Ch12 Molluscs

• Molluscs are an important animal phylum, one of the major animal phyla, Bilaterian, Protostomes, Deuterostoma, Ecdysozoa, Lophotrochozoa.
• Approximately 100,000 species of molluscs, the largest marine phylum, also land animals such as snails.
• Gastropoda is the largest class of molluscs with 70,000 species.
• Bivalves have 20,000 species of molluscs.
• Polyplacophora has 1000 species of molluscs.
• Cephalopoda has 900 species of molluscs.
• Molluscs have a closed circulatory system, soft body, usually protected by a shell, sense organs such as tentacles, and can have male/female or hermaphroditic reproduction.
• Nautilus has a peripheral sinus and lacunae where gas exchange takes place, notably in the gills.
• Blood leaves the heart through three or four vessels, the two main ones being a cephalic artery (anterior aorta) and a posterior aorta, in addition to smaller genital and renal arteries.
• The coleoid systemic heart consists of a single powerful, but rather compact, ventricle fed by the two ‘auricles’ with muscular valves that receive blood from a gill.
• The pharmacological characterization of Conus venoms has revealed a strong selection pressure for very rapid prey immobilization and a biological system able to mount a sophisticated response in evolutionary time to address this pressure.
• Early work by Endean and Annu Rev Ecol Syst 2002.33:25-47 demonstrated that there are striking differences in potency; in particular, venoms of fish-hunting Conus are much more lethal than those of the other groups when tested in vertebrates.
• Conus venom components are generated through a combinatorial library search strategy.
• Conus peptides are initially translated as larger prepropeptide precursors; a mature Conus peptide of 20 amino acids is generally processed from a 70 to 80 amino acid precursor, with a single nonrepeated copy of the toxin encoded at the C-terminal end.
• Conus venoms have a significant amount of information collected for several gene families expressed in the venom ducts, but most of the data are for a small subset of Conus species and were originally acquired for reasons unrelated to either the taxonomy or ecology of these snails.
• A first comprehensive study of the effects of different Conus venoms demonstrated that there are striking differences in potency; in particular, venoms of fish-hunting Conus are much more lethal than those of the other groups when tested in vertebrates.
• The lethality of Conus geographus envenomation to humans attracted the attention of the toxinology community, and a physiological and pharmacological characterization of the venoms of a few cone snails was carried out.
• Three aortae, with valves, leave the ventricle, the largest (dorsal or cephalic) running anteriorly to the head region mainly to supply the brain and the majority of the body.
• The pericardium is much reduced in octobrachians, represented only by a pair of lateral, thin-walled, semi-transparent, flat-shaped pouches in octopods.
• The morphological and gonadal transition from male to female is under hormonal control.
• Tissue/tissue contact can be maintained by both the cephalic tentacles and the mantle margins.
• Males release young male older male transitional young female female developing penis fully developed penis rudiment of penis penis nearly absorbed immature male mature male transitional stage mature female mout h testis gonad transf orming to ovar y seminal receptacle ovar y euspermatozo a mature & developing paraspermatozo a oocyte oocyte germinal epithelium spermatogonia spermato cyte s euspermatids disintegrating debris mature ovum follicle cell pallial genital duct Crepidula fornicata - stack of individuals showing sex change from young males to older females.
• Early euspermatids mature female mature male transitional stages Crepidula fornicata - sections of gonad at each stage.
• Crepidula onyx animals removed from shell, showing changes in reproductive system with increasing size.
• Crepidula stack and live in close association with one another due to factors that are not water soluble and require the direct contact of individuals.
• In contrast, the degeneration of the penis during the onset of the female phase is controlled by a second hormone produced by neurosecretory cells in the pleural ganglia (Moffett 1991).
• Le Gall and colleagues documented a masculinising factor in female mantle tissue sensed by males through tentacular contact.
• The pedal factor appears to be activated by a factor produced by neurosecretory cells associated with the cerebropleural ganglion in Crepidula.
• The presence of this factor produces neural and hormonal changes in the cerebral ganglion which then initiates morphogenetic activity in male pedal ganglia and the release of neurohormone that accumulates near the right tentacle (and penis) and prevents sex change; a second neurohormone inactivates somatic growth of male individuals.
• Animals fed limited rations lived much longer and showed a lower initial mortality rate than those with unlimited food, suggesting that ‘caloric restriction’ prolongs lifespan, as shown in other species such as the worm C. elegans.
• The relative numerical simplicity of the Aplysia nervous system provides, potentially, an opportunity to understand the animal’s entire behavior in terms of the operation of distinct neural circuits.
• Aplysia, as a model organism, fills an important niche and together with other established experimental preparations undoubtedly will tell us more about the principles of the organization of the nervous system, animal development and evolution.
• Veligers undergo a rapid metamorphosis, dramatically changing both their physical organization and behavior, with loss of larval characteristics and formation of juveniles that resemble mature animals in all major features except reproductive organs.
• Single-cell transcriptome and epigenomic tests can be done from individually identified Aplysia neurons, and less than 0.1% of such a cell is sufficient for direct microchemical analysis of dozens and even hundreds of metabolites and peptides using the tools of modern analytical chemistry.
• Aplysia research can be very informative for a broad spectrum of questions in evolution and development, particularly for understanding the organization of animal body plans.
• Many genes from a common bilaterian ancestor (urbilateria) seem to be lost in representatives of the Ecdysozoa lineages, for example in nematodes and flies, which have extremely short lifecycles and particularly derived genomes.
• Sexual maturity in Aplysia is generally completed within three to four months; life expectancy is about 150–380 days under various conditions.
• The most obvious advantage of Aplysia is the large neurons that facilitate physiological, biochemical and genomic studies at the level of single cells and their compartments.
• The coordinated activity of an entire genome within a single neuron can be linked to the operation of a given neural circuit and a learned behavior.
• These pouches lie behind the bases of the ureters, and each opens anteriorly into the corresponding kidney by way of a long, narrow renopericardial duct.