World Congress of Malacology Antwerp ... - Unitas Malacologica

esource with high commercial value; this species is distributed from the north of the Pacific of
Canada, the United States of America to South Baja California, Mexico. Due to the lack of biological
information and to the increasing demand mainly for exportation, it has being established as the study
object of this research the characterization of the reproductive cycle by means of the histological
analysis of the gonad and comparing the phases of gonadic maturity with the organism’s sizes to
establish the reproduction size. The clams were collected from September, 2005 to September of
2006 in Magdalena Bay at South Baja California. In order to select a subsample for histological
study, the analysis of frequency of sizes and weight of the whole organisms was made by means of
kernel density estimators with the bandwidth from the Silverman multimodality test; the frequency
estimators obtained with a significant number of modes per month permitted the characterization of
Gaussian components by Bhattacharya method. Consecutively the visceral mass was extracted and
fixed to carry out the histological technique. With the histological analysis five phases of
development were observed: undifferentiated, developing, ripe, partially spawned and spent; the
undifferentiated phase occurred in September and October; the phase of development were present in
November and December; the ripe phase was observed in January, February and March; by April and
May, the phase of partially spawned was observed and from June, July and August occurred the spent
phase. This stage sequence is in agreement with the climatic seasons: autumn, winter, spring and
summer indicating the growth, reproduction and recruitment periods respectively.
Temporal variation of bivalve assemblages in sandy subtidal sediments of the Ensenada de
Baiona (Galicia, NW Spain)
Moreira, Juan 1,2 ; Troncoso, Jesús S. 3
1. Estación de Bioloxía Mariña da Graña, Universidade de Santiago de Compostela, rúa da Ribeira 1,
A Graña, E-15590 Ferrol, Spain,
Email: ebmgjuan@usc.es
2. Departamento de Bioloxía Animal & Instituto de Acuicultura, Universidade de Santiago de
Compostela, Campus Sur, E-15782 Santiago de Compostela, Spain,
3. Departamento de Ecoloxía e Bioloxía Animal, Facultade de Ciencias, Campus de Lagoas-
Marcosende s/n, Universidade de Vigo, 36310 Vigo, Spain,
Email: troncoso@uvigo.es
The Galician rías are complex estuarine systems with high primary productivity and great economic
and social importance because of the presence of fisheries and diverse shellfish resources. In
addition, the rías have a large variety of sediments inhabited by a diverse benthic fauna. In some rías,
a number of human activities, such as the culture of bivalves on rafts, the construction of harbour
facilities and the disposal of sewage, have already induced large alterations in composition of benthic
assemblages through organic enrichment and changes in sedimentary composition. These
disturbances may also affect the variability of individual populations of benthic organisms.
Therefore, in order to identify the impact of anthropogenic disturbances in the marine benthos it is
necessary to study the distribution and natural spatio-temporal variations of the benthic infauna.
Despite the existing wealth of information on the benthic fauna of many Galician rías, little is known
of the spatio-temporal variation of the subtidal soft-sediment fauna from the Ensenada de Baiona.
This inlet is located in the mouth of the Ría de Vigo and its sediments are mostly sandy.
Bivalves are a key component to the structure of marine benthic assemblages, the diet of a number of
demersal fishes of economic interest and may serve as indicators of the conditions of the marine
bottoms. In the Ensenada de Baiona, bivalves are one of the dominant taxa in shallow subtidal soft
sediments. This study describes the temporal dynamics of the bivalve assemblages on sandy
sediments during a 1-year period from three sampling stations in the Ensena de Baiona. Sampling
was done monthly by means of a Van Veen grab from February'96 to February'97. Sampling stations
were at depths of between 4 and 7 m and had a similar sediment with low content in silt/clay and
organic matter.
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