Effects of reduced salinities on metamorphosis of a freshwater-tolerant sesarmid crab, Armases roberti: Is upstream migration in the megalopa stage constrained by increasing osmotic stress?

Abstract

Numerous species of estuarine and freshwater-tolerant crabs show an "export strategy", i.e. an early larval downstream transport towards coastal marine waters, later zoeal development at higher salinities, and a return of the last larval stage, the megalopa, into estuaries or rivers. The speed and extent of the upstream migration of the megalopa through strong salinity gradients may be constrained by increasing hypo-osmotic stress. In an experimental laboratory study with Armases roberti, a freshwater-inhabiting sesarmid crab from the Caribbean region, we studied in the megalopa stage (after zoeal rearing at 25‰) the tolerance of reduced salinities.In the first experiment, the larvae were exposed directly to various constant salinities (1-25‰). For the second experiment, they were transferred stepwise to strongly diluted media (within 6 days from 25‰ to ≤ 3‰), simulating differential scenarios of upstream migration into brackish or freshwater habitats.When postmoult megalopae were exposed directly to salinities ≤3‰, they all died within 24 h. A slightly higher salt concentration (5‰), however, allowed for considerable survival (46%) through metamorphosis to the first juvenile crab stage. In treatments with continuous exposure to 10-15‰, as well as in a control group (25‰), survival to metamorphosis was significantly higher (83-96%), and the average duration of development was shorter compared to 5‰ (12-13 vs. 16 days). In the second experiment, with stepwise salinity reductions, gradual acclimation to decreasing osmotic pressures permitted a successful development to metamorphosis at ≤3‰ and even in freshwater (b 0.2‰).This strong physiological adaptability enables the megalopa of A. roberti to cross during its upstream migration, within a short time (6 days), strong osmotic gradients, so that metamorphosis is possible also in freshwater habitats where the conspecific adult crabs live. The speed of migration appears to be limited by physiological constraints related to changes in the capability for osmoregulation occurring during the course of the moulting cycle.

Effects of osmotic stress on crustacean larval growth and protein and lipid levels are related to life-histories: The genus Armases as a model

“…Protein Lipids term cultivation, see Anger et al, 2000;Torres et al, 2006). Ovigerous females were isolated in individual aquaria and kept under otherwise identical conditions until larvae hatched.…”

“…their physiologically sensitive larvae are transported to waters with more stable conditions (e.g. Armases roberti: Diesel and Schuh, 1998;Torres et al, 2006;Chasmagnathus granulata: Anger et al, Comparative Biochemistry and Physiology, Part B 148 (2007) 209 -224 www.elsevier.com/locate/cbpb 1994; Charmantier et al, 2002;Eriocheir sinensis: Herborg et al, 2003;Cieluch et al, 2007). A lesser number of species develops through euryhaline, osmoregulating larval instars, which can successfully cope with strong salinity variations (e.g.…”

“…to coastal waters and a later re-immigration of the Megalopa to freshwater habitats (Diesel and Schuh, 1998;Torres et al, 2006;Anger et al, 2006).…”

High gene flow due to pelagic larval dispersal among South Pacific archipelagos in two amphidromous gastropods (Neritomorpha: Neritidae)

“…Although the adults of amphidromous species live and reproduce in streams, rivers or estuaries, their planktotrophic larvae are released downstream to the ocean, where marine salinities are required for their successful development (Anger et al, 1990;Diesel and Schuh, 1998;Crandall, 1999;Diele and Simith, 2006). After metamorphosis and recruitment to river mouths, juveniles migrate upstream to freshwater habitats (Schneider and Frost, 1986;Blanco and Scatena, 2005;Torres et al, 2006).…”

Influence of salinity on the larval development of the fiddler crab Uca vocator (Ocypodidae) as an indicator of ontogenetic migration towards offshore waters

“…When migrating upstream, they encounter low salinities, which they now are able to tolerate Torres et al 2006). Reduced salinity can in fact be important for guiding megalopae back to estuaries and/ or stimulating settlement and metamorphosis.…”