Ricardo Guiñez

A Tridimensional Self‐Thinning Model for Multilayered Intertidal Mussels

Intertidal mussels usually form complex multilayered matrices with density‐dependent effects on survival and growth, and self‐thinning scaling between biomass (B) and density (N) is expected. This article develops a tridimensional model of space‐driven self‐thinning that in addition to B‐N explicitly includes the degree of packing of the mussels, measured as the number of layers (L). The structure of our model (B‐N‐L) encompasses previous bidimensional models (B‐N) of self‐thinning as special cases and enables comparisons between mono‐ and multilayered populations. We contrast the predictions of the bi‐ and tridimensional models using data obtained from Perumytilus purpuratus mussel beds on the rocky shores of central Chile monitored during a 28‐mo period. The tridimensional model suggests that density dependence is much more frequent than hitherto indicated by bidimensional models. We propose that our space‐driven tridimensional model may be applied not only to mussels but also to other species where spatial overlapping configurations occur.

Disjoint geographical distribution of intertidal and nearshore benthic invertebrates in the Southern Hemisphere

Se revisa y analiza las posibles explicaciones para la distribucion geografica disyunta, presente y pasada, en el hemisferio sur de cinco especies de invertebrados bentonicos marinos litorales: Gaimardia trapesina, Ostrea chilensis, el complejo taxonomico Pyura stolonifera, Aulacomya ater y Concholepas concholepas, con estrategias reproductivas y caracteristicas de historia de vida distintas. Se discute y pone a prueba, usando informacion original o publicada, las siguientes hipotesis: a) procesos historicos de vicarianza, b) dispersion de larvas epi-planctonicas, c) dispersion de juveniles o adultos por transporte pasivo y d) dispersion antropogenica planificada o accidental. La hipotesis de dispersion transoceanica de juveniles o adultos fue la unica imposible de rechazar para las especies analizadas. Se discute las direcciones futuras de investigacion en esta area

AN ALLOMETRIC TRIDIMENSIONAL MODEL OF SELF‐THINNING FOR A GREGARIOUS TUNICATE

A tridimensional allometric model of self-thinning was tested in a tunicate population of Pyura praeputialis in the Antofagasta Bay, northern Chile. The theory tested follows the bidimensional allometric theory of Osawa and Allen for the self-thinning rule proposed for plants, except that in addition to mean individual mass we included three new concepts: the number of layers (an equivalent of the leaf area index, [LAI] used in plants), the effective unit area, and the density per unit effective area. We assumed constant tunicate mass per effective unit area and allometry for mean dimensions of the population. The tridimensional model can be reduced to a bidimensional one if density is expressed as the density per unit effective area, which corresponds to the number of individuals relative to the effective area occupied by individuals when they conform a monolayer. The model allows for random sampling and rules out the selection for samples or stands at maximum crowding. The self-thinning of P. praeputialis through a tridimensional model showed an exponent of −1.518 (95% ci,−1.635 to −1.401) for mean tunicate visceral mass, and an exponent of −1.489 (95% ci, −1.588 to −1.390) for the corrected bidimensional model. The two exponents were identical to those predicted from tridimensional allometric theory. The results of this new approach increase the variance explained in comparison to that of classical bidimensional models.

A review on self-thinning in mussels

Los bivalvos marinos de la familia Mytilidae, mas conocidos como choritos, se destacan por su abundancia y capacidad de ser especies dominantes en las costas rocosas de todo el mundo. En general, las especies de choritos tienden a formar matrices (o mantos) altamente densas y multiestratificadas, en cuyo interior ocurre una intensa competencia intraespecifica, siendo esperable la existencia de procesos de auto-raleo. En este trabajo, reviso la literatura de auto-raleo en mitilidos, con especial referencia al desarrollo de nuevos modelos. Es evidente la necesidad de realizar mas trabajo experimental y teorico para comprender apropiadamente el rol de la competencia por espacio o por explotacion sobre la dinamica del auto-raleo en especies gregarias. Se concluye que el desarrollo de nuevos modelos tridimensionales, que incluyen explicitamente los efectos del modo en que se ocupa el espacio, esta permitiendo abrir nuevas avenidas teoricas y experimentales para explorar la relacion entre densidad, tamano corporal y biomasa no solo en choritos, sino que tambien en animales gregarios en general.

