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Rev. chil. hist. nat. vol.79 no.1 Santiago Mar. 2006
Revista Chilena de Historia Natural 79: 13-27, 2006
Geographic distribution and description of four pelagic barnacles along the south east Pacific coast of Chile - a zoogeographical approximation
Distribución geográfica y descripción de cuatro especies de cirripedios pelágicos a lo largo de la costa chilena del Pacífico sur este - una aproximación zoogeográfica
IVÁN HINOJOSA1, SEBASTIÁN BOLTAÑA1, DOMINGO LANCELLOTTI1, ERASMO MACAYA1, PABLA UGALDE1, NELSON VALDIVIA1, NELSON VÁSQUEZ1, WILLIAM A. NEWMAN2 & MARTIN THIEL 1,3*
1 Facultad Ciencias del Mar, Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile
The majority of zoogeographic studies along the Chilean Pacific coast have focused on benthic organisms and oceanographic conditions are considered the main factors influencing their distributions. Herein we examined the geographic distribution of pelagic barnacles of the family Lepadidae collected from floating macroalgae at seven sampling areas between 23 and 50° S. Four species were encountered and they are briefly described herein. The most abundant northern species was Lepas anatifera, and it diminished in abundance towards the south (33° S). Moreover, this species was not found in waters with a sea surface temperature (SST) of less than ~18 °C. Lepas australis, primarily a circumpolar West Wind Drift species, diminished in abundance towards the north (33° S). This species was restricted to waters of < 18 °C SST. A third species, L. pectinata, was encountered throughout almost the entire study area, but it was most abundant between 29 and 33° S. The fourth species, Dosima fascicularis, was only found at two sampling areas, namely at 27° S and 33° S, and this is the first record of this species from the central coast of Chile. The distributional pattern of the pelagic barnacles found herein corresponds to the three main zoogeographic regions as revealed by the majority of previous studies based on littoral organisms: the northern Peru-Chilean Province, the southern Magellanic Province, and the central Chilean Transition Zone where the two provinces overlap. Even though the present study only considers four species of pelagic barnacles, the results support the hypotheses on the importance of oceanographic conditions (in particular SST) in determining the zoogeographic patterns along the south east Pacific coast of Chile.
Key words: zoogeography, Lepas, oceanography, pelagic, Cirripedia.
La mayoría de los estudios de zoogeografía a lo largo de la costa chilena del Pacífico sur este se han enfocado en organismos bentónicos y las condiciones oceanográficas se sugieren como factores principales que modulan su distribución. Aquí examinamos la distribución geográfica de especies de cirripedios pelágicos de la familia Lepadidae, asociadas a macroalgas flotando a la deriva, en siete áreas de muestreo entre los 23 y 50° S. Se encontraron cuatro especies de cirripedios lepádidos y cada una se describe brevemente. La especie del norte más abundantes fue Lepas anatifera y disminuye su abundancia hacia el sur (33° S). Además, esta especie no se encontró en temperatura superficial del mar (SST) inferior a ~18 °C. Lepas australis, que posee una distribución circumpolar, atenúa su abundancia hacia el norte (33° S). Esta especie se restringe a SST < 18 °C. La tercera especie, L. pectinata se extiende en la mayor parte del área de estudio, pero posee su mayor abundancia relativa en el área central, entre 29 y 33° S. La cuarta especie, Dosima fascicularis, solo se encontró en dos lugares de muestreo, en 27 y 33° S, y es el primer registro para la costa central de Chile. El patrón de distribución de estas especies de cirripedios pelágicos corresponde a las principales zonas geográficas reveladas por la mayoría de estudios previos basados en organismos bentónicos: la Provincia Peruano-Chilena del Norte, la Provincia Magallánica del Sur y una Zona Central de Transición, donde las dos provincias se sobreponen. A pesar que el presente estudio solo considera cuatro especies de cirripedios pelágicos, nuestros resultados apoyan la hipótesis de la importancia de las condiciones oceanográficas (en particular: SST) en la modulación del patrón zoogeográfico a lo largo de la costa sur este del Pacífico, Chile.
Palabras clave: zoogeografía, Lepas, oceanografía, pelágico, cirripedios.
