Diego Giberto

THE INVASIVE RAPA WHELK RAPANA VENOSA (VALENCIENNES 1846): STATUS AND POTENTIAL ECOLOGICAL IMPACTS IN THE RÍO DE LA PLATA ESTUARY, ARGENTINA-URUGUAY

Abstract Recent range extensions of the invasive rapa whelk Rapana venosa, biological data of the population and possible ecological impacts on the food webs of the Río de la Plata estuary are presented. A total of 41 rapa whelks and 21 egg capsules were collected between February 2004 and March 2006. Specimens were found all over the mixohaline waters of the estuary, with records off Montevideo, Samborombón Bay and off Punta Rasa. Specimens ranged between 28 and 120 mm shell length. Almost all R. venosa presented epibionts all over the shell, suggesting an exposed lifestyle. Egg-masses were attached to specimens of R. venosa, debris and plastic garbage. The spatial distribution of R. venosa within the estuary was coupled with the spatial distribution of Mactra isabelleana, a typical subtidal bivalve of mixohaline waters. The implications for the food webs of the estuary, including possible predators, are discussed. The presence of R. venosa in muddy sediments together with the finding of egg-capsules over the specimens, and the low salinity values at which it is found in the Río de la Plata estuary, confirmed the high plasticity of R. venosa and the potential capability of successfully invading novel environments.

Porifera from the Argentine Sea: Diversity in Patagonian scallop beds

Porifera are a frequently collected taxon in the invertebrate by‐catch of the Patagonian scallop Zygochlamys patagonica (King & Broderip 1832) fishery. In order to prevent the destruction of the exploited environments in the Argentine Sea, it is important to know in detail the taxonomic composition of the benthic assemblages. We present here the first study on the sponge assemblage associated with the mollusc species exploited. Thirteen species of Demospongiae (four of which are new records for the Argentine Sea) have been identified, confirming the importance of this phylum as a major component of the scallop bed community.

Reproductive biology of Lychnorhiza lucerna (Cnidaria: Scyphozoa: Rhizostomeae): Individual traits related to sexual reproduction

Abstract We studied individual traits related to the sexual reproduction of Lychnorhiza lucerna and reviewed earlier studies of sexual maturation in scyphomedusae, focusing on non-brooding species. Lychnorhiza lucerna is a gonochoric species and sexual dimorphism was noted in the gonadal colour. There were no brooding structures or any other distinguishable features enabling macroscopic determination of sex. Gametogenesis resembled descriptions available for other rhizostomes and semaeostomes. Both processes are asynchronous, with gametes at all stages of development occurring together. Oocytes arose from the gastrodermis and maintained contact with it via trophocytes throughout vitellogenesis. Spermatogenesis occurred within spermatic follicles arising from nested primary spermatogonia. Population features of sexual reproduction were defined by characterizing and quantifying individuals at different stages of sexual maturity. In Scyphozoa, sexually mature medusae can be detected by (1) the presence of fully developed gametes in the gonads indicating incipient spawning, or (2) the presence of spent follicles as evidence of ongoing or recent spawning. Whereas the former allows more detailed study of sexual reproductive patterns of any species, the latter constitutes an option for non-brooders (as in L. lucerna) equivalent to the search of fertilized eggs or planulae for brooder species.

Benthic survey of natural and artificial reefs off Mar del Plata, Argentina, southwestern Atlantic