Ciclo gonadal del chorito Mytilus chilensis (Bivalvia: Mytilidae) en dos localidades del sur de Chile

A qualitative and quantitative analysis was carried out of the gonadal cycle of the bivalve Mytilus chilensis from Chaihuin and Yal bay, southern Chile, between October 2007 and June 2008. Four gametogenic stages were determined using histological analysis of the gonads, and quantitative estimates were made of the Gametic Volume Fraction (VFG), percentage of inter follicular connective tissue, and the Gonadosomatic Index (IG). The quantitative analysis (VFG) was the best indicator of spawning. Two spawning events, one in October and one in March, were observed simultaneously in both sexes of mussels from Chaihuin. However, for specimens from bahia Yal, four spawning events were registered, principally from March to June (autumn), when the water temperature decreased. The relationship between the IG and the gametogenic stages was very low, as was that between the IG and the percentage of mature oocytes. Therefore, the IG is not a good indicator of spawning in this species. A re-evaluation of the ban period established for Mytilus chilensis (1 November to 31 December) is suggested since most individuals from the populations studied mature mainly in October. At both sites, the percentage of connective tissue for the analyzed mussel individuals ranged between 15 and 70% of gonadal coverage. The results obtained in the present study showed differences in the reproductive cycles of Mytilus chilensis between the sites sampled.

Geographical Variation of Shell Thickness in the Mussel Perumytilus purpuratus Along the Southeast Pacific Coast

At broad geographical scales, the variation in bivalve shell thickness can be modulated by environmental factors that vary with latitude, such as sea surface temperature (SST), seawater pH, or calcium carbonate availability. Mussels usually form multilayered beds, and shell thickness is also expected to be affected by density and layering due to intraspecific competition. In this work, we explored the geographical variation of shell thickness in the intertidal mussel Perumytilus purpuratus between 18° and 42°S along the southeastern Pacific coast. We tested the hypothesis that there was a positive relationship between shell thickness and SST, and then we explored other variables that could have an effect on thickness, such as density, number of layers, and others environmental variables (pH and calcite concentration). The expected positive linear relationship between shell thickness and sea surface temperature was not found, but when the other population variables were included in the analysis, an unexpected inverse SST-thickness relationships appeared as significant, probably because this species could be adapted to colder and more acid seawater as are those of the tips of South America. Thickness was also negatively affected by density, which was expected for a gregarious species showing high intraspecific competition. Finally, our results highlight the importance of including density and crowding effects when macroscale patterns are explored, particularly in gregarious species, since these patterns could also be modulated by density-dependent processes, which might then override latitudinal trends of shell thickness when they are not included in the analyses.

Análisis comparativo del ciclo gametogénico de Perumytilus purpuratus (Bivalvia: Mytilidae), en las localidades de Taltal y Huasco, norte de Chile

A qualitative and quantitative study of the gametogenic cycle from natural populations of Perumytilus purpuratus at the localities of Taltal and Huasco was carried out between October 2007 and October 2008. By means of histological analysis a gonadal maturation table was constructed according to the microscopic characteristics. Also the Gametic Volume Fraction (GVF) (% of previtelogenic oocytes, vitelogenic oocytes and mature oocytes), percentaje of interfolicular connective tissue and Gonadic Index (GI) were determined. According to the qualitative analysis, P. purpuratus has a semiannual reproductive cycle starting in January with spawning mainly during spring-summer, even do some winter spawning were detected. However, the quantitative analysis of mature oocytes percentajes found that at Taltal population four spawnings were detected (December, March, July and October) while in Huasco population three spawnings were detected (December, February and September). The relationship between GI with the gametogenic stages and the mature oocyte percentajes were significant at the localities studied. Also significant differences of the condition index between males and females were observed. Along the latitudinal gradient the reproductive cycle of P. purpuratus showed variations according to the timing and synchrony.

