I am generally interested in the chemical ecology of aquatic organisms, including the chemical basis for habitat choice by larvae, sperm navigation, foraging by scavengers, and defense from predation. Currently, I am developing a molecular phylogeny of the genus Elysia to test whether speciation has occurred via host shifts among algae with similar secondary metabolites. Other on-going studies explore the chemical basis for selective larval settlement onto host algae in Alderia and Caribbean sacoglossans.
Most marine invertebrates spend their adult life on the sea floor. Dispersal, which permits gene flow between populations, is accomplished by tiny larvae that spend anywhere from minutes to months in the plankton. Eventually, larvae face the challenge of locating a suitable adult habitat before settling to the bottom. Finding a good area is critical for species that depend on one host or prey species; if the adult host is not found soon after the larva completes its metamorphosis, the juvenile will starve.
We study how chemical cues mediate settlement in laboratory and field experiments. Research focuses on larvae of the sea slug Alderia willowi, which settle in response to carbohydrates produced by its host alga Vaucheria. This work employs chemical techniques such as NMR and GC/MS to characterize bioactive oligosaccharides, and genetic techniques to study how populations along the coast have diverged due to local differences in host chemistry.
Botello, G. and Krug, P.J. 2006. Desperate larvae revisited: Age, energy and experience affect sensitivity to settlement cues in larvae of the gastropod Alderia sp. Mar Ecol Prog Ser 312: 149-159. PDF
Krug, P.J. 2006. “Defense of benthic invertebrates against surface colonization by larvae: A chemical arms race.” Marine Molecular Biotechnology, vol 2. Springer, Berlin. PDF
Krug, P.J. and Zimmer, R.K. 2000. Larval settlement: chemical markers for tracing production, transport, and distribution of a waterborne cue. Mar Ecol Prog Ser 207: 283-296. PDF
Krug, P.J., and A.E. Manzi, 1999. Waterborne and surface-associated carbohydrates as settlement cues for larvae of the specialist marine herbivore Alderia modesta. Biol Bull 197: 94-103. PDF
From coral reefs to human beings, chemical communication between eggs and sperm plays a critical role in reproduction. Sperm cells can orient and accelerate towards an egg by tracking diffusing signal molecules, thereby enhancing fertilization success. This is especially important for broadcast-spawning organisms that shed their gametes into the sea, where they are rapidly diluted in the surrounding seawater. Species-specific sperm attractants may also function as pre-zygotic agents driving reproductive isolation and speciation, by allowing sperm to faithfully distinguish eggs of their own from related species.
We identified the amino acid L-tryptophan as the sperm attractant from eggs of red abalone, a commercially valuable but endangered resource. Although such chemical signals might maintain species boundaries in these free-spawning animals, evidence indicates that soluble signals are less evolutionarily important that are rapidly evolving gamete recognition proteins. Ecologically, this work has implications for conservation, as low population densities of abalone may prevent appreciable fertilization from occurring due to gamete dilution. Improved understanding of chemical and biological mechanisms affecting reproductive success may thus aid management and restoration efforts for threatened species.
Riffell, J.A., Krug, P.J., and Zimmer, R.K. 2004. The ecological and evolutionary consequences of sperm chemoattraction. Proc Nat Acad Sci 101: 4501-4506. PDF
Riffell, J.A., Krug, P.J., and Zimmer, R.K. 2002. Fertilization in the sea: The chemical identity of an abalone sperm attractant. J Exp Biol 205: 1439- 1450. PDF
For predators and their prey, chemistry is a double-edged sword. Chemical plumes allow predators to track their prey, and help scavengers locate fresh foodfalls. On the other hand, vulnerable prey organisms can exploit chemistry to avoid being eaten. Many aquatic animals produce defensive structures upon sensing predators in the area. Seemingly vulnerable taxa like sea slugs and sponges can produce or acquire powerful toxins to deter would-be predators.
One on-going project measures the natural flux of metabolites from live and dead prey organisms, to identify cues that trigger foraging behavior and aid navigation in predators and scavengers. To understand the attraction of isopods, common marine scavengers, we quantified the flux of amino acids and peptides from naturally aged prey flesh. Field studies are currently measuring the attraction of isopods to synthetic odor plumes that mimic the release of foraging cues. Another collaborative project with researchers at UCLA examines how newt larvae avoid being cannibalized by hungry adults, by recognizing adult defensive toxins and seeking refuge. These studies will provide new insights into the evolution of predator-prey interactions, and will further our understanding of sensory systems in important model organisms.
Zimmer, R.K., Schar, D.W., Ferrer, R.P., Krug, P.J., Katz, L.B., and Michel, W.C. 2006. The scent of danger: Tetrodotoxin (TTX) as an olfactory cue of predation risk. Ecol Monogr 76: 585-600. PDF
Krug, P.J., Boyd, K.G., and Faulkner, D.J. 1995. Isolation and synthesis of Tanyolides A and B, metabolites of the nudibranch Sclerodoris tanya. Tetrahedron 51: 11063-11074.