Heidi E. Block, M.S. 2011

Dr. Mark Steele's Fish Ecology Lab

Department of Biology
18111 Nordhoff Street
Northridge, CA 91330-8303

M.S. Thesis

Selection on larval traits in early post-settlement temperate and tropical reef fishes

In marine organisms with complex life histories it is possible for events occurring during earlier life stages to affect an individual’s subsequent survival.  In fishes, it has been proposed that settlement, the transition from a pelagic habitat to a benthic one, is a critical period in determining survivorship.  It is thought that there may be selection (i.e., non-random mortality) on specific larval traits, which could influence an individual’s probability of survival after settlement.  In my thesis I examine the effects of larval traits on the survival of juveniles shortly after settlement in two temperate and two tropical reef fishes.  This was achieved by comparing traits of individuals sampled just after settlement with those of individuals from the same cohort sampled one month later, which evaluated selective mortality occurring over a cohort’s first month of life on the reef.

 The two temperate species studied were señorita, Oxyjulis californica, and blackeye goby, Rhinogobiops nicholsii.  Collections were completed at four sites at Santa Catalina Island, California, and two times between June and August of 2009.  For señorita, I examined four larval traits: planktonic larval duration (PLD), size at settlement, metamorphosis band width, and pre-settlement growth rates.  For the blackeye goby I examined two traits: size at settlement and pre-settlement growth rates.  This study revealed that selection was inconsistent in both species.  There were differences in both the direction and intensity of selection between the different sites as well as times. Differences in selection intensity were not related to variation in habitat characteristics or conspecific densities.  Unlike selection, I found some consistent differences in larval traits between cohorts in señorita.  For the señorita I found that individuals settling earlier in the summer had longer PLDs and slower growth rates than those settling later in the summer.    

In Moorea, French Polynesia I studied the blue green chromis, Chromis viridis, and the Gnatholepis species complex, Gnatholepis scapulostigma/anjerensis.  This study was conducted at two sites on the North side of the island.  Unlike the temperate study, only one cohort was examined due to logistical constraints.  I examined three larval traits for both “species”: PLD, size at settlement, and pre-settlement growth rates.  For the Gnatholepis species complex, I did not find any evidence of selection or any differences between the two study sites for any of the larval traits measured.  For Chromis viridis,I found that there was selection for larger sizes at settlement at both sites.  However, selection on the other two larval traits was inconsistent between the two sites, with one site showing selection for faster pre-settlement growth rates and the other showing selection for longer PLDs.  For this study I also explored whether differences in habitat characteristics or predator densities between the two sites might be influencing these patterns.  I found that these two sites were not significantly different in terms of habitat or predator densities and therefore these factors could not explain differences in selection between sites.

These two studies reveal that selection on larval traits can vary greatly on relatively small scales: with sites less than 3 km apart exhibiting differences in the direction or strength of selection on a larval trait.  This small-scale variation in selection may be one reason why variability in larval traits is maintained in these species.