The temperature and salinity values reported in this study and those recorded by Koettker et al. (2010) 1 year earlier are similar to those that have been found previously in the area from in situ and remote sensing observations (Pérez et al. 2005; Macedo-Soares et al. 2012). These values are compatible with the characterization of Tropical Surface Water, described as the dominant water mass in the surface layer of the area, which presents temperature and salinity of about 27.0 °C and 36.0, respectively (Stramma and Schott 1999; Medeiros et al. 2009).
The presence of larvae of G. grapsus in all samples and the high abundance figures found at night in the inlet, demonstrate a predominance of nocturnal spawning. Preference of spawning during the night has been documented for several benthic species and is related to the lower predation rates, favouring the survival of newly hatched larvae (e.g. Christy 1982; Yamaguchi 2001). Ovigerous females of G. grapsus were caught during all seasons at Saint Paul’s Rocks (Freire et al. 2011), corroborating the results found for larvae and indicating that this species’ reproduction must be continuous at Saint Paul’s Rocks.
Crab larvae are a major food resource for planktivorous fish (e.g. Morgan 1990; Morgan and Christy 1997; Hovel and Morgan 1997), since their larvae have few short spines and small bodies making them most vulnerable to and preferred by fish, while spines can difficult ingestion by fish (Morgan and Christy 1997). Larvae of G. grapsus are relatively small compared with other crab larvae and have no lateral spines. These morphological characteristics and the high abundance observed in the inlet suggest that larvae of G. grapsus are an important food resource for planktivorous fish that inhabit the area, contributing to the pelagic marine food chain and acting as a keystone species in this ecosystem. The contribution of crab larvae for the pelagic food chain has already been recorded in the mangrove/platform relationship (Schwamborn et al. 1999) and in very high concentrations (100–100,000 larvae/100 m3) of newly hatched larvae of Pachygrapsus transversus Gibbes, 1850 at a tropical rocky shore (Flores et al. 2007).
An adult population of G. grapsus has existed on the emerged rocks of Saint Paul’s Rocks since Charles Darwin’s first reports on the area until the present day (e.g. Holthuis et al. 1980; Freire et al. 2011). Although we did not conduct vertical or oblique hauls in order to sample deeper layers, the gradual decrease in abundance of larvae of G. grapsus as the distance from the archipelago increased and because only the first larval stage were sampled lead one to believe that the parent population of these larvae is the Saint Paul’s Rocks population. In fact, oblique tows sampling the 200 m water column at the Canary Islands recorded much lower values of Grapsidae larvae than those at Saint Paul’s Rock’s (Landeira et al. 2010). In that study, larvae of the benthic Calcinus tubularis Linnaeus, 1767 and Scyllarides latus Latreille, 1802 were sampled in the third zoeal stage, but taking into account long distances to other upstream archipelagos and low densities of larvae. Here, the authors proposed that the most probable origin of the larvae was the Canary Islands themselves. Off the California coast, most of the early larval stages (92 %) of 45 crustacean species were also most abundant close to the adult habitat, up to 6 km from the shore (Morgan et al. 2009).
Although so far only the first larval stage of G. grapsus is known, the complete larval development of other Grapsidae species has been described (e.g. Cuesta and Rodríguez 2000; Cuesta et al. 2011) where the duration of their zoeal phase varies from about 30–60 days. Furthermore, the newly hatched larvae remain in the zoeal I stage for about 4 days before moulting to their next larval stage. This being said, it is unlikely that these larvae of G. grapsus spawned anywhere else, since Saint Paul’s Rocks is over 500 km from the nearest archipelago (Fernando de Noronha). Many benthic marine populations have only a portion of their recruitment ‘subsidized’ from external sources (Pascual and Caswell 1991). In addition, the more isolated a site, the more likely it is that the present population is maintained entirely by self-recruitment (Sponaugle et al. 2002). Marine populations around oceanic islands with no clear source of recruitment could be washed away if mechanisms to retain and maintain the larvae near their natal populations are absent (Landeira et al. 2009).
If we assume that a larva behaves in the same way as an inert particle and that the speed at which the equatorial branch of the South Equatorial Current (SEC) reaches Saint Paul’s Rocks (5.6 km/h) (Edwards and Lubbock 1983b) is constant, we could conclude that it is possible for a larva in zoeal stage I to reach Fernando de Noronha after 4 days. However, the small percentage (0.04 %) of larvae of G. grapsus found 2 km away from Saint Paul’s Rocks (D4) indicates that there is no connectivity between the population of this archipelago and other oceanic islands of the Equatorial Atlantic. In an efficient connection, for example, between coast/platform/coast, larval dispersal can reach distances up to 15 km, as found along the northeast coast of Brazil (Schwamborn et al. 1999).