This study describes the macrofaunal assemblages of subtidal rocky reefs off Mar del Plata in order to compare the macro invertebrate assemblages settled on shipwrecks with those of nearby reef sand to characterize the fish fauna associated with natural (NR) and artificial (AR) reefs. Topographic characterizations and surveys of invertebrates and fishes were performed in November and December 2005, using SCUBA diving. A non-parametric multivariate analysis was used to analyze the environmental and biological data. The reefs were mainly distinguished by their depth and the position of the substrate. The red calcareous algae, Corallina officinalis, dominated the shallowest NR, whereas conspicuous mytilid assemblages of Mytilus platensis were present at depths over 3 m, sea anemones, Anthothoe chilensis, were more abundant between 6 and 10 m, and the soft coral, Tripalea clavaria, was found at nearly 20 m depth. No differences were found between horizontal ARs and the adjacent NRs. The greatest differences were found between the communities of vertical and horizontal substrates, both in NRs and ARs. Fifteen fish species were recorded in the analyzed area. Species having strong site fidelity, e.g., Acanthistius brasilianus and Pseudopercis semifasciata, were clearly more abundant and/or frequent in ARs (shipwrecks) than in NRs. This pioneer study in the surveyed area showed that reef sallow the settlement of diverse benthic assemblages. ARs also provide refuge for fishes. As sport fishing and diving are activities being carried out in Mar del Plata, where tourism is one of the main economic resources, the protection of such areas should be considered in conservation plans.

New records of two species of Sabellaria (Polychaeta: Sabellariidae) from the Argentinean Biogeographic Province

Sabellaria bellis Hansen, 1882 y S. wilsoni Lana & Gruet, 1989 fueron registradas en diversos habitats costeros de Uruguay y Argentina, entre 34°S y 37°S, Provincia Biogeografica Argentina. Se discuten los patrones de distribucion local en relacion con la hidrografia. Nuestros resultados indican que ambas especies son tolerantes a los cambios de salinidad.

From coexistence to competitive exclusion: can overfishing change the outcome of competition in skates (Chondrichthyes, Rajidae)?

Competition for food could be a major force driving changes in the community structure of skates (Rajidae) subjected to fishing exploitation. Under this hypothesis, small skates are released from competition with larger skates after fishing has depleted the larger species. Here, we compare the abundance patterns of two sympatric skates with similar niches but different life histories, Bathyraja albomaculata (larger and slow-reproducing) and Bathyraja macloviana (smaller and faster-reproducing), before (1971, 1978) and after (1998-2004) a 108% increase in industrial bottom trawling on the southeastern South American shelf in order to test the prediction that B. macloviana should competitively exclude B. albomaculata after the increase in fishing mortality. In 1971 and 1978, there was no relationship between the abundance of both species, indicating that they coexisted over large scales. In 1998-2004, the relationship between the abundances of these skates was bell-shaped, indicating that both species increased in abundance at low densities until peaking, after which B. albomaculata decreased when B. macloviana became more abundant, consistent with resource competition. We tested whether food may be a potential limiting resource by comparing the diet of both species. The two species consumed mostly polychaetes, differing only in the consumption of polychaetes from the family Nephthyidae, which was much higher for B. macloviana. Bathyraja macloviana could replace B. albomaculata at high densities when food resources may become scarce. These results support the hypothesis that competition release is an important factor explaining the changes in skate communities in overexploited areas.

The Patagonian scallop fishing grounds in shelf break frontal areas: the non assessed benthic fraction

In this study, a Picard dredge was used to sample the benthic community in shelf break frontal areas off Argentina in order to detect species that could be sensitive to fishing activities but are not usually caught during the annual monitoring of Patagonian scallop and associated fauna. The present results show at least 62 taxa not reported previously as components of the benthos in the scallop grounds that are potentially subjected to trawling disturbance.

DIET AND DAILY CONSUMPTION RATES OF RAPANA VENOSA (VALENCIENNES, 1846) (GASTROPODA: MURICIDAE) FROM THE RIO DE LA PLATA (ARGENTINA-URUGUAY)

