Effects of ultraviolet radiation and water motion on the reef coral Porites compressa Dana: a flume experiment

The effects of water flow and ultraviolet radiation (UVR, 280–400 nm) on the reef coral Porites compressa Dana were explored in a manipulative flume experiment. The aim of this study was to determine whether this coral responds to changes in the UVR environment by adjusting the tissue concentration of UV-absorbing compounds (mycosporine-like amino acids, MAAs), and to see whether such an acclimation is affected by water flow. Also, calcification rate and chlorophyll-a concentration were measured during the experiment to estimate the potential costs (in the form of slowed growth and/or reduced photosynthetic capacity) to the coral–alga symbiosis of being exposed to UVR and producing MAAs. Branches of P. compressa from a single male colony were exposed to high or low flow (15 cm s⁻¹ and 3 cm s⁻¹, respectively) and ambient or no UVR in an outdoor, continuous-flow seawater system. Chlorophyll-a and MAA concentrations were determined after zero, 3 and 6 weeks of exposure to the experimental conditions. Increase in buoyant weight during the two 3-week periods was used to calculate calcification rate. The presence of UVR had a significant positive effect on total MAA concentration in the P. compressa colonies; however, there were significant interactions present. In colonies exposed to UVR, MAA concentration increased and then decreased to initial levels in high water flow, and increased steadily in low water flow. In colonies receiving no UVR, MAA concentration decreased steadily, declining 23% in 6 weeks. The absence of UVR did not result in higher chlorophyll-a concentrations, but the calcification rate was slightly affected by UVR. This study supports the putative photoprotective role of MAAs in P. compressa, and suggests that the costs of mitigating the effects of ambient UVR are detectable, but they are very small. Received: 29 February 2000 / Accepted: 20 September 2000     

Tolerance to low oxygen and sulfide in Amphiura filiformis and Ophiura albida (Echinodermata: Ophiuroidea)

The brittle stars Amphiura filiformis (Müller) and Ophiura albida (Forbes) were exposed to different oxygen saturations (100, 10, 5, 3, and <1% oxygen saturation) and to physiological anoxia (<1% oxygen saturation) at different total sulfide concentrations (0, 2, 20, 200 μM). The mortality was followed during experiments and the median survival time (LT₅₀) was determined. The infaunal A. filiformis had a significantly higher tolerance to both hypoxia and sulfide than did the epibenthic O. albida. After exposure to 10% oxygen saturation for a month, only 2.0% A. filiformis and 0% O. albida were dead. In oxygen saturations <1% A. filiformis and O. albida had a LT₅₀ of 7.5 and 2.5 d, respectively. The presence of even very small concentrations of sulfide decreased the survival significantly. Sulfide is shown to be the key factor for the survival of the two species. Received: 11 October 1996 / Accepted: 12 November 1996     

Behaviour and tolerance to hypoxia in juvenile Norway lobster (Nephrops norvegicus ) of different ages

