming the hypothesis, bite rate decreased with an increase in size of blennies, suggesting an ontogenetic variation in energy demand, which is expected to be higher for developing juveniles (Hernaman et al. 2009). Medeiros et al. (2010b) studied the feeding habits of the juveniles of two territorial damselfishes and found similar evidence of ontogenetic shifts in the bite rate of these fishes, but these reports remain somewhat scarce and underreported for most reef fish families.
Feeding rate peaked between 1 and 2 pm and was concentrated in the afternoon. Nursall (1981), studying O. atlanticus, reported that individuals of this species also concentrated their feeding in the afternoon period and employed time-minimized strategies. Our observations support the findings of Mendes (2006), who stated that individuals alternate between fast foraging and resting inside the crevices, as a means of minimizing predation risk during feeding. Nursall (1981) also stated that O. atlanticus allocated 8.5% of its time to feeding. Interestingly, time estimates from unpublished video recordings of several individuals showed that O. trinitatis individuals allocated 8.83% of their time to feeding. Furthermore, these values are considerably lower than those recorded for other reef fishes (e.g. Jones and Norman 1986, Bonaldo et al. 2005, Meekan et al. 2010), supporting Nursall’s affirmation that Ophioblennius species are highly efficient time-minimizers.
Territory size and agonistic behaviours
Territory size of O. trinitatis (0.79 m2) was considerably smaller than those reported by Rylander and Koster (1982) (between 1.17 and 2.41 m2), but slightly larger than those reported for O. atlanticus by Nursall (1977) (0.5 m2). Territories defended by O. trinitatis showed local variation in size, being larger at sites with lower fish densities (compare Figs 2a and 5a). As hypothesized, these differences suggest that territory size increases at low-density sites (a density-dependent mechanism). In addition, though food does not seem to influence territory size (see above), it seems that fish need to compensate for residing on lower quality substrates by increasing their territory coverage. Although we found little evidence that food determined territory size, availability of hiding places seems to be a major determinant (see Rylander and Koster 1982). Therefore, we highlight the importance of structural heterogeneity for site-attached fishes, as also acknowledged for several other reef fishes (e.g. McCormick 1994, Friedlander and Parrish 1998, Jones and Syms 1998). High-quality territories (sites of high structural complexity) are also important during reproduction, being a prerequisite during mate selection by females (Labelle and Nursall 1992).
Larger individuals of O. trinitatis defended larger territories, as previously observed for other territorial fishes (Ebersole 1980, Rylander and Koster 1982, Medeiros et al. 2010b). Our findings suggest that territory size is driven by hierarchical and life-stage forces. Therefore, to achieve typical territorial adult habits, recruits need to first take up interstitial space (i.e. unoccupied area between territories) and gradually increase its area during their development (see Nursall 1977, Robertson 1984). Nonetheless, field experimental studies are necessary to elucidate these processes.
Regardless of the implications concerning local differences in territory size, optimal territories should be those with the smallest possible area, but including a sufficient food supply and suitable hideaway places. As acknowledged by Rylander and Koster (1982), these territories should increase the fitness of residents by reducing the risk of foraging away from shelter and the energy spent in territory defence.
Resident O. trinitatis individuals responded differently to fishes recorded inside their territories (i.e. intruders), given that common species such as H. parra, T. noronhanum, A. saxatilis and A. chirurgus were rarely or never attacked. On the other hand, S. rocasensis, Malacoctenus sp. and conspecific individuals were those most subjected to intruder-directed agonistic interactions. Stegastes rocasensis is a highly territorial species with microhabitat and food preferences similar to those of O. trinitatis (Souza et al. 2010). Malacoctenus sp., however, has different food habits but is highly site-attached, sharing substrates with O. trinitatis (Rangel and Mendes 2009), though sandy areas are also common microhabitats for these fish (author’s personal observation). Therefore, conspecifics, S. rocasensis and, to a smaller extent, Malacoctenus sp., were the most important competitors of O. trinitatis. Focusing agonistic displays towards potential intruders should be energy-efficient, given the unnecessary cost of territory defence towards non-competitive species, which are more often tolerated inside territories (Warner and Hoffman 1980).
Agonistic interactions towards resident O. trinitatis were mostly made by S. rocasensis and conspecific individuals. In general, for each agonistic attack directed towards an S. rocasensis individual, resident O. trinitatis received over 3.5 counterattacks. Thus, S. rocasensis is a far more aggressive species than O. trinitatis. Mendes (2006) and Nursall (1977) reported a low frequency of both intra- and interspecific agonistic interactions for O. trinitatis and O. atlanticus, respectively, but Rylander and Koster (1982) stated that interspecific agonistic interactions between O. atlanticus and two damselfishes were common.
Evidence of size-dependent territory dominance was not observed in the present study. In fact, among conspecifics, residents were more successful at defending territories than intruders were at overtaking them, regardless of individual size. A similar mechanism was acknowledged by Picciulin et al. (2006), who stated that resident territorial gobies had a greater chance of maintaining their territories when attacked by an intruder conspecific, regardless of size.
Fish defending larger territories were subject to higher levels of aggressive interactions from intruders, supporting the findings of Rylander and Koster (1982). Given the obvious dislocation constraints of defending distant borders, larger territories are more vulnerable to intruders. Furthermore, agonistic interactions are expected to be higher at complex sites due to higher fish densities (McCormick 1994), but in the present study agonistic interactions were higher at low-complexity sites. This was due to the increased competition for refuge places (i.e. crevices), which are scarce at low-complexity sites.
Despite its aggressive territorial behaviour, O. trinitatis showed some degree of tolerance towards conspecifics. Densities of 2.5 and 3 ind. m–2 were frequently observed and, in many cases, without intraspecific agonistic interactions, suggesting a possible intraspecific symbiotic sharing of territories (see Robertson and Polunin 1981). Since food did not seem to be a limiting factor for the abundance of these blennies, the benefits of symbiotic sharing might overcome the net costs of co-habiting with potential competitors. These benefits appear to be the improved territory defence (given that the task is split among multiple individuals) and the underlying reduced energy cost of employing agonistic displays (Robertson and Polunin 1981, Norman and Jones 1984, Foster 1985, Iguchi and Abe 2002). Evidence of symbiotic sharing has been previously acknowledged for blennids (e.g. Roberts 1987, Townsend and Tibbetts 2004). Furthermore, Mendes (2006) recorded, at several occasions, up to six ‘tolerating’ Ophioblennius individuals inhabiting a 1 m2 area. Future studies, however, should elucidate whether territorial sharing is not simply a temporary sex-related process (i.e. sexual aggregation or harem formation).