MARB 435 1st Edition Lecture 11 Current Lecture Mollusks part 2 Class Gastropoda Contains over of all living mollusc species Inhabit marine freshwater and terrestrial environments Large diversity of lifestyles and habitats Gastropods undergo torsion which is a 180 twist of the visceral mass shell and mantle cavity relative to the foot and head during early development Why does this happen Benefits Head can withdraw into shell first Ventilation of gills Tasting of water with osphradium Better hydrodynamics Drawbacks Fouling waste from anus and excretory pore falls on the head Gastropods can be herbivores carnivores suspension feeders and also parasites Excretion Marine gastropods are usually ammonotelic i e produce ammonia as end products of protein metabolism Terrestrial gastropods are ureotelic or uricotellic i e produce urea or uric acid as end products of protein metabolism Freshwater and amphibious species may be either ammonotelic or ureotelic These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Intertidal species may switch between both modes depending on tides Gastropod larva Trichophore stage is often skipped Veliger larva is more common However many are direct developers Gastropod Systematics Has changed dramatically over the last decades Traditionally three subclasses prosobranchia opisthobranchia pulmonata None of the subclasses is supported by phylogenetic analysis No consensus yet on new classification Use names informally prosobranchs opisthobranchs pulmonates Prosobranchs Mostly marine Gonochoric Mantle cavity anterior Generally solid shell Often operculum Cone Snails More than 500 extant and several hundred extinct species in the hyperdiverse genus Conus Morphologically relatively uniform but very diverse in feeding ecology Prey types Worms mainly annelids Other gastropods Fishes Most species are very specialized in their diet Conotoxins Mix of neurotoxic peptides highly variable even within species Can lead to a variety of neurological symptoms in humans sometimes to death Mainly used for prey capture Injected into the prey through a hollow radula tooth Toxins work by blocking receptors and ion channels e g sodium potassium calcium Some prey become immune by changing the structure of the receptor or channel proteins Evolutionary arms race between predator and prey Euthyneura Opisthobranchs and pulmonates Hermaphrodites Characterized by Detorsion Torsion is partially reversed by about 90 Mantle cavity is on the right side of the body or is absent Visceral nerve cords no longer crossed over Opisthobranchs Marine Shell reduced or lost No operculum Hermaphrodites Detorsion Secondary bilateral symmetry Nudibranchia Shell mantle cavity and sometimes gills absent Body secondarily bilaterally symmetrical Anus posterior Pulmonates Terrestrial or freshwater Loss of ctenidia Tendency for detorsion Mantle cavity on the right Hermaphrodites Loss of shell common