The annual occurrence of hypoxia (<25% oxygen saturation) in the bottom waters along the Swedish west coast coincides with the postlarval settlement of Norway lobster, Nephrops norvegicus (L.). This study investigates behaviour and the experimental effects of low oxygen concentrations in juvenile N. norvegicus of different ages. All experimental individuals were reared to the juvenile (postlarval) stage in the laboratory and then given sediment as a substratum. Behavioural responses to low oxygen concentrations were tested in early and late Postlarvae 1 exposed to normoxia (>80% oxygen saturation, pO₂ > 16.7 kPa), moderate hypoxia (30% oxygen saturation, pO₂ = 6.3 kPa) and hypoxia (25% oxygen saturation, pO₂ = 5.2 kPa). The experiments were run for a maximum period of 24 h or until individuals died. Behaviour was studied using sequential video recordings of four behavioural activities: digging, walking, inactivity or flight (escape swimming up into the water column). Behaviour and mortality changed with lowered oxygen concentrations; energetically costly activities (such as walking) were reduced, and activity in general declined. In normoxia, juveniles initially walked and then burrowed, but when exposed to hypoxia they were mainly inactive with occasional outbursts of escape swimming. To increase oxygen availability the juveniles were observed to raise their bodies on stilted legs (similar to adults in hypoxic conditions), but oxygen saturations of 25% were lethal within 24 h. The results suggest that the main gas exchanges of early postlarval stages occur over the general body surface. Burrowing behaviour was tested in Postlarvae 1 and 2 of different ages held in >80% oxygen saturation for 1 wk. The difference in time taken to complete a V-shaped depression or a U-shaped burrow was measured. The results showed a strong negative relationship between postlarval age and burrowing time, but all individuals made a burrow. Juveniles were more sensitive to hypoxia than adults. Thus, the possible consequences of episodic hypoxia for the recruitment of Nephrops norvegicus and for the recolonization of severely affected areas are discussed. Received: 4 August 1996 / Accepted: 11 October 1996     

Horizontal movements and depth distribution of large adult yellowfin tuna (Thunnus albacares) near the Hawaiian Islands, recorded using ultrasonic telemetry: implications for the physiological ecology of pelagic fishes

We measured the horizontal and vertical movements of five adult yellowfin tuna (Thunnus albacares, estimated body mass 64 to 93 kg) near the main Hawaiian Islands, while simultaneously gathering data on oceanographic conditions and currents. Fish movements were recorded by means of ultrasonic depth-sensitive transmitters. Depth–temperature and depth–oxygen profiles were measured with vertical conductivity–temperature–depth (CTD) casts, and the current-velocity field was surveyed using an acoustic Doppler current profiler (ADCP). Large adult yellowfin tuna spent ≃60 to 80% of their time in or immediately below the relatively uniform-temperature surface-layer (i.e. above 100 m), a behavior pattern similar to that previously reported for juvenile yellowfin tuna, blue marlin (Makaira nigricans), and striped marlin (Tetrapturus audax) tracked in the same area. In all three species, maximum swimming depths appear to be limited by water temperatures 8 C° colder than the surface-layer water temperature. Therefore, neither large body mass, nor the ability to maintain elevated swimming-muscle temperatures due to the presence of vascular counter-current heat exchangers in tunas, appears to permit greater vertical mobility or the ability to remain for extended periods below the thermocline. In those areas where the decrease in oxygen with depth is not limiting, the vertical movements of yellowfin tuna, blue marlin and striped marlin all appear to be restricted by the effects of water temperature on cardiac muscle function. Like juvenile yellowfin tuna, but unlike blue marlin and striped marlin, adult yellowfin tuna remained within 18.5 km of the coast and became associated with floating objects, including anchored fish-aggregating devices (FADs) and the tracking vessel. Like juvenile yellowfin tuna, large adult yellowfin repeatedly re-visit the same FAD, and appear able to navigate precisely between FADs that are up to 18 km apart. The median speed over ground ranged from 72 to 154 cm s⁻¹. Neither speed nor direction was strongly influenced by currents. Received: 27 March 1998 / Accepted: 13 November 1998     

Photoadaptation of zooxanthellae in the sponge Cliona vastifica from the Red Sea, as measured in situ