The majority of biogeographic studies along the south east Pacific coast of Chile indicate the presence of three biogeographical zones (e.g., Knox 1960, Lancellotti & Vásquez 1999, 2000, Camus 2001, Thiel 2002; but see also Hernández et al. 2005). In general, there is evidence that the northern biota, with its temperate characteristics, extends southwards while the austral biota extends northwards, and both overlap in a central zone. In a review of various biogeographic studies, Camus (2001) suggested the following limits for the south east Pacific: (i) the Peruvian Province north of 30° S, (ii) the Magellanic Province south of 41-43° S, and (iii) the intermediate area in the central Chilean region, where the two first provinces overlap. Additionally, it has been discussed that the exact limit for each zone can vary depending on the number of taxa considered in the respective studies (Lancellotti & Vásquez 2000, Camus 2001) or the prevalent oceanographic conditions during the sampling period (Thiel 2002, Escribano et al. 2003).
Most zoogeographic studies along the SE Pacific coast have focused on littoral organisms (primarily benthic invertebrates), and oceanographic conditions have been suggested as the main factors influencing species dispersal (Knox 1960, Menzies 1962, Lancellotti & Vásquez 1999, 2000, Ojeda et al. 2000, and references in Camus 2001). In particular, it has been inferred that the latitudinal gradient of sea surface temperature (SST) along the Chilean coast is responsible for the distribution of many littoral and benthic organisms (Lancellotti & Vásquez 1999, 2000, Astorga et al. 2003, Rivadeneira & Fernández 2005). While relatively consistent biogeographic patterns are emerging, the driving forces behind these patterns are not very well known. SST may act directly on adult organisms in benthic habitats, or it may affect transport and survival of planktonic dispersal stages. If biogeographic patterns are dependent on water column processes, we could expect similar patterns for planktonic organisms as have been revealed for littoral benthic organisms.
While most studies along the Pacific coast of Chile have focused on the latitudinal distribution of littoral benthos, there are also a few studies on pelagic species. Fagetti (1968) reported on three groups of species of Chilean Chaetognatha; a northern group consisting of tropical-equatorial species, a southern group consisting of sub-Antarctic species, and a central group of species. The author mentioned a key species within each of the three groups that acted as a bio-indicator for each of the different water masses (SST and salinity). A northern species is replaced by a southern species at 30° S, whereas another northern species ranges southward to 38° S. Antezana (1981) identified four groups of Euphausiacea for the south east Pacific, and included the geographic information of chaetognaths, medusae and calanoid copepod fauna. This author distinguished two provinces along the Pacific coast of Chile, a northern Peruvian-Chilean province and a southern Magellanic province, separated by a transitional zone between 38° S at 42° S. As the principal cause for this pelagic faunas distribution pattern he suggested that the northern limit of intrusion of subantarctic waters in the Humboldt Current lies between about 37 and 38° S. Similarly, in a recent synthesis of the pelagic fauna, Escribano et al. (2003) suggested three zoogeographical regions along the Chilean cost; Northern Upwelling Region (NUR, north of 30° S), Central/South Upwelling Region (CSUR, 30 to 40° S) and an Austral Fjord Region (AFR, south of 40° S). These authors found similarities in phyto- and zooplankton composition between NUR and CSUR, but mentioned that "physical processes may be more important in regulating pelagic populations in the CSUR than in the NUR, where biological interactions seem to play a greater role". Thus, pelagic organisms indeed appear to display a very similar distributional pattern as many benthic organisms. In order to verify whether this pattern holds for other organisms, herein we examined the distribution of pelagic goose barnacles.
All barnacles of the family Lepadidae (goose barnacles) are pelagic, commonly found associated with floating objects, e.g., ships, buoys, wood, animals, marine debris and macroalgae, and only members of one genus (Dosima) can produce a float of their own (Skerman 1958, Cheng & Lewin 1976, Arnbom & Lundberg 1995, Minchin 1996, Gollasch 2002, Sano et al. 2003). Some authors suggested that the geographic distribution of theses barnacles is restricted to particular oceanographic conditions (e.g., species-specific SST) (Zevina & Memmi 1981, Moyse 1987, Green et al. 1994).
