Luis Miguel Pardo

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.

An effective sperm competition avoidance strategy in crabs drives genetic monogamy despite evidence of polyandry

For polyandrous species where females have sperm storage structures, males develop several strategies to avoid sperm competition and thus to maximize the number of eggs fertilized. On the other hand, females may receive several benefits from multiple paternity (indirect and directly), and a potential sexual conflict can arise. This research describes the mating systems of an exploited crab species (Metacarcinus edwardsii), integrating (1) the individual level by assessing the mating behavior in a scenario of potential polyandry, (2) the organ level by examining histological sections of seminal receptacles from localities with scenarios of contrasting sex ratios, and (3) the genetic level by measuring the number of parents involved in egg clutches. We found that females can mate with multiple males under experimental conditions. Further, in all localities, we found histological evidences that sperm receptacles stored ejaculates from more than one male. However, contrary to expectations, genetic analysis revealed high probability of single male paternity of all progeny in each egg clutch. In this mating system, males compete to be the single male that mates with a receptive female, investing energy in guarding behavior and foregoing opportunities to mate with other females, all in order to ensure their paternity. However, females benefit from multiple mating (or potential for it) by prolonged guarding behavior, protecting them from predation after molt (soft-shelled period). The mating system of M. edwardsii can be defined as polygamous (where both sexes can mate multiple times) with genetic monogamy.

Field Assessment of the Predation Risk - Food Availability Trade-Off in Crab Megalopae Settlement

Settlement is a key process for meroplanktonic organisms as it determines distribution of adult populations. Starvation and predation are two of the main mortality causes during this period; therefore, settlement tends to be optimized in microhabitats with high food availability and low predator density. Furthermore, brachyuran megalopae actively select favorable habitats for settlement, via chemical, visual and/or tactile cues. The main objective in this study was to assess the settlement of Metacarcinus edwardsii and Cancer plebejus under different combinations of food availability levels and predator presence. We determined, in the field, which factor is of greater relative importance when choosing a suitable microhabitat for settling. Passive larval collectors were deployed, crossing different scenarios of food availability and predator presence. We also explore if megalopae actively choose predator-free substrates in response to visual and/or chemical cues. We tested the response to combined visual and chemical cues and to each individually. Data was tested using a two-way factorial design ANOVA. In both species, food did not cause significant effect on settlement success, but predator presence did, therefore there was not trade-off in this case and megalopae respond strongly to predation risk by active aversion. Larvae of M. edwardsii responded to chemical and visual cues simultaneously, but there was no response to either cue by itself. Statistically, C. plebejus did not exhibit a differential response to cues, but reacted with a strong similar tendency as M. edwardsii. We concluded that crab megalopae actively select predator-free microhabitat, independently of food availability, using chemical and visual cues combined. The findings in this study highlight the great relevance of predation on the settlement process and recruitment of marine invertebrates with complex life cycles.

Fishery induces sperm depletion and reduction in male reproductive potential for crab species under male-biased harvest strategy.

Sperm depletion in males can occur when polygynous species are intensively exploited under a male-biased management strategy. In fisheries involving crabs species, the effects of this type of management on the reproductive potential is far from being understood. This study tests whether male-biased management of the principal Chilean crab fishery is able to affect the potential capacity of Metacarcinus edwardsii males to transfer sperm to females. Five localities in southern Chile, recording contrasting crab fishery landing, were selected to assess the potential of sperm depletion triggered by fishery. Seasonally, male crabs from each locality were obtained. Dry weight and histological condition of vasa deferentia and the Vaso-Somatic Index (VSI) were determined in order to use them as proxies for sperm depletion and male reproductive condition. A manipulative experiment was performed in the laboratory to estimate vasa deferentia weight and VSI from just-mated males in order to obtain a reference point for the potential effects of the fishery on sperm reserves. Sperm storage capacity is significantly affected by fisheries; during the mating season vasa deferentia from localities with low fishery intensity were heavier than those from high intensity fisheries, and these differences were even more evident in large males. Histological section showed that this disparity in vasa deferentia weight was explained principally by differences in the quantity of spermatophores rather than other seminal material. VSI was always higher in males from localities with low fishery intensity. Males from localities with high fishery intensity showed little capacity to recover sperm reserves and the VSI of these males remained below the values of the just-mated males. Detriment in the capacity of males to transfer sperm is the first step to sperm limitation in an exploited population, thus detection of sperm depletion can be an alert to introduce changes in the current management of crabs.

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.

Use of novel microsatellite markers for population and paternity analysis in the commercially important crab Metacarcinus edwardsii (Brachyura: Cancridae)

Abstract In order to study fishery stocks and the effect of the male-biased fishery on multiple paternity, new genetic resources were developed for Metacarcinus edwardsii, the most important crab of artisanal fishery in Chile. A total of eight microsatellite loci were isolated and characterized in this crab, using an enriched library. Six to 27 alleles per locus were observed in two samples of 24 crab individuals. The observed heterozygosity ranged from 0.58 to 1.00 and all loci showed no deviation from Hardy–Weinberg equilibrium for either. Two sampling sites separated by 300 km showed no differences in allele frequencies, indicating that both sites are a part of the same population. Further, the genotypes of five ovigerous females and their newly hatched larvae showed that only one male was involved in the fertilization of each clutch. The variation detected in these markers is currently being used both for the study of populations of Metacarcinus edwardsii in a highly exploited geographic area and in order to elucidate the mating system of this species.

