Alvaro T. Palma

Evolutionary versus ecological success in Antarctic benthic invertebrates

The unusually high proportion of brooding compared with broadcaster species among coastal Antarctic invertebrates has been traditionally interpreted as an adaptation to local environmental conditions. However, species with a planktotrophic developmental mode are ecologically dominant (in terms of abundance of individuals) along Antarctic coastal areas. Therefore, is the apparent ecological success of broadcasters related to their developmental mode? We argue that the present shallow Antarctic benthic invertebrate fauna is the result of two processes acting at different temporal scales. First, the high proportion of brooding species compared with coastal communities elsewhere corresponds to species-level selection occurring over geological and evolutionary times. Second, the ecological dominance of broadcasters is the result of processes operating at ecological timescales that are associated with the advantage of having pelagic larvae under highly disturbed conditions.

DOES VARIABLE COLORATION IN JUVENILE MARINE CRABS REDUCE RISK OF VISUAL PREDATION

We discovered that newly settled marine rock crabs, Cancer irroratus, exhibit a variety of non-adult colors early in life. This color polymorphism predominates in populations of minute juvenile crabs living in polychromatic habitats where it apparently renders them inconspicuous to visual predators such as fish. Experiments revealed lower frequencies of non-adult color morphs in monochromatic three-dimensional habitats with predators and polychromatic habitats from which predators were excluded. These patterns result from selective predation on visually contrasting color morphs. Adult crabs are monochromatic, conspicuous, and not associated with shelters. Both polymorphic newly settled and monochromatic large individuals occur in environments dominated by small predatory fish where larger adult crabs are at low risk of predation. Behavioral and visual crypsis may only be important early in life when post-settlement mortality is high, and survival at that stage determines recruitment and ultimately population densities. The well-known examples of camouflage among insects usually apply to adults who, unlike these marine counterparts, are small relative to their predators and thus remain vulnerable throughout their lives. Many other large marine crustaceans are cryptic only early in life, suggesting that this early developmental color polymorphism might be an important difference between marine and terrestrial arthropods.

Settlement-driven, multiscale demographic patterns of large benthic decapods in the Gulf of Maine

Three decapod species in the Gulf of Maine (American lobster Homarus americanus Milne Edwards, 1837, rock crab Cancer irroratus Say, 1817, and Jonah crab Cancer borealis Stimpson, 1859) were investigated to determine how their patterns of settlement and post-settlement abundance varied at different spatial and temporal scales. Spatial scales ranged from centimeters to hundreds of kilometers. Abundances of newly settled and older (sum of several cohorts) individuals were measured at different substrata, depths, sites within and among widely spaced regions, and along estuarine gradients. Temporal scales ranged from weekly censuses of new settlers within a season to inter-annual comparisons of settlement strengths. Over the scales considered here, only lobsters and rock crabs were consistently abundant in their early post-settlement stages. Compared to rock crabs, lobsters settled at lower densities but in specific habitats and over a narrower range of conditions. The abundance and distribution of older individuals of both species were, however, similar at all scales. This is consistent with previous observations that, by virtue of high fecundity, rock crabs have high rates of settlement, but do not discriminate among habitats, and suffer high levels of post-settlement mortality relative to lobsters. At settlement, large, habitat-scale differences exist for lobsters but not for rock crabs; these are probably the result of larval settling behavior. In contrast, patterns at the largest, inter-regional, spatial scales suggest oceanographic control of larval delivery. Increased mobility and vagility with greater body size for both species reduces demographic differences among older individuals over a range of spatial scales.

Crypsis in Paraxanthus barbiger (Decapoda: Brachyura): Mechanisms Against Visual Predators

Abstract Marine organisms have evolved a suite of responses to minimize the exposure to predators. Visual crypsis is one such strategy to avoid predation. Paraxanthus barbiger (Poeppig, 1836) is a species that exhibits different color morphotypes over heterogeneous substrates as a means of protection against visual predators. Our main objectives were to quantify the occurrence of color morphotypes over a three-year period and to investigate, via an experimental approach, on the possible mechanisms involved that would provide crypsis to this species. Field surveys occurred over a three-year period at two nearby sites on the central Chilean coast. Initial observations indicated that small juvenile P. barbiger exhibited higher degrees of color polymorphism than larger (> 20 mm carapace width) conspecifics. Furthermore, survival rates of small (< 10 mm carapace width) P. barbiger exposed to predators increased on heterogeneous substrata under both natural and laboratory conditions. Laboratory experiments further demonstrated that newly settled P. barbiger actively select heterogeneous substrata. Hence, cryptic responses of this species might reduce predation-mediated mortality through color pattern disruption of individuals with respect to their environment.