In the Southern Hemisphere goose barnacles are found abundantly on rafts of two species of floating macroalgae, Macrocystis spp. (C. Agardh, 1820) and Durvillaea antarctica (Hariot 1892), (Helmuth et al. 1994, Smith & Bayliss-Smith 1998, Thiel & Gutow 2005). The time that detached macroalgae survive on the sea surface before sinking (several weeks to months) is sufficiently long for larvae of goose barnacles to settle, grow and achieve sexual maturity (Skerman 1958, Green et al. 1994, Smith & Bayliss-Smith 1998, Hobday 2000a).
In this study we present brief descriptions of the goose barnacles found on floating Macrocystis spp. and D. antarctica along the Chilean Pacific coast. Additionally, we describe the distributional pattern of these pelagic barnacles and compare it with the biogeography reported for benthic marine organisms. We expected that if the actual distribution of littoral fauna is determined by recent oceanographic processes, the distribution pattern of pelagic organisms (such as goose barnacles) should match that of the littoral fauna.
MATERIAL AND METHODS
During austral summer 2002 (January-March) we carried out nine surveys along the south east Pacific coast of Chile, between ~18 and ~50° S. For the surveys we utilized medium-sized (~15 m total length) fishing boats or research vessels. The ship track led westward for ~50 km, turning southwards for ~15 km before returning directly to the port. Each hour we recorded the SST, thus obtaining ~12 measurements for each sampling area, except for the location close to "Isla Madre de Dios" (~50° S, where we could only take one measurement due to foul weather). During navigation we surveyed the sea surface for floating Macrocystis spp. and Durvillaea antarctica. Samples were only taken at seven areas (Table 1), because no floating macroalgae were detected at the two northernmost locations (Arica and Iquique). The number of samples varied for each sampling area between six (in Mejillones) and 15 (in Concepción and "Isla Madre de Dios") (Table 1). At most sampling areas, floating macroalgae were collected between 5 to 50 km out from the coast, except for "Isla Madre de Dios" where macroalgae were sampled 2 to 4 km off the mouth of a large oceanic channel.
When floating macroalgae were encountered, randomly selected macroalgae were collected with the aid of a dip net (ring diameter r = 60 cm, mesh size of funnel = 3 mm, mesh size of cod end = 0.5 mm, see Hinojosa et al. 20031). We collected individual algal patches < 60 cm diameter, directly from the vessel. In the case of algal patches > 2 m diameter, a diver cut random samples from the whole patch that were immediately placed in the mesh bag (0.5 mm mesh). All samples were placed in plastic bags and preserved in 5 % formalin. In the laboratory, samples were rinsed thoroughly with freshwater over a sieve (0.25 mm), and all epibionts, including goose barnacles, were carefully removed. Barnacles were transferred to 70 % alcohol and later identified under a dissecting microscope. Total abundance (number of individuals) per sample was recorded in order to calculate the relative abundance (in percentage). Herein, we present relative proportions rather than absolute values (abundance per gram of macroalgae) because the abundances of goose barnacles is influenced by the floating time of macroalgae. The relative abundances of goose barnacles associated with Macrocystis spp. and Durvillaea antarctica samples were treated separately in order to account for the possibility of a species-specific association (one species of Lepas settling preferentially on either species of macroalgae).
In order to examine the latitudinal distribution pattern of the studied goose barnacles, we conducted a non-metric multidimensional scaling ordination (nMDS; Field et al. 1982, Clarke 1993) using the Bray-Curtis dissimilarity measure of relative species abundance on each of the two species of floating algae. Since the comparative analysis revealed no significant differences in the composition of Lepas spp. between the two algal species (see below), we pooled all samples to calculate relative abundance per locality for purposes of graphic presentation.
The majority of the collected macroalgae harbored barnacles of the family Lepadidae (goose barnacles). The lowest percentage of samples with goose barnacles was found near Isla Madre de Dios (60.0 %, Table 1). In total, 67.0 % of the goose barnacles were identified to species. All others were noted as cyprids since calcareous plates were missing. The relative percentage of cyprids was highest in the sampling area off Coquimbo (98.4 %, Table 1). In total, four species of goose barnacles associated to floating macroalgae were identified, for which a brief morphological description is presented in the following. The quantity and deposition number for the voucher specimens sent to the Museo Nacional de Historia Natural de Chile (MNHN) is noted following the description of each species.
Description of the goose barnacles from the Chilean Pacific coast
In general the species of the family Lepadidae possess a capitulum covered by five calcareous plates and a naked peduncle. Most species feature one or more characteristic filamentary appendages on or near the pedicel of the first cirrus (Fig. 1). All species represented herein can be distinguished using the relationship, form and texture of these plates as well as by the number and the relative length of the filamentary appendages (Fig. 1 and 3).
