Linking neuroethology to the chemical biology of natural products: interactions between cone snails and their fish prey, a case study

Abstract

From a biological perspective, a natural product can be defined as a compound evolved by an organism for chemical interactions with another organism including prey, predator, competitor, pathogen, symbiont or host. Natural products hold tremendous potential as drug leads and have been extensively studied by chemists and biochemists in the pharmaceutical industry. However, the biological purpose for which a natural product evolved is rarely addressed. By focusing on a well-studied group of natural products—venom components from predatory marine cone snails—this review provides a rationale for why a better understanding of the evolution, biology and biochemistry of natural products will facilitate both neuroscience and the potential for drug leads. The larger goal is to establish a new sub-discipline in the broader field of neuroethology that we refer to as “Chemical Neuroethology”, linking the substantial work carried out by chemists on natural products with accelerating advances in neuroethology.

Appendix

The model of Conus geographus behavior coordinated with envenomation described above comes from decades of study in multiple labs; a recent publication (Dutertre et al. 2014b) would modify this model. We believe that the interesting hypothesis proposed in this publication requires more experimental data; an explanation that rationalizes all of the present experimental observations is provided below.

The prey capture observations on Conus geographus described in the text above were based on hundreds of specimens, collected from various localities in the Philippines. These observations have been widely corroborated, including in a publicly accessible video (see C. geographus, Silent Assassin from the BBC that shows a freely moving Conus geographus engulfing an enormous fish without extending its proboscis. In this video, after being totally engulfed, the fish struggles and jerks, and then becomes paralyzed, presumably in response to the envenomating sting. See https://www.youtube.com/watch?v=FYh2zeAsRXY). However, a diversity of foraging behavior has been reported for Conus geographus from some Australian localities; as documented by Bingham and co-workers for Australian specimens (Bingham et al. 2012). Even in this survey of Australian specimens, most Conus geographus engulf their prey before envenomation, but a minority (5/27) were observed to extend their proboscis in the presence of prey. Behavioral divergence between different populations of conspecific cone snails is clearly a topic that needs to be systematically explored.

Based on their study of the minor fraction of Australian Conus geographus that extend their proboscis when presented with fish prey, Dutertre, Lewis and coworkers proposed that Conus geographus could secrete two types of venom, one for predation and another exclusively for defense. In essence, the predation venom components were equivalent to the nirvana cabal described in this review, while the defensive venom contained motor cabal components. An alternative explanation for the observations on the behavioral cadre of Conus geographus that inject venom before completely engulfing their prey is that motor cabal components are used both for prey capture, as well as for defense. This is supported by findings of Dutertre et al. (2014a, b) on the presence of motor cabal toxins (μ-GIIIA and ω-GVIA) in the defense-evoked venom and a study by Bingham et al. (2012) that showed that these compounds were also present in the predatory venom of Conus geographus. The ecological conditions of the Australian Conus geographus cadre presumably make direct injection of nirvana cabal components more effective than release into the water (but would therefore result in the snail only being able to capture one fish prey per foraging event). Conus geographus specimens from the Philippines can be fed fish sequentially in aquaria (which is not true for other fish-hunting Conus species), and after being fed one or two fish, if the snail is presented with another fish prey, it opens its rostrum and totally paralyzed fish are visible within the open rostrum. These observations are consistent with the snail being able to inject multiple prey sequentially, and suggest that paralytic components of venom are injected into each engulfed fish. In contrast, when the snail is threatened, it can clearly inject a larger amount of venom (we have observed that if the shell of a Conus geographus specimen is being broken, it will extend its proboscis and spray out several hundred microliters of venom). Thus, several experimental questions should be addressed for the Australian Conus geographus with the unusual behavior. Do these snails sting fish prey a second time after they are engulfed? Does the defense-evoked venom changes composition when the snails encounter a natrual predator? These are experimental issues that need to be resolved to elucidate the role of motor cabal components. A fundamental conceptual issue is that there is no evidence that fish are the main predators—why would venom components purely evolved for defense be so specific for the vertebrate neuromuscular junction; this suggests that selection pressure was based on the prey type.

Professor J.P. Bingham, University of Hawaii, is probably the most knowledgeable authority on the ecology of Conus geographus in Australia, and has kindly provided a potential rationale for the divergence in behavior observed in some Australian specimens. C. geographus that extend their proboscis towards fish prey are enriched in the Boult Reefs at the Great Barrier Reef in which staghorn coral is prevalent (Bingham et al. 2012). This may be an environment in which releasing venom components and engulfing prey is a less effective strategy, selecting for behavior in which the snails directly inject nirvana cabal venom components into potential prey. This possibility is made more likely by our unpublished observations that certain lineages of fish-hunting cone snails that do appear to have nirvana cabal-like components inject their prey twice, the first injection resulting in quiescent (but not paralyzed) fish.

The diversity of prey capture behavior documented above is presumably reflected in variability in venom composition. The striking interspecific divergence in the venom components of different Conus species is well documented, but the extent of intraspecific divergence is less well studied. However, a number of recent studies have been carried out, both on Conus geographus and other fish-hunting cone snails which allow a comparison between venoms of different individuals of the same species; these reports suggest considerable intraspecific variation in venom components (Dutertre et al. 2010, 2014b; Himaya et al. 2015; Rodriguez et al. 2015) when these analyses are carried out on milked venom of the same individual (Biass et al. 2009); the results also suggest that even within a single individual, there can be variation in venom components as a function of time. It is clear from the diversity of ecological responses observed in Conus geographus and some of the new ideas about the roles of its venoms in nature that much remains to be learned about the behavior of these molluscs, even from the best studied species.