Crypsis in early benthic phases of Brachyuran Decapod Crustaceans in central Chile

Aunque comunes, y en algunos casos de valor comercial, poco es lo que se conoce acerca de los mecanismos y procesos que afectan la distribucion y abundancia de muchas especies de crustaceos decapodos, particularmente en Chile. Todas estas especies poseen larvas planctonicas que deben asentarse y reclutar antes de pasar a formar parte de los segmentos adultos de sus poblaciones. En varias de estas especies hemos detectado la presencia de una variedad de morfos de coloracion en individuos recien asentados. La existencia de dichos morfos en especies chilenas es comparable con aquellos observados en otras latitudes (e.g., costa del Golfo de Maine). Por otro lado, es caracteristica la aparicion de estos patrones solo durante los estadios juveniles tempranos y, al alcanzar cierto tamano, estos desaparecen, siendo los adultos tipicamente monocromaticos. La expresion de este patron en el submareal chileno parece ser mas pronunciada en las especies detritivoras que en las carnivoras. Proponemos que la existencia de este tipo de atributos fenotipicos, aparentemente generalizado en decapodos braquiuros, podria jugar un rol importante, tanto desde un punto de vista ecologico como evolutivo

Antarctic shallow subtidal echinoderms: is the ecological success of broadcasters related to ice disturbance?

One characteristic pattern found in the marine Antarctic shallow environments is the unusually high proportion of species with protected and pelagic lecitotrophic development modes. However, species with planktotrophic development generally appear as the most conspicuous types of organisms in these environments. The Antarctic shallow benthos is considered as one of the most disturbed in the world, mainly due to the action of ice, thus one could hypothesize that such an environment should favor organisms with high dispersal capability. In order to test this general hypothesis, for two consecutive summers (2004–2005) and at two locations, we quantified the abundance and size distribution of most echinoderms present along bathymetric transects. Our results show the predominance of broadcasters (i.e., Sterechinus neumayeri and Odontaster validus) at a location where disturbances are common, while brooders (e.g., Abatus agassizii) only occurred at shallower depths of the least disturbed location. These results not only corroborate the hypothesis that local disturbance is an important factor generating these ecological patterns, but also suggest how ice-related disturbances could represent a major selecting agent behind the patterns of species diversity at an evolutionary scale in Antarctica.

Effects of La Niña on recruitment and abundance of juveniles and adults of benthic community-structuring species in northern Chile

The El Nino Southern Oscillation (ENSO) has been associated with dramatic changes in adult populations of shallow benthic community-structuring species in northern Chile. However, early ontogenetic stages are more likely to be affected by physical processes associated with ENSO (e.g. increased temperature, upwelling), potentially reflected in differences in recruitment intensity. We evaluated recruitment and abundance of juveniles and adults of the major benthic community-structuring groups (kelps, sea urchins, sea stars, crabs and rockfish) during a period affected by La Nina followed by a period of neutral conditions. Recruitment was several times higher during neutral conditions for sea urchins and sea stars. Lower recruitment during La Nina was probably caused by offshore larval transport caused by increased upwelling. Recruitment of crabs tended to be higher during neutral conditions, but this varied among sites. In contrast to invertebrates, kelp abundance seemed to be controlled by post-settlement processes, as reflected by fewer kelp juveniles after La Nina when the abundance of herbivore fishes strongly increased. Our results highlight the importance of evaluating the effects of ENSO at early ontogenetic stages rather than on adult populations, which are less likely to be affected in an upwelling-dominated system.

Restricted geographic distribution and low genetic diversity of the brooding sea urchin Abatus agassizii (Spatangoidea: Schizasteridae) in the South Shetland Islands: A bridgehead population before the spread to the northern Antarctic Peninsula?

