Active Parental Care in a Freshwater Amphipod (Crustacea: Gammarus Pseudolimnaeus): Effects of Environmental Factors

Article excerpt

ABSTRACT.-

Female amphipods carry their eggs and offspring in a ventral brood pouch. Previous research has demonstrated that females of species inhabiting harsh environmental conditions expend additional energy in active parental care such as ventilating eggs by beating their pleopods. This study investigated whether similar forms of care could be detected in amphipods collected from spring fed ponds and streams. Two behaviors reported to function in parental care in other species were observed in ovigerous females, but were absent in nonovigerous females and males. Ovigerous females were also shown to beat their pleopods significantly longer than males or nonovigerous females, suggesting that this is another form of active parental care. The duration of pleopod beating varied considerably depending on both the season and the habitat from which the amphipods were collected. Differences were most pronounced for ovigerous females. Similarly, the duration of pleopod beating was significantly longer, especially for ovigerous females, when amphipods were placed in water with experimentally reduced levels of oxygen. These results suggest that even in habitats with high water quality, amphipods can show active parental care and that females have a phenotypically plastic response to environmental conditions in determining the amount of care provided. The potential fitness effect of such response may have important implications for water quality management.

INTRODUCTION

Parental care involves trade-offs between allocating time and energy toward current offspring vs. future offspring (Trivers, 1974; Trumbo, 1996; Webb et al., 1999). As such, parents are expected to act in ways that optimize their investment patterns. Wilson (1975) identified four primary factors selecting for high levels of parental care: unusually stressful physical environments (e.g., extreme temperature, pollution, salinity or oxygen levels); high predation levels; scarce specialized food resources; or stable structured habitats. Under these conditions, parents can optimize energetic investment in their offspring either through specific behavioral adaptations promoting care in a particular habitat or through behavioral plasticity where the amount of parental care varies depending on environmental conditions (Carlisle, 1982; Clutton-Brock, 1991; Glazier, 1999).

Freshwater amphipods (Crustacea: Amphipoda) are an important component of aquatic food webs and often comprise the highest percentage of invertebrate biomass in stream systems (Waters, 1984). Female amphipods provide parental care by carrying their eggs and young in a specialized brood pouch composed of interlocking plates near the gills on the ventral side of the body (pre-emergent care: Borowsky, 1980; Thiel, 2000). More rarely, some species also display post-emergent care associated with cohabitation of mothers and young (e.g., Aoki, 1997; Thiel, 1997, 1998, 1999). In addition to carrying eggs/young as a form of passive pre-emergent parental care, some amphipod species exhibit active brood care (Dick et al., 1998). This active care includes females beating their posterior appendages (pleopods), which normally function to draw water toward the gills for self ventilation, at an increased rate to ventilate developing eggs and juveniles (Borowsky, 1980). Additional active care behaviors include "elbowing" eggs (using appendages to manipulate the position of eggs within the brood pouch) and thorasic flexion to increase the volume of the brood pouch and provide an influx of water (Borowsky, 1980; Shillaker and Moore, 1987; Dick et al., 1998). Active parental care requires more energy (Dick et al., 1998) and potentially exposes the female and her brood to higher levels of predation (Thiel, 1998). Given these costs, it is not surprising that active care has only been documented in amphipod species that inhabit harsh environments (e.g., those with low oxygen levels or high pollution; Dick et al. …