World Congress of Malacology Antwerp ... - Unitas Malacologica

each of these radiations goes back to a single invasion event, that all these radiations started relatively
simultaneously, and that they are all younger than the lake itself (i.e., intralacustrine radiation).
B) Micro-habitats and intralacustrine barriers may have supported allopatric speciation
Phylogenetic data indicate a strong horizontal (e.g. feeder springs vs. actual lake) and vertical
structuring (e.g. below and above the Chara belt) in Lake Ohrid molluscs that may have contributed
to micro-allopatric speciation.
C) Ongoing speciation
Though intralacustrine radiations started in the early Pleistocene, there is evidence for ongoing
diversification in Lake Ohrid. We hypothesize that the high degree of biodiversity seen today may
not be due to elevated speciation rates but rather due to reduced extinction rates.
D) Source function of Lake Ohrid
Within a European phylogeographical context, we identified taxa, ancestors of which possibly
evolved inside the lake and later spread over the Balkans. Examples include the mussel Dreissena
presbensis and the gastropod Pyrgula annulata.
In summary, it can be said that Lake Ohrid served as evolutionary reservoir for relic species from
different parts of Europe. However, the vast majority of the endemic faunas seen today is relatively
young and the result of allopatric intralacustrine speciation.
Receptor physiology in bivalve eyes and the role of primary inhibition
Wilkens, Lon A.
Center for Neurodynamics and Department of Biology, University of Missouri-St. Louis, One
University Blvd, St. Louis, Missouri 63121, USA,
Email: wilkensl@umsl.edu
Bivalve molluscs are known for shadow responses involving closure and/or retraction of the sipn and
valve adduction. In representative genera (Spisula, Mercenaria, Lima) the pallial nerves contain
photosensitive fibers that exhibit physiological shadow responses. These photoreceptors are mostly
spontaneously active in the dark and inhibited by light. Dimming, i.e., a shadow triggers an excitatory
burst of off-response spikes. Equivalent responses characterize the photoreceptors in bivalve eyes.
Optic nerve fibers with vigorous light-off responses are known for the siphon tentacle eyes of
Cardium, and the mantle eyes of scallops, e.g., (Argo)Pecten and Lima. Invariably these off-receptors
are ciliary based and constitute a distal retinal layer in scallops, distinct from a proximal microvillar
layer. In the ciliary off-receptors, light inhibition is the result of a hyperpolarizing receptor potential.
A rebound depolarization generates the spike burst in response to dimming. Hartline (1938) initially
attributed light inhibition to inhibitory synapses, as seen in Limulus lateral eyes. The absence of
synapses in the scallop retina concludes that the absorption of light directly inhibits the off receptors,
in contrast to a depolarizing, excitatory light response in all other invertebrate photoreceptors.
Although primary inhibition is apparently universal among bivalves, the scallop eye also features
proximal (microvillar) receptors depolarized by light with bursts of spikes at light onset. Aside from
Pecten and Lima the only other bivalve eye in which receptor potentials have been investigated are
those of the giant clam, Tridacna. Here, two types of hyperpolarizing light-inhibited receptors have
been described, one of which generates spikes at light offset, the other non-spiking. Vertebrate
photoreceptors are the only other receptors where light elicits “inhibitory” hyperpolarizing receptor
potentials. However, membrane conductance and ion specificity is unrelated. In bivalves light
triggers an increase in K + current whereas in vertebrates light triggers a decrease in the dark Na +
current.
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