the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.(Brante et al. 2004). The contrasting pattern in reproduction between populations of brachyurancrabs north and south of 30°S seems also to occur in other species that aggregate embryos. It isinteresting that despite the lack of difference in reproductive output of Concholepas concholepasbetween 28°S and 36°S (Figure 19), embryo packing shows a clear break that coincides with thebreak exhibited by brachyuran crabs (north and south of 29–30°S). The mean number of embryosper unit of capsule area was significantly lower in capsules from sites north of 29°S than souththereof (20.6 vs. 31.8, respectively), over a range of study sites located between 30°S and 42°S.Although there is no dramatic break in temperature at 30°S (Broitman et al. 2001), the pattern ofembryo packing is explained by the mean temperature at the study site shortly before egg deposition.The evidence accumulated to date suggests that the higher brooding costs north of 30°S seemto affect reproductive output and that the effect is consistent across the brooding modes and taxastudied so far. This finding suggests that environmental conditions are less favourable for broodingspecies in the northern part of the HCS. In fact, large-scale studies of the patterns of speciesdistribution in relation to larval developmental mode support the hypothesis that the spatial distributionof brooding species is explained by the cost of brooding, which is associated with the costof oxygen provision (Astorga et al. 2003, Marquet et al. 2004). It is less clear if the impact of thecost of brooding on reproductive output may change with adult body size. Recent studies haveshown that small crab species perform the same active brooding behaviours as large species (e.g.,Pisoides edwardsi, Acanthocyclus gayi; Fernández et al. 2006b). However, mean oxygen consumptionof brooding females, which is a proxy of brooding cost, is not significantly different fromoxygen consumption of non-brooding females (Fernández et al. 2006b). These results contrast withprevious studies of large species, showing a 2- to 3-fold increase in oxygen consumption by femalescarrying later-stage embryos over non-brooding females (e.g., Cancer setosus: Baeza & Fernández2002; Homalaspis plana: Ruiz-Tagle et al. 2002; Ovalipes trimaculatus: Fernández & Brante 2003).Therefore, the patterns described for brooding species may be dependent on adult body size. Infact, the small crab species Acanthocyclus gayi and A. hassleri do not show any pattern in reproductiveoutput along the region extending from 28°S to 36°S (Espinoza 2006). Although existingevidence on reproductive output along the HCS suggests that the behaviour of populations northand south of 30°S depend on the cost of oxygen provision in larger-size species, more studies areneeded in order to generalise the findings on the effects of temperature on brooding costs, the linkbetween adult size and brooding mode, and the consequences on species distribution, especially inregions influenced by upwelling where low oxygen conditions cover extended regions of the ocean(Fuenzalida et al. accepted).Larval life in the HCSOceanographic conditions in the HCS ecosystem expose planktonic larval forms to environmentalconditions in which they do not behave simply as a passive particle. Therefore, morphological,physiological and behavioural characteristics present in fish and benthic invertebrate larval formsinhabiting any particular habitat can be interpreted as effective local adaptations evolved to facethis unique ecosystem. Herein, the aim is to give a brief overview of some behavioural and feedingtraits together with transport processes described in larval forms of benthic invertebrates and fishesinhabiting the HCS.Behavioural traitsLarvae of many marine benthic invertebrates, differing from lecitotrophic short-lived larvae, spendextended periods in the plankton prior to settlement and metamorphosis (Thorson 1950, Pechenik1986, 1999). Usually, during their pelagic phase, planktotrophic larvae (PL) of marine benthic268
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEinvertebrates must remain suspended, locate and gather food, avoid predators and unfavourableconditions, disperse to new areas and select sites for settlement (Strathmann 1974, Palmer &Strathmann 1981, Scheltema 1986). The importance of larval behaviour has been recognised as animportant subject in marine ecology and the recent focus on the role of nearshore oceanographicprocesses controlling larval dispersal and recruitment of benthic organisms has revived interest inthe behaviour of larvae in the plankton (reviewed in Le Fèvre & Bourget 1992). Although larvaltransport seems to be mainly controlled by hydrographic factors (Thorson 1950), larval behaviourmay influence their final destination (e.g., Butman 1987, Pineda 1994a,b). In general, sources ofmortality in the dispersal phase for PL are food limitation, extreme conditions of temperature andsalinity, low dissolved oxygen, UV radiation and pollution (Pechenik 1986). However, because ofthe difficulties associated with larval tracking in the field, the relative importance of each potentialsource of larval mortality in nature is still largely unknown. Field and laboratory experiments haverevealed a number of behavioural mechanisms that allow larvae to contend with these selectivepressures on mortality. However, specific studies of characteristics displayed by PL throughout theHCS are largely absent. Few published studies regarding larval characteristics linked with ecologicalimportance have been conducted in Chile (e.g., Manríquez & Castilla 2007). Field evidence of aDVM pattern in competent larvae has been described for the muricid gastropod Concholepasconcholepas (Poulin et al. 2002a,b). It has been suggested that the behaviour of competent larvaeof this species may help them to avoid offshore transport during upwelling events. Studies withseveral crustacean species have shown clear synchronisation between reproduction and the hydrodynamicprocesses promoting larval transport or retention (Yannicelli et al. 2006a). Similar studieswith other PL in the HCS incorporating behavioural or physiological responses to environmentalvariables such as currents, temperature and salinity are urgently needed. However, along the HCSthose studies are precluded by the absence of basic knowledge such as larval identification in manyspecies of Chilean invertebrates. Work to investigate the scale of the source–sink dynamics of PLin the temporally heterogeneous HCS ecosystem is also largely absent. More recent techniquesinvolving chemical tags in calcified structures of PL have been successfully used to identify potentiallarval source and sink areas, and larval trajectories in the HCS (Zacherl et al. 2003).Feeding and larval food environment in the HCSRecruitment is recognised as the leading determinant of population dynamics of benthic invertebrateand fish species and it strongly adjusts the importance and intensity of species interactions in theHCS (see above). Since PL rely on food to grow and develop, availability and quality of foodparticles during larval development are important factors influencing larval recruitment (Vargaset al. 2006a). Despite the high variability in larval food composition, there are well-documented,persistent, temporal and spatial differences in plankton structure and abundance along the HCS.For instance, large and geographically persistent heterogeneity in chl-a has been documented alongthe Chilean coast (Thomas et al. 2001b, Yuras et al. 2005; and see Dominant primary producersand their role in the pelagic food web, p. 210ff.). The general pattern indicates that a large percentageof chl-a is found in the large phytoplankton fraction in permanent coastal upwelling areas offnorthern Chile (Iriarte & González 2004; see also Dominant primary producers and their role inthe pelagic food web, p. 210ff.), as well as during spring/summer at seasonal upwelling sites incentral Chile (González et al. 1989, Vargas et al. 2006b). Similarly, a large and geographicallypersistent heterogeneity in chl-a levels has been observed to the north of latitude 32°S (Yuras et al.2005).The scarcity of available published data on larval fish and invertebrate feeding makes it difficultto evaluate potential larval survival and recruitment along the HCS. Currently, the largest body ofliterature is available for larval fish of commercially exploited species. Because of their seasonal269
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