Staff Scientist Emeritus
Address: Marine Biology Research Institute 0202 Scripps Institution of Oceanography
University of California at San Diego
La Jolla, CA 92093 0202
Telephone: (858) 822 2486
Many marine "species" (traditionally identified using morphological characters) are really complexes of genetically and ecologically distinct forms. Research in my laboratory revolves around recognizing these cryptic taxa and using them to better understand the biogeography, history and ecology of marine species.
Our studies have focused on species from coral reef environments, which are the most diverse habitats in the sea.
A) The species problem in seleractinian corals The nature of species in reef building corals is a subject of continuing debate. Some believe that corals are highly variable, widely distributed, and prone to hybridizing, but data to support this view are limited. Research in my laboratory has focused on the Montastraea annularis complex, the dominant reef builders of the Caribbean. For many years M. annularis was considered a single albeit highly variable species, but morphological, reproductive, molecular and ecological studies all point to the existence of at least three species.
One major focus of this research concerns the nature of reproductive barriers (a collaborative project with Don Levitan of Florida State University). Reproduction in many reef building corals, including M. annularis sensu late, occurs on a few nights a year, a phenomenon known as mass spawning. Our observations have shown that one of the three species spawns 12 hours before the other two, and the two that spawn simultaneously do not readily hybridize because of gamete incompatibilities. The longevity of sperm in artificial crosses, and studies of fertilization rates in the field suggest that a two hour difference in spawning times will be a potent barrier to hybridization. These studies of natural fertilization rates also indicate that successful fertilization depends on both high densities and high synchrony, suggesting that reef degradation may seriously compromise the future reproductive success of these and other mass spawning species. Other work on these corals includes an analysis of amplified fragment length polymorphisms; genetic markers revealed by these molecular analyses will be used to quantify the extent of natural hybridization in the field. I also collaborate with Nancy Budd of the University of Iowa on morphological and paleontological studies of these corals, the goal of which is to trace the origins of these corals in the fossil record as well as to estimate rates of molecular evolution.
B) Snapping shrimp biediversity and biogeography Snapping shrimp in the genus Alpheus are highly diverse and abundant throughout the world's tropical and subtropical oceans. They therefore provide an ideal system for assessing whether particular regions have many endemic species, and for reconstructing the history of biogeographic connections in the sea. Our work began with an examination of mtDNA, allozyme and reproductive divergence across the Isthmus of Panama. Those
studies revealed the complex and protracted nature of biogeographic isolation between 3 and 10 million years ago associated with the gradual rise of the Isthmus. Prior to these studies, it was widely assumed that isolation occurred only around the time of final closure of the Isthmus. We have also shown that the forms we have studied that are separated by the Isthmus are all reproductively isolated and thus constitute distinct species.
We have since expanded these studies to include nuclear genes and to use rates of molecular evolution estimated from comparisons across the Isthmus to calculate the nature of the connections between the Caribbean and the tropical eastern Atlantic, between the tropical eastern Pacific and the Indo West Pacific, and between the Caribbean and tropical Brazilian waters. In broad outline, these studies have shown that many so called species with wide biogeographic ranges are really complexes of much more narrowly distributed species, and that endemicity in the sea is much higher that previously assumed.
Education and Degrees
B.A. Harvard University, 1971
Ph. D., University of California, Berkeley, 1978
Knowlton, N., E. Well, L. A. Weigt, and H. M. Guzman. 1992. Sibling species in Montastraea annularis, coral bleaching, and the coral climate record. Science 255: 330 333.
Knowlton, N. 1993. Sibling species in the sea. Annual Review of Ecology and SYstematics 24: 189-216.
Knowlton, N., L. A. Weigt, L. A. Solorzano, D. E. K. Mills and E. Bermingham. 1993. Divergence in proteins, mitochondrial DNA, and reproductive compatibility across the Isthmus of Panama. Science 260: 1629-1632.
Knowlton, N. and J. B. C. Jackson. 1994. New taxonomy and niche partitioning on coral reels: jack of all trades or master of some? Trends in Ecology and Evolution 9: 79.
Knowlton, N., J. L. Mate, H. M. Guzman, R. Rowan,and J. Jara. 1997. Direct evidence for reproductive isolation among the three species of the Montastraea annularis complex in Central America (Panama, Honduras). Marine Biology
Rowan, R., N. Knowlton, A. Baker, and J. Jara. 1997. Landscape ecology of algal symbionts creates variation in episodes of coral bleaching. Nature 388: 265-269.
Knowlton, N. and L. A.Weigt. 1998. New dates and new rates for divergence across the Isthmus of Panama. Proceedinos of the Royal Society: Biological Sciences 265:2257-2263.
Herre, A., N. Knowlton, U. Mueller and S. Rehner. 1999. The evolution of mutualisms: exploring the paths between conflict and cooperation. Trends in Ecology and Evolution 14: 49-53.
Lopez, J.V., R. Kersanach, S.A. Rehner, and N. Knowlton. 1999. Molecular determination of species boundaries in corals: genetic analysis of the Montastraea annalaris complex using amplified fragment length polymorphisms and a microsatellite marker. Biological Bulletin 196: 80