Lepas anatifera Linnaeus, 1767
General description: the carina is generally smooth or sometimes barbed, usually sharply bifurcated near the basal umbo; scutum is smooth, sometimes with concentric rays radiating up from the umbo; right scutum with one internal umbonal tooth, independent of developmental status although some individuals have a more pronounced than others; tergum is a little convex and obliquely quadrangular (Darwin 1852, Newman & Ross 1971) (Fig. 2A). With two filamentary appendages on each side of the body, a short one near the basis of the first cirrus and a large one nearby on the prosoma (Fig. 3A). Lepas anatifera shows a great similarity with L. australis. However, it can be distinguished from the latter by three main characteristics: (1) the presence of a single internal umbonal tooth on the right scutum, (2) the form of the carina is more tapered and shows bifurcation on the base and (3) the filamentary appendage of the prosoma is relatively large (relationship between length of scutum and filamentary appendage differs significantly between L. anatifera and L. australis; F = 62.286; DF = 1,20; P < 0.001, Fig. 3). Additionally, this filamentary appendage is comparatively more conspicuous in L. anatifera than in L. australis.
Inhabited substrata: Lepas anatifera has been found associated with diverse animals and floating objects such as e.g., macroalgae (Hobday 2000b), bottles (Young 1990), boats, buoys (Nilsson-Cantell 1957), wood (Minchin 1996), turtles (Kitsos et al. 2003) and even fish (Zevina & Memmi 1981). Voucher specimens, ten individuals deposited in the MNHN of Chile: MNHNCL CIRR - N°11548.
Lepas australis Darwin, 1852
General description: the carina is generally smooth, basal fork often broadly rounded, the apex not extending up between the terga; scutum smooth and flat, triangular but can be also quadrangular, both scuta have an internal umbonal tooth, the size of the teeth is variable and independent from the developmental status of individuals; Tergum, triangular, smooth and sometimes convex (Fig. 2B) (Darwin 1852, Newman & Ross 1971, Arnaud 1973, Foster 1978). With two filamentary appendages on each side of the body; one is located at the base of the first cirrus and the other near the prosoma. As noted above, this species is very similar to L. anatifera. However, L. australis generally has an internal umbonal tooth on each scutum, the carina is wider and often proportionately shorter, and the filamentary appendage on each side of the prosoma is comparatively short (Darwin 1852, Foster 1978) (Fig. 3).
Inhabited substrata: this species has been reported on floating macroalgae (Darwin 1852, Nilsson-Cantell 1957, Arnaud 1973, Zevina & Memmi 1981, Helmuth et al. 1994), volcanic pumice (Newman & Ross 1971), plastics (Barnes & Milner 2005) and on marine sea elephants (Mirounga leonia, Arnbom & Lundberg 1995, Barnes et al. 2004). Voucher specimens, ten individuals deposited in the MNHN of Chile: MNHNCL CIRR - N°11549.
Lepas pectinata Spengler, 1793
General description: the upper part of the carina is wide and gets slowly narrower bifurcating at the end, generally barbed; scutum almost triangular with radial furrows, often with spines; the right scutum with internal umbonal tooth; tergum almost triangular, radial furrowed, often with spines; both terga with a visible notch to receive the apex of the scutum; no space between scutum and tergum (Darwin 1852, Foster 1978, Young 1990) (Fig. 2C). Filamentary appendages reduced to a small protuberance at the base of the first cirrus.
Inhabited substrata: mainly associated with macroalgae and floating objects e.g. boats, buoys (Tsikhon-Lukanina et al. 1986), wood (Young 1990), tar pellets, plastics and volcanic pumice (Minchin 1996). Voucher specimens, ten individuals deposited in the MNHN of Chile: MNHNCL CIRR - N°11550.Dosima fascicularis (Ellis & Solander, 1786)
General description: the genus Dosima can be easily distinguished from the genus Lepas by the distinct angle formed at the sub-central umbo of the carina, and the very thin and brittle plates (Young 1990). During its development this species changes considerably in form; carina smooth and thin, the form is very different from the previous species, the base is almost round and not imbedded in the membrane, in the medium part (umbo) with an angle of ~100 degrees; scutum smooth and thin, also with a very different form as in comparison to all other species, it is separated from the other plates, no umbonal tooth is present; tergum is smooth and thin, with three angles, also flat (Darwin 1852, Newman & Ross 1971, Foster 1978, Young 1990) (Fig. 2D). Five filamentary appendages located at the base of the cirri on each side of the body.
