World Congress of Malacology Antwerp ... - Unitas Malacologica

epresented by 23 spp. (7.1 %) and there are 8 endemic species (2.4 %) from southern Spain or the
Ibero-Moroccan area.
The diversity of the molluscan fauna, possibly the highest in European waters for a single locality, is
favoured by variety of habitats but also by drawing from different biogeographic origins. The
presence of upwellings may also explain the rather large share of West African species, which is
surprising considering the rather cool temperatures of surface waters.
Valve ultrastructure of suborder Ichnochitonina (Polyplacophora) from the South West
Atlantic
Urteaga, Diego; Pastorino, Guido
Museo Argentino de Ciencias Naturales, Av. Angel Gallardo 470, 3º Piso, Lab. 57. C1405DJR.
Buenos Aires, Argentina,
Email: diegourteaga@macn.gov.ar; gpastorino@macn.gov.ar
The ultrastructure of the intermediate valves of Plaxiphora aurata (Spalowsky, 1795) (Mopaliidae),
Chaetopleura isabellei (d'Orbigny, 1841) (Ischnochitonidae, Chaetopleurinae), Callochiton puniceus
(Gould, 1846) (Ischnochitonidae, Callochitoninae) and Tonicia lebruni (Rochebrune, 1887)
(Ischnochitonidae, Lepidochitoninae), collected along the Argentine coast, was studied based upon
different sections, i.e., transversal, sagittal, exsagittal, diagonal and parallel to the surface. Four main
layers were found: tegmentum, articulamentum, p. hypostracum and myostracum, according to
terminology described by Hass (1972).
The tegmentum is the outermost layer. It covers the dorsal surface of the valves and eventually folds
at boundaries. This layer is made up of micro crystals without evident order, although in the upper
portion it seems to form spherulitic sectors. The p. hypostracum shows a greater development in the
jugal area, occupying most of its thickness. It has a crossed lamellar arrangement with its long
horizontal axis normally to the sagittal axis of the chiton, although this orientation becomes
somewhat disorganised when thin prismatic layers pass through the p. hypostracum. Muscles are
inserted in the myostracum, which is made up of crystals arranged prismatically with their longer axis
almost normal to the ventral surface of the valve. The articulamentum, which is imbedded into the
soft tissues of the chiton, is made of micro crystals that diverge from the centre towards the dorsal
and ventral sides, becoming almost parallel to each other. Differences among the species studied are
discussed. In addition, these features are compared with those observed by Bøggild (1930) and Haas
(1972) for other chiton species.
Genetic basis of adaptive evolution of left-right reversal under developmental constraint
Utsuno, Hiroki; Asami, Takahiro
Department of Biology, Shinshu University, Matsumoto 390-8621, Japan,
Email:t04h151@shinshu-u.ac.jp; asami99@shinshu-u.ac.jp
The animal rule of directional asymmetry in developmental polarity indicates the presence of
selection against left-right reversal. If developmental constraint stabilizes the left-right polarity in
snails, how sinistral snails have repeatedly evolved? Our study answers this question. Sinistral
variants of Lymnaea stagnalis hatch poorly, and hatchlings do not grow in the exact mirror-image of
dextrals, even if they share nuclear genome from the same parents. This example of developmental
constraint against sinistrality may result from distortion due to pleiotropy (side-effects) of the leftright
determinant or to left-right reversed morphogenesis. To determine the cause, we crossed a
racemic mutant with the wild dextral type in a simultaneous hermaphrodite Bradybaena similaris.
The racemic mutant produces both dextral (RD) and sinistral (RS) offspring that share cytoplasmic
factors from the same mother. The wild type produces only dextrals (WD). We found that only RS
exhibit significantly reduced hatching success and flattened shell shape compared to RD and WD,
despite sharing nuclear genes from the same parents. Because RD does not exhibit those effects of
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