ABSTRACT In this study we describe the feeding biology of the exotic predatory whelk Rapana venosa from the Río de la Plata when offered representative coastal bivalves from both soft and hard bottoms. Furthermore, we estimate daily consumption rates using two mussel species commonly found on local rocky outcrops and artificial reefs. R. venosa gastropods (n = 21; shell height (SH), 45–117 mm) were obtained by trawling in the Río de la Plata, and were kept in controlled laboratory settings. To describe its feeding behavior, 10 potential prey species were offered to the gastropods, including Mactra isabelleana, Erodona mactroides, Pitar rostratus, Donax hanleyanus, Mesodesma mactroides, Mytilus platensis, Brachidontes rodriguezi, Mytella charruana, Amiantis purpurata, and Crassostrea gigas. All prey species were consumed. The empty shells left after consumption frequently showed broken shells and rasp signatures on the edges, suggesting a prey attack by marginal chipping or a similar mechanism. This common pattern was not observed in A. purpurata or C. gigas. To explore potential effects of size on feeding behavior, the gastropods were separated into three size classes and offered M. platensis, B. rodriguezi, and M. charruana mussels. Small, intermediate, and large gastropods (SH <60 mm, 60–90 mm, and >90 mm, respectively) consumed comparable mussel size ranges. Small gastropods fed on prey sizes up to 30% larger than their size, whereas large gastropods consumed the largest mussel offered (SH, 78 mm). To estimate daily consumption rates, the 21 gastropods were fed with M. platensis and B. rodriguezi in individual tanks for 12 days. They ate 1.45 ± 0.67 SD mussels and 0.68 ± 0.65 g/day per gastropod. The proportion of tissue wet weight consumed did not change with prey species, but varied among the size classes of R. venosa. Small (<60 mm) and intermediate (60–90 mm) gastropods consumed a similar proportion of prey tissue (up to 12% of their tissue wet weight). In contrast, large gastropods (>90 mm) ate up to 5% of their tissue wet weight. This work is the first report of R. venosa feeding behavior and daily consumption rates in Argentina. The gastropods succeeded in attacking and feeding on a broad variety of local bivalve species characterized by noticeable differences in shell shape, size, and life habits.

EBAEIA ROTUNDATA (A. MILNE-EDWARDS, 1880) (BRACHYURA, LEUCOSIIDAE) IN MARINE WATERS OFF ARGENTINA AND URUGUAY

The present paper reports the occurrence of Ebalia rotundata (A. Milne-Edwards, 1880) in marine waters off Argentina and Uruguay, and thus extends its known geographical distribution in the Argentinean Zoogeographical Province more than 700 km northwards. Feasible dispersal mechanisms of this species are discussed. Este trabajo reporta la ocurrencia de Ebalia rotundata (A. Milne-Edwards, 1880) en aguas marinas de Argentina y Uruguay, extendiendo su distribucion geografica en la Provincia Zoogeografica Argentina mas de 700 km hacia el norte. Se discuten los posibles mecanismos de dispersion de esta especie.

Synoptic Post Fishery Structure of Invertebrate Bycatch Associated to Zygochlamys patagonica Fishing Grounds at the Southwest Atlantic Shelf-Break Front (39° S, Argentina)

ABSTRACT Biological data were collected in a Patagonian scallop fishing ground, between 39° 00′ and 39° 50′ S and between 87 and 130 m depth. During 2007, we sampled the following stations where the trawling exerted could be precisely determined: 11 locations subjected to continuous fishing effort between 1996 and 2006 (continuous fishing effort, 878 commercial tows) and 12 locations subjected to fishing effort between 1996 and 2002 (interrupted fishing effort, 302 commercial tows). Univariate (species richness, Shannons diversity index, and Pielous equitability index) and multivariate analysis, analysis of variance, and Kruskal—Wallis tests were applied to analyze species composition and biomass. This study reflects the significant higher density of sessile taxa (Porifera and the ascidean Paramolgula gregaria) and two small ophiuroids (Ophiacantha vivipara and Ophiura lymani), all of them conspicuous during the exploratory cruises conducted in 1995, in the sites where fishing effort was interrupted 4 y before the sampling in 2007 than in those sites continuously exploited and discusses the results considering previous knowledge on the damage that the process on board scallopers imparts on invertebrates bycatch.