Variaciones temporales en la composición y abundancia de cuatro especies de Cancer (Decapoda: Brachyura: Cancridae) capturadas con trampas en bahía San Vicente, Concepción (Chile central)

This study analyzes the composition and distribution of the main brachyuran decapod crustacean species caught by small-scale fishermen in the coastal area off the Hualpen Peninsula (Concepcion). These patterns were then compared and related to variations in temperature and extraction depth. The catches throughout the year were basically represented by four Cancer species, especially in autumn 2003, when C. porteri was most abundant. This species was under-represented during the rest of the year and dominant again the following summer. C. coronatus, C. setosus, and C. edwardsi were not very abundant in summer, but appeared in greater quantities during the rest of the year. This variation in species composition and spatial distribution of the catches was related to seasonal migrations, which were associated with the occurrence of bad weather fronts that affect the area in winter. The significant relationships between abundance and temperature support the occurrence of these migrations. Moreover, experiments (catch, marking, re-catch) were done to determine the population parameters of the species under study. Although the general re-catch rate was very low, the results suggest a high degree of mobility.

Description of a new species of Asymmetrione (Isopoda: Bopyridae: Pseudioninae) infesting the hermit crab Paguristes tomentosus (Anomura: Diogenidae) from Peru, with a key to species and a review of southeastern Pacific bopyrids

A new species of bopyrid isopod is described from a host hermit crab Paguristes tomentosus collected from Pisco, Peru and assigned to the pseudionine isopod genus Asymmetrione Codreanu, Codreanu and Pike, 1965. This is the second species in the genus with less than 20° asymmetry in females, but agrees with all other species in the genus in having the characteristic “socket” on the propodus of the females pereopods, and in all characters of the males. A key is provided to the 10 species now in Asymmetrione. New records of an Anathelges sp. from Pagurus villosus collected in central Chile are given but additional material is required to determine whether these specimens represent the eastern Pacific A. thompsoni or the western Atlantic A. hyptius. A review of all the bopyrid species known from the western South American coast, with remarks on their taxonomy and biology, is provided.

Asentamiento diario de megalopas de jaibas del género Cancer en un estuario micromareal

RESUMEN. El asentamiento de crustaceos decapodos en ambientes estuarinos, es la ultima etapa de un complejo proceso en que las especies meroplanctonicas cierran la fase de vida pelagica. Este proceso tiene etapas sucesivas, que comienza con la formacion de parches larvales frente a la costa, su transporte hacia las zonas de crianza, exploracion de sustrato en busca de un lugar adecuado para el asentamiento y la metamorfosis a juvenil. Las interacciones entre los factores fisicos y el comportamiento de las larvas competentes regulan las diversas etapas de este proceso, determinando finalmente las tasas de asentamiento locales. En este estudio se analiza la importancia relativa de las forzantes fisicas que actuan a meso-escala (i.e. estres del viento, nivel del mar y caudal del rio) y micro-escala (condiciones locales de temperatura, salinidad y tipo de sustrato) sobre el asentamiento de megalopas de los braquiuros Cancer edwardsii y C. coronatus, principales jaibas comerciales en la pesca artesanal chilena. Para esto se instalaron colectores larvales pasivos de fondo por 73 dias consecutivos en el periodo de maximo reclutamiento (i.e. abundancia de juvenil I), en la desembocadura del estuario del rio Valdivia y se dispuso de arena gruesa y un simil de tapiz algal como sustrato. Los resultados mostraron que: a) el asentamiento esta caracterizado por extensos periodos de escasos asentados alternados por pulsos de maximos, que no necesariamente coinciden entre las especies estudiadas, b) el asentamiento de C. edwardsii mostro estar relacionado con multiples forzantes que generan la adveccion de aguas calidas hacia la costa, c) el asentamiento de C. coronatus tuvo relacion con las forzantes que generan la surgencia costera, d) las larvas de ambas especies no difieren en el asentamiento sobre sustratos heterogeneos. Este estudio mostro que los patrones de asentamiento larval son especie especificos, incluso en especies simpatricas y estrechamente emparentadas. Asi, cualquier generalizacion por taxa debe ser bien soportada por estudios comparativos en la misma escala espacio-temporal. Palabras clave: estuario, rio Valdivia, Metacarcinus, Brachyura, reclutamiento, caudal, estres del viento, Chile.

High fishing intensity reduces females’ sperm reserve and brood fecundity in a eubrachyuran crab subject to sex- and size-biased harvest

High fishing intensity reduces females’ sperm reserve and brood fecundity in a eubrachyuran crab subject to sexand size-biased harvest Luis M. Pardo*, Marcela P. Riveros, Juan Pablo Fuentes, Ramona Pinochet, Carla C ardenas, and Bernard Sainte-Marie Facultad de Ciencias, Instituto de Ciencias Marinas y Limnol ogicas, Universidad Austral de Chile, Valdivia, Chile Centro de Investigaci on de Din amica de Ecosistemas Marinos de Altas Latitudes (IDEAL), Valdivia, Chile Institut Maurice-Lamontagne, Pêches et Océans Canada, 850 Route de la Mer, C.P. 1000, Mont-Joli, QC G5H 3Z4, Canada *Corresponding author: tel: þ56 63 2293551; fax: þ56 63 2221315; e-mail: luispardo@uach.cl.