Osmotic and body density response in the harpacticoid copepod Tigriopus brevicornis in supralittoral rock pools

Studies of the osmotic and ionic responses of the supralittoral rock pool inhabitant Tigriopus brevicornis (Crustacea: Copepoda) showed that this species of orange harpacticoid copepod is an unusually euryhaline osmoconformer. Body fluid osmolalities were 297-3691 mOsm kg^-1 over a medium range of 151-3440 mOsm kg^-1. Whole body sodium increased from 3.65 to 15.0 μmol Na⁺ mg DW^-1 (dry weight) over the medium range 80-1900 mM Na⁺. Comparative studies on Artemia nauplii and the prawn Palaemon elegans revealed the characteristic hyper-hyporegulation of both organisms. Body density of T. brevicornis remains greater than that of the external medium over the salinity range 5-100 psu because of changes in body fluid osmolality and body volume. Median body density values for T. brevicornis ranged from 1.0359 g ml^-1 in 5 psu (median density of 5 psu seawater=l.0017 g ml^-1) to 1.0848 g ml^-1 in 100 psu (median density of 100 psu seawater=l.0751 g ml^-1). Tigriopus brevicornis is principally a benthic forager and needs to remain negatively buoyant over a wide salinity range. Environmental monitoring of five supralittoral rockpools was conducted over a year. Live or dormant T. brevicornis were found in salinities of 4-150 psu; temperatures of -1 to +32°C and oxygen concentrations of 0 to > 20 mg l^-1. It is probably of advantage to T. brevicornis to be an osmoconformer; it is perhaps energetically efficient for such a small organism (~1 mm) and avoids buoyancy problems at high salinity.