Inhabited substrata: Dosima fascicularis is mainly found associated on small plastic fragments and tar pellets (Minchin 1996) or simply without substrate (Zevina & Memmi 1981). The species gains buoyancy due to the secretion of gas bubbles into a foam-float at the base of the peduncle (Darwin 1852, Boetius 1952-1953, Newman & Ross 1971, Zevina & Memmi 1981, Minchin 1996). Voucher specimens, two individuals deposited in the MNHN of Chile: MNHNCL CIRR - N°11551.
Distribution of goose barnacles along the Chilean Pacific coast
We found a similar faunal composition of goose barnacles on the two species of floating macroalgae (Macrocystis spp. and Durvillaea antarctica) from the respective sampling areas. The total abundances of Lepas spp. were similar on the two collected macroalgae with the highest abundances being found at Concepción (Table 2). Lowest total abundances were recorded in Coquimbo and Isla Madre de Dios (Table 2). A distinct distribution pattern of the assemblages of goose barnacles was observed along the Chilean coast, showing a strong break between San Antonio and Concepción (Fig. 4). In both areas, the relative abundances of barnacles growing on Durvillaea antarctica showed similarities with that on Macrocystis spp. in the sampling areas immediately to the north, which contrasts with the southern sampling areas where no differences in the barnacle assemblages from both algal species were observed.
Lepas anatifera was mainly distributed in the northern region of the study area, with highest relative abundances in Mejillones (23° S) and Caldera (27° S) of 95.9 and 79.6 %, respectively, but declining in abundance before dropping out at 33° S (Fig. 5). In both locations of abundance, the average SST fluctuated between 18.8 and 21.2 °C (Table 1). In contrast, L. australis showed a southern distribution, with declining abundances from "Isla Madre de Dios" (50° S, 100 %) north to Ancud (41° S, 92.7 %) and Concepción (36° S, 63.8 %), before dropping out at ~33° S (see Fig. 5) where the average SST fluctuated between 9.6 and 14.5 °C (Table 1). Lepas pectinata was encountered at all sampling areas except for "Isla Madre de Dios", with the highest relative abundance in the central region, off Coquimbo (29° S, 72.2 %) and San Antonio (33° S, 81.8 %) (Fig. 5), where the average SST varied between 16.7 and 18.2 °C (Table 1). Dosima fascicularis found herein is the first definite record for the SE-Pacific coast. This species was only detected at two sampling areas, namely Caldera (27° S, 0.2 %, 27 individuals) and San Antonio (33° S, 0.01 %, only one individual). In general, in the present study we identify a northern zone dominated by L. anatifera, a central zone where L. pectinata was most abundant and a southern zone where L. australis showed the highest relative abundance.
The described distribution pattern of Lepas spp. along the Chilean coast fits well with the global distribution of the studied species (Fig. 6). In general, we found L. anatifera in highest proportion in warm waters corresponding to their general distribution in tropical and subtropical waters. In contrast, L. australis was found primarily in colder waters, which fits the reported pattern of this species in subantarctic water masses. Lepas pectinata was found in waters with intermediate SST, matching the published distributional records of this species (Fig. 6).
Pelagic barnacles along the south east Pacific coast of Chile
There exist only a few studies that examine the presence of pelagic barnacles along the Chilean coast. In a review of the barnacles of Chile, Nilsson-Cantell (1957) reported three species of pelagic barnacles from the family Lepadidae; namely, Lepas anatifera, L. pectinata and L. australis. In a previous study, this author had already reported these species for the Islands "Juan Fernández" (Nilsson-Cantell 1929), suggesting they were the three most common lepadid species in Chilean waters. Memmi (1980) described what he considered a new species, Lepas annabellae, from the Chilean coast (Antofagasta, 23°19' S, 71°01' W). While it shows certain similarities with previously described varieties of L. australis, according to the author it does not present internal umbonal teeth on the scuta. Since the description of L. annabellae is only based on a single report of seven individuals, which were found stranded on the shore, the status of this species for the Chilean coast must be considered as uncertain.
