A recent paper in Proceedings of the Royal Society B (O’Dea et al. 2014) documents a decrease in the mean size of maturity of West Indian Fighting Conch off the coast of Panama. The study compared fossilized material from time periods prior to (7000 YBP) and following early human occupation of the area as well as materials from contemporary populations.Conch are and have been an important food throughout the Caribbean. While Queen conch seems to be preferred by contemporary and historical populations, other species such as the fighting conch were and are consumed as well. Because humans preferentially harvest the largest sexually mature conch, selection against large mature individuals is thought to drive the evolution of populations towards smaller size, an effect of size-selective harvest seen in many different species including fishes and mammals.
The purpose of this study was to examine evidence for human mediated evolutionary change in the size of harvested conch populations due to human harvest.
To evaluate size selective harvest on conch the authors collected size measures from three different periods: Pre-human, Prehistoric, and Contemporary. Pre-human shells were collected from lagoon sediments at a site dated to 7187–5711 years before present. Prehistoric shells were collected from middens dated to 690-1410 years before present. And contemporary shells were collected from areas around these sites.
They estimated size at maturity using the same types of indicators used today for queen conch, i.e., shell lip presence and thickness (Figure 1). They used these data along with width and length of shells to examine how the mean size of populations change over time and whether the size at maturity changes as well.
Their results demonstrate that thousands of years of low-intensity subsistence harvesting caused the evolution of reduced size at maturity from about 77mm in Pre-human deposits to 67 mm in the contemporary populations, a 12% decline in size at maturity (Figure 2). Interestingly the data show that most of this shift occurred rather quickly following human habitation in the area. Presumably, this indicates that size selective harvests affect conch populations soon after incurring relatively low harvest pressures associated with the prehistoric peoples.
The evidence provided by O’Dea et al. show that size selective conch harvesting drove the evolution of 1) smaller conch, and 2) smaller size at maturity (Figure 2).
What are the consequences of reduced size? First, reduced size is associated with reduced fecundity, that is to say fewer offspring per individual. That means that such a reduction in size can affect population growth rates by reducing the number of animals recruiting into the population, an important parameter for the maintenance of healthy populations. Second, smaller conch means less meat for consumption. Estimates from this study suggest that the reduction in conch size reduced the amount of consumable meat by 40% since Pre-human times.
The authors offer a bright spot noting that the largest contemporary shells were found in an area subject to low harvest intensity for several years. This observation suggests that reduced harvesting may result in recovery of prehistoric size at maturity due to rapid evolution. Therefore, while persistent selection due to harvest can cause the reduction in size, relaxation of these pressures due to protection through Marine Protected Areas or other management strategies could help mitigate the evolutionary consequences of harvest.
So, do the shifts seen in fighting conch in Panama indicate the fate of queen conch in The Bahamas? Well there are several things to consider. First, the duration of human habitat in Panama was much longer, so perhaps phenotypic shifts have not happened yet in The Bahamas. That said, the intensity of conch harvest in The Bahamas is much higher than the Panamanian fighting conch ever experienced (a purely subsistence scale harvest). Queen conch is a high-value commercially harvested species and increasingly high efforts are expended to harvest them suggesting that while the duration of harvest is not similar between the two scenarios, the harvest intensity is certainly high enough to drive evolutionary shifts in size, and size at maturity in Queen conch. Anecdotally, I have heard of increases in the abundance of the ‘samba conch’, a smaller, thick-shelled form of Queen conch. This is consistent with the evolutionary dynamics described in this paper, i.e. selective harvest of larger conch results in decreased size and decreased size at maturity in harvested populations. Perhaps some readers have more to add about the changing sizes of conch in The Bahamas?
O’Dea, A, M. L. Shaffer, D. R. Doughty, T. A. Wake, F. A. Rodriguez. 2014. Evidence of size-selective evolution in the fighting conch from prehistoric subsistence harvesting. Proceedings of the Royal Society B: Biological Sciences, 281. (Link)