Population Variation in the Life History of a Land Fish, Alticus arnoldorum, and the Effects of Predation and Density

Abstract

Life history variation can often reflect differences in age-specific mortality within populations, with the general expectation that reproduction should be shifted away from ages experiencing increased mortality. Investigators of life history in vertebrates frequently focus on the impact of predation, but there is increasing evidence that predation may have unexpected impacts on population density that in turn prompt unexpected changes in life history. There are also other reasons why density might impact life history independently of predation or mortality more generally. We investigated the consequences of predation and density on life history variation among populations of the Pacific leaping blenny, Alticus arnoldorum. This fish from the island of Guam spends its adult life out of the water on rocks in the splash zone, where it is vulnerable to predation and can be expected to be sensitive to changes in population density that impact resource availability. We found populations invested more in reproduction as predation decreased, while growth rate varied primarily in response to population density. These differences in life history among populations are likely plastic given the extensive gene flow among populations revealed by a previous study. The influence of predation and density on life history was unlikely to have operated independently of each other, with predation rate tending to be associated with reduced population densities. Taken together, our results suggest predation and density can have complex influences on life history, and that plastic life history traits could allow populations to persist in new or rapidly changing environments.

Land colonisation by fish is associated with predictable changes in life history

“…Conducting similar experimental manipulations is likely to be impractical for blennies. However, we do know that populations of the land species A. arnoldorum on Guam vary in predation pressure and density (Morgans et al 2014), and that these populations exhibit differences in reproductive effort and growth rate in a way consistent with the notion that increases in reproductive effort (at the expense of grow rate) reflect a reduction in predation [and to some extent competition (Platt and Ord 2015)]. These data provide support for the general hypothesis in the current study that improvements in female survival may have prompted the observed changes in life history following the transition to land, with this probable change in survival potentially reflecting some form of ecological release such as a reduction in predation.…”

“…This might occur, for example, if the transition onto land released fish from resource competition or predation that tended to reduce the survival of younger individuals in the aquatic environment (e.g. Osenberg et al 1992;Belk 1995;Platt and Ord 2015). Marine fishes are subject to extremely high mortality during the juvenile stage (Victor 1986;Myers and Cadigan 1993), usually due to size-selective predation (Almany and Webster 2006), but also because of competition for resources (Forrester 1990), such as habitat (Shulman 1985).…”

“…Future study would then be justified in attempting to document the specific changes in age-specific survival, and whether the colonisation of land has been associated with factors related to an ecological opportunity or release [e.g. a change in the availability of key resources or likelihood of predation (see Platt and Ord 2015)]. However, if life history characteristics were broadly consistent among blenny species, or differed among species in a pattern unrelated to their degree of terrestriality, then this would suggest that fish made the transition onto land in response to factors other than an ecological release or ecological opportunity [e.g.…”

Repeated evolution of amphibious behavior in fish and its implications for the colonization of novel environments

“…In the case of the amphibious fishes of Blenniidae, the likelihood of encountering novel predators in a terrestrial environment appears to have been circumvented through the evolution of cryptic body coloration in aquatic ancestors (i.e., blennies were already well camouflaged in their aquatic environment such that they should still be cryptic to most visual‐oriented predators on land as well; Morgans and Ord ). There is also some evidence of changes in life history that might be consistent with shifts in competition and predation, but these would reflect an ecological release from these factors following the transition to land, not vice versa (Platt and Ord ; Platt et al. ).…”

Reproductive life-history strategies in a species-rich assemblage of Amazonian electric fishes

“…We removed the right sagittal otolith via dissection of the otic capsule, following removal of the operculum. Each otolith was then washed in distilled water, air-dried, and glass-slide mounted in a thermoplastic mounting adhesive (Crystalbond 509, Aremco Inc., Valley Cottage, NY), with the transverse plane oriented parallel to the slide [60, 64]. The otolith was then filed down on one side with increasingly fine grades of wet sandpaper (from 25 to 8 μm), the plastic melted to allow the entire otolith to be inverted, and the filing processing repeated on the other side until a thin and translucent transverse section remained.…”