Life history and distribution of mysids (Tenagomysis spp.) in estuaries of southern New Zealand: an eco-physiological approach

Physiological flexibility influences the life-history strategies and spatial-temporal distribution of species. Spatial and temporal variation in salinity, temperature, nutrients, and the opening and closing regime of estuaries make estuarine systems physiologically challenging environments to inhabit. Estuarine species may adapt their body size to buffer physiological challenges associated with osmoregulation. However, adjusting body size may have consequences for life-history parameters, including growth, fecundity, timing of maturity, and eventually influence distribution. I investigated the interplay between these life-history conflicts using the dominant estuarine mysids in New Zealand, i.e. Tenagomysis chiltoni and T. novaezealandiae. Southern New Zealand estuaries are cold in winter and comprise a mix of intermittently and permanently open systems. T. chiltoni is more dominant in tidal lakes and upper reaches of estuaries, whereas T. novaezealandiae occurs more in lower reaches and in intermittently open estuaries. The adults of these species differ significantly in body size, with T. chiltoni growing larger than T. novaezealandiae. This fueled my interest to find multiple working hypotheses for my thesis, in which I explored the tolerance, osmoregulation capacity, respiration, growth and fecundity of these two species. My aim was to clarify the energetic, growth and fecundity trade-offs, and the mechanisms driving key life history features that determine where these species live. My results showed that the salinity tolerance and osmoregulation of Tenagomysis chiltoni and T. novaezealandiae were poor in a cold, low salinity environment, especially for juveniles. This might have different implications for the two Tenagomysis species, depending on their habitat. In cold hypo-saline habitats, T. chiltoni is possibly more restricted by osmoregulation than by food, and forced to prolong growth and delay maturity to achieve a larger body size to survive winter. Hence its breeding period is limited, and juveniles can only be released in warmer seasons to reduce mortality from cold. However, its larger body may also have advantages over T. novaezealandiae. First, it keeps the per unit mass metabolic expenditure lower, which is crucial to meet the higher osmotic cost in upper reaches of estuaries, which is possibly difficult for T. novaezealandiae. Further, T. chiltoni females carry larger broods than T. novaezealandiae. However, T. novaezealandiae grows better in brackish (i.e. salinities 10 to 20), and nutrient-rich intermittently open estuaries (nutrient rich due to eutrophication associated with limited marine exchange) in southern New Zealand. By taking advantage of such productive, often euryhaline environments, T. novaezealandiae can reach maturity at smaller body size and produce multiple cohorts, compensating for lower individual fecundity than T. chiltoni. This strategy may enable T. novaezealandiae to attain very high densities, and allow them to out-compete and exclude T. chiltoni from the lower reaches of estuaries. Results suggest mysids and similar crustaceans in temperate estuaries may be constrained by osmoregulation, especially in winter months. Such physiological constraints could explain some aspects of habitat-specific life-history specialization, distribution and seasonal changes in many estuarine communities. However, in future, synergistic changes in the estuarine environment (natural and anthropogenic) may trigger responses that are non-linear, and less predictable