Marine populations range from entirely closed (self-sustaining populations, such as endemic island species where 100 % of recruitment is due to the settlement of offspring produced by that population) to fully open populations, recruiting only a relatively small proportion of their own offspring and receiving a high number of recruits from other populations (Sponaugle et al. 2002). It has become increasingly apparent that larvae are more likely to recruit closer to natal populations and in higher abundances than generally realized (Morgan et al. 2009; Morgan and Fisher 2010). Islands normally have coastal retention zones that allow for maintenance of the larvae near their natal sites, a result of the interaction between the main current flow and topographic relief (capes, banks, headlands and embayments) (Landeira et al. 2009). In Saint Paul’s Rocks, the inlet must act as a site not only for retention, but must also be a location where the larvae of benthic species can maximize growth and survival.
Although late larvae were not found in samples, the geographical isolation of the area and the low levels of larval abundance found at the points farthest from the archipelago indicate that some type of physical process may be maintaining the larvae of G. grapsus in the Saint Paul’s Rocks region, while another type of biological process causes late larvae to inhabit the deepest layers of the water column. Late larvae tend to keep to deeper waters, while initial larvae are most abundant in surface waters (Queiroga and Blanton 2004). Eddies serve as retention-favourable areas for a variety of larvae in regions near islands (Sponaugle et al. 2002; Queiroga and Blanton 2004), increasing plankton production by pumping nutrients in the euphotic zone (Arístegui et al. 1994), thereby providing a suitable environment for larval survival. However, little is known about this type of phenomenon in the Saint Paul’s Rocks area.
Larvae of S. edwardsi were more frequent in the open ocean samples. Larvae and adults of Sergestidae are fully adapted to areas of the open ocean (Omori 1974). The abundance figures for S. edwardsi at Saint Paul’s Rocks were similar to figures that have been reported for other dominant species of Sergestidae from other oceanic regions (e.g. Calazans 1994; Landeira et al. 2009; Mujica et al. 2011). Dendrobranchiata larvae, mainly represented by Sergestidae, were dominant around Saint Paul’s Rocks at different stages of development (Koettker et al. 2010). Larvae of the genus Sergestes occurred in oceanic stations around Easter Island (Mujica 2006). This is probably related to the distribution of adults in pelagic oceanic waters, where they spawn (Landeira et al. 2009).
The relatively homogeneous distribution across the different sampling stations suggests that larvae of S. edwardsi can benefit from the proximity of the archipelago, but since they are independent of any substrate, they did not exhibit a strong association with the archipelago. Larvae distribution of Sergestidae was also found homogeneous around Easter Island (Mujica 2006).
The larvae of benthic species also exhibited the opposite tendency of pelagic species around the Canary Islands, in that the larvae of benthic species decreased in abundance in offshore directions, while those of pelagic species increased. While the distribution of Gennadas spp., Sergestes cornutus Krøyer, 1855 and S. atlanticus H. Milne Edwards, 1830 was a clear example of the pelagic pattern, other species were strongly associated with the area close to the island [e.g. Galathea intermedia Liljeborg, 1851, Pagurus spp., Munida spp. and Alpheus macrocheles Hailstone, 1835] (Landeira et al. 2009).
It is likely that the populations of other local benthic decapods (Plagusia depressa, Platypodiella spectabilis and other Grapsidae, Xanthidae and Alpheidae) follow the same pattern as G. grapsus, while pelagic decapods (Sergestes curvatus, Sergestes henseni and the genera Gennadas and Lucifer) are distributed in a similar manner to that of S. edwardsi.
In summary, the two species analysed, once adults inhabit different biological compartments, resulting in the maintenance of different ecological adaptations. Species with high parental investment, such as G. grapsus, have increased survival of eggs, larvae and even juveniles, which favours self-recruitment, promoting retention near the parental population. On the other hand, species whose fertilized eggs are spawned, like Sergestidae shrimps, have relatively smaller eggs and larvae with less ability to remain close to the source population (Sponaugle et al. 2002). The results of this study suggest that there is a possible retention mechanism acting on the larvae of meroplanktonic species at Saint Paul’s Rocks, while larvae from holopelagic species are widely distributed through the oceanic area around the archipelago. Although later larvae of G. grapsus were not recorded in the 200 m water layer off the Canary Islands (Landeira et al. 2010), vertical hauls combined with descriptions of currents should elucidate the retention process.