Néstor J. Cazzaniga

Mass mortality of the yellow clam, Mesodesma mactroides (Bivalvia: Mactracea) in Monte Hermoso beach, Argentina

Yellow clam was the most abundant species in the sandy mesolittoral community of Monte Hermoso (Buenos Aires Province, Argentina). In November 1995, a sudden massive mortality almost decimated its population. The number of dead individuals was estimated in near 63 million, with a total biomass of 428.03 ton (fresh weight with shells). Simultaneous samples of sediments, seawater and clams did not allow detecting the probable cause of this phenomenon. No abnormality was found in the physicochemical parameters that could justify the mortality. Several specific mortality factors were discarded, as the analysis for heavy metal contamination, phytotoxins, abnormal phytoplankton composition, and protozoan tissue-parasites gave negative results. The meteorological conditions were rather stringent during a period immediately previous to the mortality, this accounting for a probable switch condition. This sole factor can not however account for the progressive decimation of the species observed from 1993 on: in the term of 3 years mass mortalities of Mesodesma mactroides occurred in Brazil, Uruguay and Argentina, covering the entire area of distribution of the species. The yellow clam is now in a risk condition as for its conservation status.

Genetic exchange between two freshwater apple snails, Pomacea canaliculata and Pomacea maculata invading East and Southeast Asia

Two species of apple snails, Pomacea canaliculata and Pomacea maculata (formerly Pomacea insularum), have invaded many countries of East and Southeast Asia from their native range in South America. This study investigated the genetic structure of the two species invading these areas. Phylogenetic analysis based on sequences of the nuclear gene elongation factor 1-alpha (EF1α) detected two well-supported clades (Clade C and Clade M). Both P. canaliculata and P. maculata were represented in each clade. Some snails had both Clade C and Clade M EF1α sequences. These results suggest genetic exchange between snails of the two clades. A mating experiment between P. canaliculata with Clade C EF1α sequences and P. maculata with Clade M EF1α sequences resulted in viable F1 progeny under laboratory conditions. The genetic exchange was also inferred in some populations collected from Argentina, suggesting an existence of hybrid in the native range. Simple identification of EF1α types using a restriction enzyme, ApaLI, detected significant geographical structure of the EF1α variants in the invaded area. The divergent geographical structure could have resulted from either the founder effect or the bridgehead effect, although further genetic analysis is needed to clarify this. Average individual egg weight, which is an indicator of egg size, was higher in P. canaliculata than P. maculata in both field and laboratory reared samples, suggesting that some (probably most) P. canaliculata and P. maculata invading East and Southeast Asia still maintain species-specific populations.

Anatomical studies of sibling species within neotropical lymnaeids, snail intermediate hosts of fascioliasis

Several anatomical parameters of the reproductive system have been used to distinguish Lymnaea cubensis from L. viatrix, the snail hosts of fascioliasis in South America and the Caribbean area. Three samples have been collected in the type localities of L. cubensis (Cuba), L. viatrix var. A ventricosa (Argentina, Río Negro Lower Valley), and L. viatrix var. B elongata (Peru, Lima), respectively. Only one parameter, the relative lengths of the penis sheath and preputium, showed significant differences between L. viatrix var. ventricosa and the two other taxa. None of the studied parameters separated L. cubensis from L. viatrix var. elongata.

Asiatic Clam, Corbicula fluminea, Reaching Patagonia (Argentina)

ABSTRACT The invasive Asiatic clam, corbicula fluminea (Muller, 1778), is reported from the Colorado River, at Rio Colorado city, in the northern boundary of Patagonia. This locality is deemed to be on the southernmost latitude the species can reach due to cold constraint.

Triatoma patagonica (Hemiptera, Reduviidae), a New Host for Triatoma virus

Previous authors demonstrated that Triatoma virus (TrV) is able to infect several species of triatomines when injected with viral inoculum obtained from its original host, T. infestans. Both vertical (transovarian) and horizontal (faecal-oral) mechanisms of viral transmission were also described. In this paper we report the experimental TrV infection of a wild species from southern Argentina, T. patagonica. The inoculum consisted of clarified gut contents of infected T. infestans rubbed on the chicken skin whereupon T. patagonica individuals were fed. The results demonstrate that this is another potential host for the virus, and that the oral route is also effective for experimental interspecific infections.

Winter distribution, density and size of Mesodesma mactroides (Bivalvia, Mactracea) in Monte Hermoso Beach (Argentina)

The yellow clam Mesodesma mactroides (Deshayes, 1854) is a seasonal migrant that moves in spring to the sandy upper intertidal level. In this paper we analyze the spatial distribution of density and mean shell size of the yellow clam population in Monte Hermoso beach (Argentina) in winter 1995, i.e., three months before the mass mortality occurred in November 1995. Sampling covered 32 km of beach, with a regular design of 22 transects. The major environmental gradient in the beach was determined using principal component analysis (PCA) on the correlation matrix of the environmental data (beach morphology, slope, and sand granulometry). Correlation analysis was used to assess the relationship between the score of a site (transect) on the first and second principal component, and clam mean density and mean shell size. Most of the beach seems to be habitable for clams, their spatial heterogeneity not having been explained by the measured variables since, although the first axis of the PCA has demonstrated an E-W physical gradient, clam density was not in correlation with it. Density was maximum near the piers, even though these are points with high tourist activity. It seems that non-extractive touristic activities do not affect population density but rather mean shell size, probably due to reduction of growth rates. The abundance of the winter population, as compared with the assessment done after the mass mortality of November, strongly suggests that a great part of the population was overwintering in the intertidal fringe.