In the present study four species of pelagic barnacles from the family Lepadidae were found on floating macroalgae (Lepas anatifera, L. pectinata, L. australis, and Dosima fascicularis). While the first three species were very common and had been previously reported (Nilsson-Cantell 1929, 1957), D. fascicularis is reported herein for the first time for the Chilean Pacific coast (see Newman & Ross 1971 for a record near Cape Horn). This species was found in low abundances and only at two localities. Several authors mentioned that D. fascicularis is commonly associated with small plastic fragments (Boetius 1952-1953, Cheng & Lewin 1976, Zevina & Memmi 1981, Minchin 1996). Considering that the examined substrata in the present study were exclusively floating macroalgae, it could be possible that the abundance of D. fascicularis was underestimated, but it also cannot be ruled out that this species has recently increased in abundance, because of increasing availability of attachment substrata. Floating plastic fragments are abundant in the oceans and the high floating property of these items may favor dispersal of D. fascicularis and possibly of other species (Minchin 1996, Thiel et al. 2003, Barnes & Milner 2005, Thiel & Gutow 2005).
Floating Macrocystis spp. and Durvillaea antarctica with associated populations of goose barnacles may serve as a good indicator for local supply of their larvae because the floating period of these macroalgae is sufficiently long for larval settlement and growth (Skerman 1958, Green et al. 1994), but may not permit long distance dispersal of adults. In contrast, substrata that have the potential to remain afloat for long time periods (months or years) such as plastic litter or buoys, would have the chance to be dispersed to distant sites and thus far away from the place where initial settlement occurred. Thus, populations of goose barnacles on floating macroalgae may be a good indicator for local larval supply, whereas individuals on plastics (e.g., Barnes & Milner 2005) and volcanic pumice (e.g., Bryan et al. 2004) may rather indicate distant larval pools.
Distribution of goose barnacles along the Chilean Pacific coast
Barnacles of the genus Lepas are reported to be widely distributed in the world's oceans, but most species have an optimum temperature range favoring reproduction and larval development (Fig. 6). Green et al. (1994) revealed that L. anatifera from the North Atlantic developed reproductive organs within ~120 days after settlement at SST between 10.2 to 18.4 °C. The reproductive rates observed by these authors were low, possibly because penis activity is limited at temperatures < 19 °C SST (Patel 1959). Evans (1958) showed that at temperatures of ~ 25 °C this species can reach sexual maturity within ~ 30 days. Zevina & Memmi (1981) mentioned that L. australis can be found in warmer waters, but the optimal water temperature for reproduction is between 5-15 °C. In the Atlantic, Lepas pectinata is abundant at temperatures between 10-27 °C and larvae are typically found in waters higher than 16 °C (Moyse 1987).
Herein a clear SST gradient was found along Chilean coast and the encountered goose barnacles were differentially distributed within particular temperature ranges. Lepas anatifera declined in abundance between 21.2 and 18.2 °C, while L. australis increased in abundance below 18.2 °C. Lepas pectinata was not found at temperatures < 13.2 °C, was most abundant at temperatures between 16.7 and 18.2 °C, and declined at higher temperatures (Fig. 5, Table 1). The possible reason for the low abundances outside the temperature-specific range is probably due to these species being unable to settle, grow and/or reproduce successfully under sub-optimal conditions. As an alternative explanation, Moyse (1987) suggested that the spines of the larval states (nauplii) of Lepas spp. may be correlated with the different viscosity of water masses: the relatively shorter and vestigial spines of L. pectinata and L. australis respectively, may be correlated with the higher viscosity of the colder waters inhabited by these species, while the large spines of L. anatifera may be correlated with the lower viscosity of warm waters. Therefore, it can be expected that the larvae of these three barnacles show highest abundances in their respective species-specific SST range and are restricted to the water masses to which they are morphologically as well as physiologically adapted.
