Ontogeny of osmoregulation of the embryos of two intertidal crabs Hemigrapsus edwardsii and Hemigrapsus crenulatus.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The effects of variations in salinity on the developing embryos of two intertidal crabs, Hemigrapsus edwardsii and Hemigrapsus crenulatus were studied in relation to short term (6 - 96 h) and long term (continuous) exposure to low salinities from spawning to hatching. For the purpose of describing physiological and morphological changes during development, developing embryos of both species were grouped into 5 stages that were timed under standard conditions in the laboratory; Stage (1) Cleavage to Blastula, (2) Gastrula, (3) Eyespot & chromatophores, (4) Yolk in 4 lobes and (5) Yolk in 2 lobes. The total incubation periods of embryos from spawning to hatching of H. edwardsii and H. crenulatus were 62.0 ± 3.1 and 43.0 ± 1.8 days respectively at a constant temperature of 15°C. Embryos of H. edwardsii (20 nL stage 1 to 35 nL stage 5) are larger than the embryos of H. crenulatus (7 nL to 18 nL). Ultrastructural studies revealed the presence of two distinct egg membranes in these embryos. Silver staining demonstrated a distinct patch on the surface of the embryos of both species, presumably corresponding to an area of high chloride permeability. This structure appeared at the gastrula stage and was present throughout the embryonic development until hatching when it disappeared. It is suggested that this structure corresponds to the "embryonic dorsal organ" and plays a role in either the uptake or excretion of salts. Salinity tolerance experiments revealed that the postgastrula stage embryos of both species are remarkably tolerant of periods of dilution, even down to 1% seawater, for many hours and that they hyperosmoregulate throughout development. Pre gastrula embryos (stage 1) were nearly isosmotic in all salinities and were less tolerant of hyposaline exposure. Normal development and successful hatching occurred in 50% seawater for both species provided the exposure commenced after gastrulation. Thus, the gastrula stage appears to be the critical stage when the capacity to osmoregulate commences. The four cations Na+, K+, Ca2+ and Mg2+ (and associated anions) were major osmotic effectors in the embryos of H. crenulatus but contributed only about half of the total osmolality. Na+ and K+ were the main cations in these embryos. The average concentrations of all four cations were better regulated than was total osmolality indicating the involvement of other osmolytes in osmoregulation. Na+/K+ ATPase activity was undetectable in stage 1 embryos but rose steadily between gastrulation and hatching. A relatively larger increase in the activity of this ion transport enzyme at the Yolk in 4 lobe stage in embryos incubated in 50% seawater supports its involvement in osmoregulation. Tritiated water and 22Na efflux data indicated that relative to the size of the embryos, the egg membranes of H. crenulatus are very permeable to water and ions throughout development. There were two components of water and sodium exchange with the seawater in all embryos; a rapidly exchanging pool and a slowly exchanging pool. This rapidly exchanging pool may represent the external embryonic membranes and compartments while the slowly exchanging pool is thought to represent the embryo/yolk. Half times for turnover of embryonic (slow pool) water and sodium ranged from 0.25 to 1.25 h and 1 to 6 h respectively, throughout development. As the embryos maintain a hyperosmotic state for much longer times than these they must be actively hyperosmoregulating. That is they must be in a dynamic steady state involving the active uptake of salts and excretion of osmotically entering water. Oxygen consumption rates of both H. edwardsii and H. crenulatus increased during development and the rate increase was greater at the end of the incubation period. Rates of oxygen consumption of embryos in acute exposure to dilute seawater varied with salinity and exposure time for all developmental stages. There was a significant increase in the total metabolic cost of development, in terms of oxygen consumed, for embryos developing in 50% seawater compared with development in normal seawater. This could be attributed primarily to an extended development time rather than an increased metabolic rate at any stage.