Allopatry and Anatomical Distinctiveness of Two Puzzling Land Snails in Genus Plagiodontes, from Argentina (Gastropoda: Orthalicidae, Odontostominae)

ABSTRACT Plagiodontes daedaleus (Deshayes, 1851) is the most widespread species in the genus Plagiodontes Doering, 1877. Although some 19th century authors recorded some differences in shell shape between the populations living to the east or to the west of the Pampean Sierras, Argentina, such differences were ignored by later authors. A multivariate statistical analysis on 608 adult shells collected at 29 localities in and around these mountains was used to test if such narratively described shell differences were statistically significant, irrespectively of their taxonomic status. Shells classified as collected from the east or the west of the sierras were correctly identified in 91.9% of the cases by Multivariate Discriminant Analysis, leading to recognizition of two allopatric shell morphologies. These external divergences were found to correspond with anatomical differences in the reproductive system. We conclude that two different, allopatric species were merged under the name Plagiodontes daedaleus in the literature and raise the subspecies Plagiodontes daedaleus strobelii (Doering, 1877) to the specific level. Another subspecies living to west of the sierras, Plagiodontes daedaleus salinicola (Doering, 1877), also showed a simple penis papilla and a non-swollen vagina, which shows that salinicola is not a member of P. daedaleus but rather a population of P. strobelii that does not deserve subspecific status. The taxonomic significance of the presence of either a simple penial papilla or a papilla bearing an accessory lobe was also confirmed for these Plagiodontes species.

Biological studies and phenology of the slug Deroceras reticulatum (Müller, 1774) (Pulmonata: Stylommatophora)

Summary In the southeast of Buenos Aires Province (Argentina), slugs cause reductions in crop yield by killing seeds and seedlings, by destroying stems and growing points, and by reducing leaf area. Deroceras reticulatum (Müller, 1774) was introduced into Argentina a long time ago, but the biology and ecology of this species in this country are still unknown. The aim of this study was to determine how temperature affects growth rate, reproduction and survival of D. reticulatum and also to assess the seasonal phenology of this species. The life cycle of D. reticulatum was investigated at 12°C, 20°C and at a temperature which alternated between 12°C for 16 h and 20°C for 8 h. The light/dark regime during the experiment was LD: 8/16 h. Slug populations were sampled from grassland located at the Experimental Station of INTA Balcarce. Eggs hatched after 16.3 to 39.8 days and fertility varied between 83.9% and 91.1%. The mean time from hatching to oviposition was longer and fecundity lower at 20°C than 12°C and 12/20°C (p <0.05). The net reproductive rate was 1.49 to 70.53 and the mean generation time varied between the 27 and 30 weeks. In natural conditions, eggs of D. reticulatum were found from early winter until the end of the spring, when soil temperature was 8–15°C and humidity was above 15%. These results indicate that D. reticulatum has one generation per year, univoltine phenology, with peak densities of slugs from the beginning of winter to the end of spring, which coincides with the sowing and emergence of winter and summer crops.

Asiatic Clam, Corbicula fluminea, in Northwestern Patagonia (Argentina)

ABSTRACT The invasive Asiatic clam, Corbicula fluminea (Muller, 1778), is reported from the Negro River, at Chimpay, in the Patagonian shrubland. The invasion seems to have occurred in 1997–1998, as revealed by its absence in the field assessments made up to January 1997 and the presence of a layer of Corbicula empty shells in the dry sand banks left by the flood of the river in winter 1998.

Barrels and pagodas: clarifying the identity and variability of two nominal taxa of carinated land snails in the genus Plagiodontes (Gastropoda: Orthalicidae: Odontostominae)

Plagiodontes weyenberghii (Doering, 1877) has a shell with a strong suprasutural keel; the species inhabits a restricted area in Central Argentina. Plagiodontes weyenberghii minor (Parodiz, 1939) was described as a subspecies with obese shells and broader geographic distribution; some populations were sympatric with Plagiodontes daedaleus (Deshayes, 1851). A discriminant analysis on 244 adult shells combined with original information on the internal anatomy confirmed the distinctiveness of P. weyenberghii weyenberghii, whereas P. weyenberghii minor showed a broad overlap with P. daedaleus and no evidence of anatomical differences. As a consequence, P. weyenberghii minor is considered a junior subjective synonym of P. daedaleus daedaleus (Deshayes, 1851). Plagiodontes weyenberghii (Doering, 1877) has unique characteristics such as the weak vascularization of the pallial system, the relative size of the penial papilla and its accessory lobe, the markedly swollen distal portion of the bursa copulatrix duct, the spermoviduct with two whorls, and the vagina with a medial depression.