the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.Winter5129Summer−20°810634−25°−30°−35°Southern latitude−40°LowMid High Low Mid HighFood supplyHabitat harshnessFigure 10 Zonation scheme of sandy beach community along the coast of Chile during winter and summer;shaded background areas represent intertidal gradients in food supply (FS, dotted area) and habitat harshness(HH, grey area), showing both latitudinal and seasonal variation in these physical factors; size of boxes withanimal sketches represents proportional abundances of the respective species; note the latitudinal differencein species composition in the upper intertidal zone; species shown are 1, Ocypode gaudichaudii; 2, Phalerisidamaculata; 3, Orchestoidea tuberculata; 4, Tylos spinulosus; 5, Excirolana braziliensis; 6, E. hirsuticauda; 7,E. monodi; 8, Emerita analoga; 9, Nephtys impressa; 10, Mesodesma donacium. (Figure inspired by a drawingfrom Jaramillo 1987.)McLachlan & Jaramillo 1995, Jaramillo et al. 2001). The lower and middle shore have similarspecies composition (i.e., Emerita analoga, Mesodesma donacium, Nephtys impressa, Excirolanabraziliensis, E. hirsuticauda and E. monodi) along the continental coast of Chile, while the uppershore has species-composition differences between northern, central and southern Chile (Jaramillo1987) (Figure 10). The upper zone, extending from the drift line up to the dunes, is a very dryenvironment, occasionally moistened by sea mist. Inhabitants of this zone, such as tenebrionidbeetles (Phalerisida maculata), talitrid amphipods (Orchestoidea tuberculata) and oniscoid isopods(Tylos spinulosus) feed on plant and animal remains washed up on the beach (Jaramillo 1987). Inthe northern zone (18–25°S) and off Peru, the amphipod and isopod species are replaced by theghost crab Ocypode gaudichaudii (Quijón et al. 2001), which together with Phalerisida maculatainhabits the upper shore (Figure 10). To which degree species replacement is due to physical factorsor to species interactions is not well known at present. However, the fact that these faunal changesin the supralittoral zone are accompanied by a gradient in burrowing depth and proximity to thedrift line of the main organisms suggests that physical factors (in particular desiccation risk) mayplay an important role. Guppy (1906) expressed that “the beaches are of dry loose sand in whichthe hand fails to find on scooping below the surface that refreshing coolness which is the characterof beaches in all latitudes where the land is vegetated”.228
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEWithin the beach, there are differences not only in the interspecific but also in the intraspecificzonation patterns, which may be accompanied by behavioural differences. In particular, locomotoractivity and position on the beach varies depending on the developmental stage and on the time ofthe day. For example, juvenile Orchestoidea tuberculata and Phalerisida maculata are most activeduring the day, while adults show highest activities at night (Jaramillo et al. 1980, 2000). This hasbeen suggested as a strategy to avoid intraspecific competition for food and additionally the riskof intraspecific predation. Furthermore, Orchestoidea tuberculata shows an aggregated distributionon kelp patches stranded in the supralittoral zone (Duarte et al. 2004), which is related mainly tofeeding behaviours. For Emerita analoga and Mesodesma donacium, similar ontogenetic differencesin activity or habitat use have been reported. Juvenile Emerita analoga typically occur in highestabundances in the low intertidal zone slightly above the adults (Contreras et al. 2000). Similarly,juvenile Mesodesma donacium inhabit the lowest intertidal zone of the beach (in the swash zone),while adults live in permanently water-covered sediments of the subtidal zone (mainly in the surfzone) (Jaramillo 1994).Most sandy beach studies have centred on the effect of physical factors, in particular waveexposure, grain size and tides, on the community structure. Species richness, biomass and abundancedecrease from dissipative to reflective beaches (Jaramillo 1994, McLachlan & Jaramillo 1995).Biological interactions are more intense on dissipative beaches than on reflective beaches, wherethe population dynamics are mainly controlled by physical factors (Defeo & McLachlan 2005).Strong competition between mole crabs Emerita and surf clams Donax has been suggested as thecause for the aggregated distribution of the surf clams (Leber 1982), and negative interactionsbetween Emerita analoga and Mesodesma donacium were observed along the Chilean coast (Duganet al. 2004), but aggregations may also be a response to sediment characteristics (i.e., grain size).On the other hand, organisms inhabiting the upper shore (i.e., talitrid amphipods) are not associatedto one particular beach type (Defeo & McLachlan 2005), which suggests that their populations arenot influenced by beach morphodynamics but rather by other physical factors or food availability.To which degree resource or interference competition (or direct predation) is responsible for thelatitudinal changes in the species composition of this zone remains to be explored.The species that inhabit sandy beaches have diverse reproductive strategies, which can be relatedto the habitat characteristics. For example, most of the species of the upper and middle shore (i.e.,Orchestoidea tuberculata, Tylos spinulosus and Excirolana spp.) have direct development, whichmight suggest that the connectivity between populations is low. In northern-central Chile (29°S)the reproductive peak of Excirolana hirsuticauda is in spring–summer (Contreras & Jaramillo2003), and it is suggested that the other species from the upper/middle shore follow a similarpattern. The lower intertidal and subtidal zone is inhabited by organisms with indirect development(Emerita analoga, Mesodesma donacium, Nephtys impressa), which have planktonic larvae possiblypermitting higher connectivity between local populations. For Emerita analoga, distinct recruitmentpeaks (see also Arntz et al. 1987 for Peru) have been reported for autumn, spring or early summerin northern (22°S) and southern-central Chile (39°S), but ovigerous females are usually foundthroughout the year (Contreras et al. 1999, Contreras et al. 2000). In northern-central Chile (30°S)the surf clam Mesodesma donacium has two spawning peaks during the year (usually during theearly summer and autumn), but females with mature gonads have been observed throughout mostof the year (Alarcón & Navea 1992, Stotz et al. 1999). These observations suggest continuousreproductive activity, but spawning or successful recruitment seems to occur only infrequently,possibly depending on particular environmental factors (triggering gamete release) and oceanographicconditions (affecting larval survival and supply).There is also a high temporal variability of faunal abundance throughout the year. For example,Orchestoidea tuberculata reach highest abundances during the winter months, both in central (30°S)and southern Chile (40°S) (Sánchez et al. 1982, McLachlan & Jaramillo 1995). Furthermore, during229
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