Fish-hunting species

In the venom of C. geographus and other fish-hunting species, the conotoxins isolated so far can be divided into three major classes (1-4) o -conotoxms which block neuronal calcium channels at the presynaptic terminus of the neuromuscular junction, a-conotoxins which inhibit the acetylcholine receptor at the postsynaptic terminus, and x-conotoxins which block Na channels on the muscle membrane. 

Figure 1 Conus purpurascens, the only fish-hunting species of the Eastern Pacific region, (a). The animal showing the siphon, eyes, and mouth, (b) The animal swallowing a fish (photograph from the bottom of the animal displaying its muscular foot). Photos courtesy of Nicole Vanderweit and Storm Stilmann, Boca Raton, FL.

With the exception of a-conotoxin SII from Conus striatus, all a-conotoxins have the cysteine pattern, CC—C—C (Myers et al., 1993) (see Table 4). Peptides from the fish-hunting species, C geographus, C. striatus and C. magus, have the consensus core sequence CC(N/H)PACGXX(Y/F)XC and two disulfide bonds that connect Cys to Cys and Cys to Cys. Comparison with the a-conotoxins recently isolated from C. pennaceus, a mollusc-hunter, and C. imperialism a worm-hunter, indicates variations in the size of the intercysteine loops. The second loop has seven amino acid residues in a-conotoxins PnIA and PnIB from C pennaceus (Fainzilber et al., 1994), and only three residues in a-conotoxin Iml from C. imperialis (McIntosh et al., 1994), compared to five residues in the other a-conotoxins. 

Among a-conotoxins, SII from C. striatus venom (Ramilo et al., 1992) is unique in having the unusual 6-Cys/4-loop framework (CCC—C—C—C) in place of the regular a-type pattern (CC—C—-C). However, its main structure clearly fits the core consensus sequence (CC(N/H)PACGXX(Y/F)XC) of the a-conotoxins from fish-hunting species discussed above. 

Conus. Thus, Figure 6c shows a group of fish-hunting cones, the Conus magus-striatus group. Other piscivorous species such as Conus geographus are less closely related to the species shown in Figure 6c. 

Between 6 and 10 homologous peptides have been extensively characterized for each toxin class. Although uj- and a-conotoxins have been isolated from several fish-hunting Conus species, x-conotoxins have so far been isolated only from C. geographus venom. 

In the fish-hunting cone snail venoms, a- and w-conotoxins are ubiquitously distributed. As noted above, z-conotoxins have only been found in one species. Conus geographus. In addition to these three well-characterized classes, however, a fourth class of paralytic conotoxins has been found. In contrast to the a-, z-, and... 

Predatory fish locate their prey primarily by scent or vision. Here, we are concerned with the chemical sense. Chemical hunting is particularly adaptive for nocturnal species or those living in turbid waters. Many marine and freshwater fishes hunt by smell. The chemical compounds responsible for this attraction have been identified. Most of them are amino acids, and particularly active as mixtures of several amino acids. 

As indicated, co-conotoxins are found in almost aU fish-hunting Conus species examined up to now. The most thoroughly studied are the venoms from C. geographus, C. magus, and C. striatus. Several of these peptides have been purified, sequenced, and synthesized (with similar potency as the natural toxins) and they have become important tools for the identification and characterization of the different subtypes of voltage-dependent Ca channels found in neuronal tissues. 

A hunting cone snail (Conus textile). Currently, more than 2,000 species are known. They occur in every ocean in the World. From the phylogenetic viewpoint, cone snails have a strikingly broad spectrum of prey. This stretches from other snails via worms to fish. Each species has its own toxins, consisting of diverse cocktails of up to 200 (from a pool of ca. 50,000) different neuroactive peptides, some of which can also be dangerous to humans. 

The common structural feature of the first intercysteine loop of a-conotoxins from different species is the proline residue, which is involved in a P-turn. For this loop, the consensus sequence of the a-conotoxins from the fish-hunters is —(N/H)PA-. The corresponding sequence in the peptide of the worm eater C. imperialis (Iml) is —SDPR— and in the mollusc-hunting species, C pennaceus (PnIA and PnIB) the sequence is —SLPP— (Fainzilber et al., 1994 McIntosh et al., 1994). Replacement of this proline residue by Gly was found by Nishiuchi and Sakakibara (Hashimoto et al., 1989 Nishiuchi and Sakakibara, 1984) to completely abolish the activity of a-conotoxin MI on i.p. injection in mice. 

The impact of chemical pollution on the reproductive success and population sizes of wildlife species is often difficult to assess. In many cases, environmental factors such as habitat restriction, stress due to human intrusion and changes in natural food supplies owing to hunting, fishing and restocking policies may have a significant, even predominant, effect on population size. This makes it difficult to determine to what extent, if any, environmental endocrine disrupters may be contributing to observed effects on reproduction or population size in wildlife species. 

U.S. Fish and Wildlife Service (USFWS). 1987. Migratory bird hunting zones in which lead shot will be prohibited for the taking of waterfowl, coots and certain other species in the 1987-88 hunting season. Federal Register 52(139) 27352-27368. 

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