Los periodos glaciales del Pleistoceno han promovido los principales cambios climaticos del oceano Austral. Han generado el interes cientifico sobre las estrategias desarrolladas por la fauna bentonica para tolerar y superar la extension y contraccion de la capa de hielo sobre la plataforma continental. Un estudio reciente acerca de la zonacion batimetrica y distribucion de macro-invertebrados en una zona submareal somera de la Bahia Fildes (Isla Rey Jorge, islas Shetlands del Sur), resalta la presencia de una importante agregacion del erizo incubante Abatus agassizii, cuya distribucion geografica es conocida solo para localidades al sur de la convergencia antartica (Peninsula Antartica, islas Shetland del Sur e Isla Georgias del Sur en el Arco de Escocia). Sin embargo, su presencia es atipica dado a que, primero, sus poblaciones someras debieron haber sido extintas desde la vecindad de la Peninsula Antartica por los avances y retrocesos de la capa de hielo y segundo, la ausencia de estadios larvales asociado a la estrategia de incubacion como modo de desarrollo deberia limitar una recolonizacion desde areas subantarticas mas al norte. El objetivo de este estudio fue evaluar si A. agassizii puede haber sobrevivido a las glaciaciones en su estrecho rango batimetrico en las islas Shetland del Sur o ha recolonizado recientemente el area, a pesar de su baja capacidad de dispersion. A pesar de un intensivo esfuerzo de muestreo, la baja ocurrencia de A. agassizii indica que su distribucion es muy escasa a lo largo de las islas Shetland del Sur y la Peninsula Antartica y parece estar limitada a densas poblaciones someras en areas protegidas y libres del hielo en la Bahia Fildes en la Isla Rey Jorge. Ademas, estas poblaciones presentan una muy baja diversidad genetica asociada a una senal de una expansion demografica reciente. Finalmente, la reconstruccion de las relaciones filogeneticas entre las especies de Abatus, con secuencias mitocondriales del gen COI, establecieron la afinidad de la especie antartica A. agassizii con especies subantarticas. De acuerdo a los resultados obtenidos, consideramos que la presencia de esta especie en las islas Shetlands del Sur probablemente corresponde a una reciente recolonizacion desde las islas antarticas situadas mas al norte.

Effect of prolonged exposition to pulp mill effluents on the invasive aquatic plant Egeria densa and other primary producers: a mesocosm approach.

The recent disappearance of the aquatic plant Egeria densa, a Brazilian native invasive species, from a wetland in southern Chile prompted several efforts to unveil the origin of this phenomenon. Because these changes occurred by the time a newly built pulp mill started its operations in the area, a reasonable doubt for a cause-effect relationship is plausible. We implemented a mesocosm approach to directly evaluate the effect of treated pulp mill effluents (PMEs) on several growth-related parameters of E. densa as well as other primary producers. We hypothesize that effluent, at a dilution similar to that detected in the zone of the wetland where the negative environmental impacts were evident, has a significant negative effect on this aquatic plant as well as on other primary producers inside a mesocosm system. After a prolonged (months) exposure to both 0% PME with pure river water and a 4 to 5% (v/v) dilution of treated PME, no effect on E. densa was measured. Furthermore, plants exposed to effluent exhibited a significantly greater general growth rate. Coincidently, chlorophyll a concentration in the water column and periphyton biomass also changed over time, but without any pattern attributable to the effluent. Values of the autotrophic index obtained from the periphyton growth pattern did not suggest enrichment of the system with organic matter. Our results only refer to the direct effect of mill effluents on several biotic responses, but they represent an important advance toward generation of the scientific knowledge necessary to understand how the ecosystem functions while receiving this and other unquantified sources of water.

Description, seasonal morphological variation, and molecular identification of Paraxanthus barbiger megalopae obtained from the natural environment

Larval identification represents a powerful tool for detailed studies on recruitment and population dynamics in marine invertebrates. However, intra-specific morphological variation can become a serious limitation for the correct identification at species level. High morphological variation can be expected in species with continuous breeding periods because larvae are exposed to seasonal fluctuations in physical and biological factors during their development in the plankton. We describe, for the first time, the megalopae of Paraxanthus barbiger, one of the most common and abundant brachyuran crabs along the coast of Chile. To validate larvae identification, the 16S rRNA gene was sequenced from both megalopae and adults, and was compared with sequences of three sympatric species. In addition, size, body shape, and appendage setation pattern variations were analyzed with a year-round sampling scheme. The results demonstrated high seasonal phenotypic plasticity in size. Despite these differences, certain conservative characteristics exist which can be very useful for identification at species level.