Asimetría bilateral de la forma de las valvas y posición espacial en matrices del chorito Perumytilus purpuratus (Lamarck, 1819) (Bivalvia: Mytilidae)

De las distintas formas de la asimetria bilateral, la asimetria direccional (AD) ha recibido considerablemente menos atencion en la literatura que la asimetria fluctuante (AF). Si bien existen buenos ejemplos de AD conspicua en algunos grupos de moluscos (e.g., Ostreidae y Pectinidae), para otros como los mitilidos la AD no ha sido reportada. En matrices del chorito Perumytilus purpuratus de tres localidades del intermareal rocoso de Chile norte y central, comparando individuos de los estratos superior e inferior, y usando dos indices de forma de la valva, reportamos evidencia de AD en relacion con la forma de las valvas, tendiendo la derecha a ser consistentemente mas elongada que la valva izquierda. Sin embargo, no solo en estratos inferiores de la matriz los choritos tienden a tener valvas menos elongadas sino que tambien tienden a ser mas asimetricas, en relacion con el estrato superior. Los resultados sugieren que el cambio de forma y de asimetria podria estar siendo modulado por la densidad o la estratificacion. Las explicaciones causales de este fenomeno estan abiertas a la investigacion

PERMANENT GENETIC RESOURCES: Development of microsatellite markers for the ecosystem bioengineer mussel Perumytilus purpuratus and cross‐priming testing in six Mytilinae genera

Eight microsatellite markers have been characterized from the Perumytilus purpuratus genome. Their gene diversity ranged from 0.057 to 0.873 and significant interpopulation genic heterogeneity was observed between two populations of southeastern Pacific (Chile) and southwestern Atlantic (Argentine). Distinct cross‐priming amplification rates were recovered on nine additional species belonging to six Mytilinae genera. The microsatellites developed herein would likely be a powerful intraspecific genetic tool to undertake fine population studies in the intertidal ecosystem bioengineer P. purpuratus along the South American shoreline.

Variación fenotípica y genética en el tunicado Pyura praeputialis (Heller, 1878) en el área norte de la Bahía de Antofagasta, Chile

The morphology of “sea squirts” (Urochordata: Tunicata) is highly variable and is expressed in habitat-specific differences intra- and interspecific variation pattern that had hindered the interpretation of such differences as well as their taxonomical, ecological or evolutionary meaning. Here we describe the population genetic structure of three morphotypes (squat, intermediate and tall “sea squirt”) observed in populations of the “piure de Antofagasta”, Pyura praeputialis by enzyme electrophoresis. The study was carried out in three localities in the northern sac of the Bay of Antofagasta, Chile, and distributed along a wave exposure gradient. A total of 17 enzymatic loci were assayed by horizontal starch-gel electrophoresis, three of which (IDH-1, MDH-1 and PGM-1) were polymorphic. The results showed significant genetic differences among localities, but not among morphotypes. These results are contrasted with similar studies conducted on the same species in Australia, on the base that P. praeputialis has recently colonized Antofagasta from Australia. It is suggested that the differences among the morphotypes from Antofagasta are intraspecifics and those from Australia are interspecifics. The possible explanations for the genetic and morphological differences among localities are discussed.The morphology of “sea squirts” (Urochordata: Tunicata) is highly variable and is expressed in habitat-specific differences intra- and interspecific variation pattern that had hindered the interpretation of such differences as well as their taxonomical, ecological or evolutionary meaning. Here we describe the population genetic structure of three morphotypes (squat, intermediate and tall “sea squirt”) observed in populations of the “piure de Antofagasta”, Pyura praeputialis by enzyme electrophoresis. The study was carried out in three localities in the northern sac of the Bay of Antofagasta, Chile, and distributed along a wave exposure gradient. A total of 17 enzymatic loci were assayed by horizontal starch-gel electrophoresis, three of which (IDH-1, MDH-1 and PGM-1) were polymorphic. The results showed significant genetic differences among localities, but not among morphotypes. These results are contrasted with similar studies conducted on the same species in Australia, on the base that P. praeputialis has recently colonized Antofagasta from Australia. It is suggested that the differences among the morphotypes from Antofagasta are intraspecifics and those from Australia are interspecifics. The possible explanations for the genetic and morphological differences among localities are discussed.