Ammonia excretion in mytilid mussels is facilitated by ciliary beating.

The excretion of nitrogenous waste products in the form of ammonia (NH3) and ammonium (NH4 (+)) is a fundamental process in aquatic organisms. For mytilid bivalves, little is known about the mechanisms and sites of excretion. This study investigated the localization and the mechanisms of ammonia excretion in mytilid mussels. An Rh protein was found to be abundantly expressed in the apical cell membrane of the plicate organ, which was previously described as a solely respiratory organ. The Rh protein was also expressed in the gill, although at significantly lower concentrations, but was not detectable in mussel kidney. Furthermore, NH3/NH4 (+) was not enriched in the urine, suggesting that kidneys are not involved in active NH3/NH4 (+) excretion. Exposure to elevated seawater pH of 8.5 transiently reduced NH3/NH4 (+) excretion rates, but they returned to control values following 24 h acclimation. These mussels had increased abundance of V-type H(+)-ATPase in the apical membranes of plicate organ cells; however, NH3/NH4 (+) excretion rates were not affected by the V-type H(+)-ATPase specific inhibitor concanamycin A (100 nmol l(-1)). In contrast, inhibition of ciliary beating with dopamine and increased seawater viscosity significantly reduced NH3 excretion rates under control pH (8.0). These results suggest that NH3/NH4 (+) excretion in mytilid mussels takes place by passive NH3 diffusion across respiratory epithelia via the Rh protein, facilitated by the water current produced for filter feeding, which prevents accumulation of NH3 in the boundary layer. This mechanism would be energy efficient for sessile organisms, as they already generate water currents for filter feeding.

Thermal physiology of the common eelpout (Zoarces viviparus).

We investigated the temperature dependence of some physiological parameters of common eelpout (Zoarces viviparus) from different locations (North Sea, Baltic Sea and Norwegian Sea) on acclimation temperature (3 degrees C and 12 degrees C) and acute temperature variation. The lethal limit of 12 degrees C-acclimated eelpout was determined as the critical thermal maximum [loss of equilibrium (LE) and onset of muscular spasms (OS)] and it was found to be 26.6 degrees C for LE and 28.8 degrees C for OS for all populations. However, these parameters do not have any relevant ecological interpretation. We therefore investigated the effect of gradually increased water temperature on standard metabolic rate (measured as resting oxygen consumption Mo2) and critical oxygen concentration ([O2]c) of eelpouts. Acclimation to low temperature (3 degrees C) resulted in partial compensation of Mo2, paralleled by a decrease of activation energy for Mo2 (from 82 kJ mol(-1) at 12 degrees C to about 50 kJ mol(-1) at 3 degrees C) in North Sea and Baltic Sea eelpouts. At the same time, Norwegian eelpout showed no acclimation of oxygen demand to warm temperature (12 degrees C) at all. The scope for eelpout aerobic metabolism shrank considerably with increased acclimation temperature, as [O2]c approached water oxygen concentrations. At 22.5+/-1 degrees C the [O2]c reached air saturation, which is equivalent to the upper critical temperature (TcII) and at this temperature the aerobic scope for the metabolism completely disappeared. In line with previous insight, the comparative analysis of the temperature dependence of Mo2 of Z. viviparus from different populations suggests that a pejus (sub-critical) temperature for this species is about 13-15 degrees C. In conclusion, the capacity to adjust aerobic metabolism relates to thermal tolerance and the bio-geographical distribution of the species. Global warming would thus be likely to cause a shift in the distribution of this species to the North.

Temperature effects on hemocyanin oxygen binding in an antarctic cephalopod.

The functional relevance of oxygen transport by hemocyanin of the Antarctic octopod Megaleledone senoi and of the eurythermal cuttlefish Sepia officinalis was analyzed by continuous and simultaneous recordings of changes in pH and hemocyanin oxygen saturation in whole blood at various temperatures. These data were compared to literature data on other temperate and cold-water cephalopods (octopods and giant squid). In S. officinalis, the oxygen affinity of hemocyanin changed at deltaP50/degrees C = 0.12 kPa (pH 7.4) with increasing temperatures; this is similar to observations in temperate octopods. In M. senoi, thermal sensitivity was much smaller (<0.01 kPa, pH 7.2). Furthermore, M. senoi hemocyanin displayed one of the highest levels of oxygen affinity (P50 < 1 kPa, pH 7.6, 0 degrees C) found so far in cephalopods and a rather low cooperativity (n50 = 1.4 at 0 degrees C). The pH sensitivity of oxygen binding (delta log P50/delta pH) increased with increasing temperature in both the cuttlefish and the Antarctic octopod. At low PO2 (1.0 kPa) and pH (7.2), the presence of a large venous oxygen reserve (43% saturation) insensitive to pH reflects reduced pH sensitivity and high oxygen affinity in M. senoi hemocyanin at 0 degrees C. In S. officinalis, this reserve was 19% at pH 7.4, 20 degrees C, and 1.7 kPa O2, a level still higher than in squid. These findings suggest that the lower metabolic rate of octopods and cuttlefish compared to squid is reflected in less pH-dependent oxygen transport. Results of the hemocyanin analysis for the Antarctic octopod were similar to those reported for Vampyroteuthis--an extremely high oxygen affinity supporting a very low metabolic rate. In contrast to findings in cold-adapted giant squid, the minimized thermal sensitivity of oxygen transport in Antarctic octopods will reduce metabolic scope and thereby contribute to their stenothermality.