Contribution to the understanding of the biogeography of the south east Pacific
The present study only considers the distribution of three principal species of pelagic barnacles. Even though this is a very limited number of species for a biogeography study, the distribution of these three species revealed a clear separation of three zoogeographic zones, similar to those reported for littoral organisms. While the low number of species included in this study could be considered a drawback, it allows on the other hand to link knowledge of the biology of each species to the water masses in which it was found. The warm-water species, L. anatifera, was not found south of 33° S in the Peruvian-Chilean Province, and the West Wind Drift species, L. australis, was not detected north of 33° S in the Magellanic Province, whereas Lepas pectinata was most abundant in the Chilean Transition Zone where these two provinces overlap. A study by Foster (1978), including the lepadids from New Zealand, revealed a similar northern distribution for L. anatifera and L. pectinata, whereas L. australis was only encountered south of 39° S. This author noted the overlap of these species in a central region and suggested that the surface currents and the prevailing SST were responsible for maintaining the distribution pattern observed, because the surface currents drive the distribution of floating substrata and SST directly affects the life cycle of individuals or restricts larval distribution. This together with the suggestion by Moyse (1987) on temperature-specific adaptations of larvae, indicates that indeed the larval states of these goose barnacles are restricted to certain water masses.
The distribution pattern reported herein for goose barnacles closely matches that of littoral benthic fauna. The species-specific SST association of goose barnacles suggests that a clear oceanographic influence exists along Chilean coast, and that SST itself and its influence on planktonic stages is the principal causal factor for the biogeographic patterns observed along the south east Pacific coast. For littoral mollusks, Rivadeneira & Fernández (2005) found that 20 % of the analyzed species changed their distributional range in accordance with changing gradients of SST. Astorga et al. (2003) also demonstrated that the pattern of SST is inversely proportional to the diversity of decapod crustaceans and argued that SST may affect the larval phases of these species. While these considerations provide support for the direct influence of SST (possibly via planktonic stages), we can not exclude the possibility that other factors modulate the observed distribution pattern (e.g., Knox 1960, Briggs 1995). Lancellotti & Vásquez (1999) also revealed a link between SST and the distribution of benthic species, but they additionally mentioned that freshwater influx along the Chilean coast exhibits a clear latitudinal gradient, which may also influence the distribution of benthic species (highest amount of freshwater towards the south). Zvyagintsev & Mikhajlov (1985) reported a massive death of L. anatifera at decreasing salinity, which could also contribute to the distributional patterns observed herein. Larvae of parasitic barnacles exhibit varying developmental times at different salinities (Kashenko & Korn 2002), suggesting that salinity may also affect the distribution of goose barnacles.
While most authors agree on three geographical zones along the Chilean coast (e.g., Knox 1960, Lancellotti & Vásquez 1999, 2000), the exact limits of these zones can vary substantially (see Camus 2001), possibly due to the effects of "El Niño-Southern Oscillation" ENSO (Thiel 2002, Escribano et al. 2003). In the present study, we found all four species of goose barnacles near San Antonio (33° S) in the center of the transition zone, while at the other sampling areas only one or two of these species were observed. Thus, our results agree with previous biogeographic studies of benthic organisms, which report a peak of species richness in the transition zone (Lancellotti & Vásquez 2000, Ojeda et al. 2000, Camus 2001, Rivadeneira et al. 2002, Thiel 2002, Hernández et al. 2005).
Along the south east Pacific coast there appears to exist a close match between the biogeography of planktonic and of littoral benthic organisms (Fagetti 1968, Antezana 1981, Ojeda et al. 2000, Camus 2001, Thiel 2002, Escribano et al. 2003, this study), just as there is in the north east Pacific (Newman 1979). Herein, we observed that the oceanographic conditions, mainly SST, are shaping the distribution of pelagic goose barnacles. This supports the hypothesis that the dispersal of organisms and their distribution along the Chilean Pacific coast is modulated by specific oceanographic conditions (SST, see Briggs 1995). Based on these considerations we suggest that the temporal distribution patterns of pelagic organisms and larvae may be highly variable in the central transition zone, in accordance with interannual variations in oceanographic conditions (e.g., ENSO). Studying interannual variability in the latitudinal distribution of the pelagic biota may contribute to a better understanding of the biogeographic limits along the Chilean coast.
We sincerely acknowledge to all boat crews for their cheerful assistance during the surveys. We also acknowledge Juan José Garrido from Puerto Natales for logistic support. Eva Rothäusler helped with the initial translation of this manuscript. Two anonymous reviewers provided highly constructive comments that contributed significantly to the final manuscript. Funding provided by FONDECYT 1010356
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Received May 16, 2005; accepted October 19, 2005