World Congress of Malacology Antwerp ... - Unitas Malacologica

Shape variability of Mactra isabelleana (Heterodonta: Bivalvia) from two different habitats off
Buenos Aires province coast
Signorelli, Javier H.; Pastorino, Guido
Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” Av. Ángel Gallardo 470,
C1405DJR, Buenos Aires, Argentina,
Email: jsignorelli@macn.gov.ar
The frequent species Mactra isabelleana (d’Orbigny, 1846) can be found in several locations along
the coast of the province of Buenos Aires, including the Rio de la Plata estuary. The different
environmental processes present at these locations can produce morphological variation in the
specimens. In order to prove this hypothesis we analyze, using geometric - morphometric methods,
the shell shape variation of Mactra isabelleana. In addition, we also compare this species with the
cogeneric M. marplatensis to verify the generic variability. All specimens were collected by a
trawling net in two locations that represent the estuary and the open sea. They were San Clemente
(36º20’50’’ S, 56º 43’ 40’’ W) and Mar del Plata (38º01’45’’S, 57º30’05’’W) respectively. Sixteen
ad hoc landmarks were digitalized from shell pictures. Principal Component and Canonical Variation
analysis were carried out with IMP software, in order to investigate the shape variability between
specimens and samples from different habitats. Samples from Mar del Plata were the most variable
and those from San Clemente were the most homogeneous, perhaps because all of them belonged in
the same population. Principal Component analysis allowed us to unquestionably separate the two
species. However, Canonical variation analysis was necessary to reveal the shape variation in M.
isabelleana. According to Bartlett’s test, we found significative differences between specimens from
San Clemente and Mar del Plata. This analysis also allowed us to verify the distance between the two
species. Finally, to prove the cause of this variability, tests will be carried out of the different
environmental factors like salinity, sediments and tide and wave dynamic, among others.
Radiation and diversity of primitive living chitons – unravelling the base of the
polyplacophoran family tree
Sigwart, Julia D.
Collections-based Biology in Dublin, National Museum of Ireland, Natural History Division, Merrion
Street, Dublin 2, Ireland,
Email: julia.sigwart@ucd.ie
Chitons are often referred to as “living fossils” in part because they are proposed as one of the
earliest-diverging groups of living molluscs but also because the gross morphology of the
polyplacophoran shell has been conserved for hundreds of millions of years. As such, the analysis of
evolution and radiation within polyplacophorans is of considerable interest not only for resolving the
shape pan-molluscan phylogeny but also as model organisms for the study of character evolution.
This study presents a new rigorous cladistic analysis of the morphological characters used in
taxonomic descriptions for chitons in the suborder Lepidopleurina (the most basal living group of
chitons). Shell-based characters alone entirely fail to recover any recognised subdivisions within the
group, which may raise serious questions about the application of fossil data (from isolated shell
valves). New analysis including characters from girdle armature and gill arrangements recovers some
genera within the group (Hanleya, Ferreiraella) but also points to new and previously unrecognised
affinities within the main genus Leptochiton. Additional characters from molecular data and soft
anatomy are clearly needed to resolve questions of chiton relationships. However, the datasets
currently available already provide interesting insights into the analytical power of traditional
morphology, as well as some knowledge about the early evolution and radiation of this group.
205