Cold tolerance and the regulation of cardiac performance and hemolymph distribution in Maja squinado (Crustacea: decapoda).

Elevated Mg(2+) levels in the hemolymph ([Mg(2+)](HL)) of brachyuran crabs have recently been demonstrated to limit cold tolerance by reducing motor and circulatory activity. Therefore, the limiting function of elevated [Mg(2+)](HL) on circulatory performance and arterial hemolymph flow was investigated by the pulsed-Doppler technique in the spider crab Maja squinado during progressive cooling from 12 degrees to 0 degrees C. [Mg(2+)](HL) were reduced from control levels of 39.9 mmol L(-1) to levels of 6.1 mmol L(-1) by incubation in magnesium reduced seawater. At 12 degrees C cardiac output was 13.9+/-2.4 mL kg(-1) min(-1) and stroke volume 0.2+/-0.04 mL kg(-1) min(-1) in control animals. In [Mg(2+)](HL)-reduced animals cardiac output increased to 43.6+/-5.0 mL kg(-1) min(-1) and stroke volume rose to 0.6+/-0.1 mL kg(-1) min(-1). Temperature reduction in control animals revealed a break point at 8 degrees C linked to a major redirection of hemolymph flow from lateral to sternal and hepatic arteries. Cardiac output and heart rate dropped sharply during cooling until transiently constant values were reached. Further heart rate reduction occurred below 4.5 degrees C. Such a plateau was not detected in [Mg(2+)](HL)-reduced animals where the break point decreased to 6 degrees C, also indicated by a sharp drop in heart rate and cardiac output and the redirection of hemolymph flow. It is concluded that progressive cooling brings the animals from a temperature range of optimum cardiac performance into a deleterious range when aerobic scope for activity falls before critical temperatures are reached. Reduction of [Mg(2+)](HL) shifts this transition to lower temperatures. These findings support a limiting role for [Mg(2+)](HL) in thermal tolerance.

Haemolymph Mg(2+) regulation in decapod crustaceans: physiological correlates and ecological consequences in polar areas.

Reptant decapod crustaceans are almost absent from the Southern Ocean south of the Antarctic Convergence. We tested the hypothesis that this may be due to the reduced ability of this group to regulate Mg(2+) levels in the haemolymph ([Mg(2+)](HL)). Mg(2+) acts as an anaesthetic in marine invertebrates and its level is higher in Reptantia (crabs such as Cancer spp., Chionoecetes spp., Maja spp., 30-50 mmol l(-)(1)) than in Natantia (prawns such as Pandalus spp., Palaemon spp., Crangon spp., 5-12 mmol l(-)(1)). We varied [Mg(2+)](HL) in three species of reptant decapod crustaceans, Carcinus maenas, Hyas araneus and Eurypodius latreillei, and investigated heart rate, the rate of oxygen consumption and levels of spontaneous and forced activity at different temperatures. The rate of oxygen consumption and heart rate increased significantly with reduction in [Mg(2+)](HL) over the entire temperature range investigated in E. latreillei. In H. araneus, an increase in metabolic and heart rates compared with control values was found only at temperatures below 2 degrees C. Forced and spontaneous activity levels increased significantly in the group of [Mg(2+)](HL)-reduced animals below 0 degrees C, at which control animals were mostly inactive. At a reduced [Mg(2+)](HL) of 5-12 mmol l(-)(1), which is the [Mg(2+)](HL) of caridean shrimps in the Southern Ocean, Q(10) and activation energy were reduced for all these variables and extended the temperature range over which physiological functions were maintained. We suggest that the high [Mg(2+)](HL) in Reptantia causes relaxation of the animals and reduces their scope for activity, especially at temperatures below 0 degrees C. The hypothesis that the synergistic effects of high [Mg(2+)](HL) and low temperature probably prevented the Reptantia from recolonizing the permanently cold water of polar areas is discussed.