In the Red Sea, the zooxanthellate sponge Cliona vastifica (Hancock) is mainly present at >15 m depth or in shaded areas. To test whether its scarcity in unshaded areas of shallower waters is linked to the functional inefficiency of its photosymbionts at high irradiances, sponges were transferred from 30 m to a six times higher light regime at 12 m depth, and then returned to their original location. During this time, photosynthetic responses to irradiance were measured as rapid light curves (RLCs) in situ by pulse amplitude modulated (PAM) fluorometry using a portable underwater device, and samples were taken for microscopic determinations of zooxanthellar abundance. The zooxanthellae harboured by this sponge adapted to the higher irradiance at 12 m by increasing both their light saturation points and relative photosynthetic electron transport rates (ETRs). The ETRs at light saturation increased almost fourfold within 15–20 days of transfer to the shallower water, and decreased back to almost their original values after the sponges were returned to 30 m depth. This, as well as the fact that the photosynthetic light responses within an individual sponge were in accordance with the irradiance incident to specific surfaces, shows that these photosymbionts are highly adaptable to various irradiances. There was no significant change in the number of zooxanthellae per sponge area throughout these experiments, and the different photosynthetic responses were likely due to adaptations of the photosynthetic apparatus within each zooxanthella. In conclusion, it seems that parameters other than the hypothesised inability of the photosymbionts to adapt adequately to high light conditions are the cause of C. vastifica's rareness in unshaded shallow areas of the Red Sea. Received: 25 April 2000 / Accepted: 13 October 2000     

Movements of tiger sharks (Galeocerdo cuvier) in coastal Hawaiian waters

The giant clam Tridacna crocea harbors in the mantle tissue symbiotic microalgae commonly called zooxanthellae. Isolated zooxanthellae release glycerol into the medium in the presence of mantle tissue homogenate (MH), but it is not clear whether the cells do so in situ. In order to determine the photosynthetic products released by zooxanthellae in the mantle of the giant clam we traced photosynthetic fixation products from ¹³C- and ¹⁴C-bicarbonate both in the clam and in isolated zooxanthellae (IZ) in the presence or absence of MH. After 15 min incubation in the absence of MH the IZ released less than 0.6% of the fixed labeled carbon, mainly as glucose. The major intracellular photosynthates were neutral lipids, which constituted 20 to 40% of the total extractable ¹⁴C. In the presence of MH, the IZ released up to 5.6% of the total fixed ¹⁴C, mostly as glycerol, and the major intracellular photosynthate was glucose. In an intact clam incubated in sea water containing ¹⁴C-bicarbonate, 46 to 80% of the fixed ¹⁴C was translocated from the zooxanthellae to the host tissues. Most of the ¹⁴C in the hemolymph, in the isolated zooxanthellae and in intact mantle tissue (containing zooxanthellae) was recovered as glucose. No ¹⁴C-glycerol was detected in the mantle after 1 to 30 min incubation, and, even after 60 min, far less ¹⁴C-glycerol was synthesized than by IZ in the presence of MH. The possibility that in clam tissue glycerol is converted to glucose was examined by tracing the labeled carbon from ¹⁴C-glycerol injected into the adductor muscle. After 5 min incubation, no labeled glucose was found in the hemolymph, but after 60 min, some 20% was found as glucose. Thin slices containing zooxanthellae, cut from the surface of the mantle, fixed inorganic carbon supplied as NaH¹⁴CO₃ in the medium and mainly released ¹⁴C-glucose. The addition of MH to the surrounding medium did not affect the release rate or form of release product. When the slices were cut into smaller pieces, however, the ratio of glycerol to glucose in the release product increased. These results indicate that in the presence of MH the metabolism of isolated zooxan- thellae was different from that of zooxanthellae in the mantle. In the presence of MH, isolated zooxanthellae release mostly glycerol, whereas in the mantle they release glucose. Received: 18 February 1998 / Accepted: 4 December 1998     

Microsensor studies of photosynthesis and respiration in the symbiotic foraminifer Orbulina universa

Oxygen and pH microelectrodes were used to investigate the microenvironment of the planktonic foraminifer Orbulina universa and its dinoflagellate endosymbionts. A diffusive boundary layer surrounds the foraminiferal shell and limits the O₂ and proton transport from the shell to the ambient seawater and vice versa. Due to symbiont photosynthesis, high O₂ concentrations of up to 206% air saturation and a pH of up to 8.8, i.e. 0.5 pH units above ambient seawater, were measured at the shell surface of the foraminifer at saturating irradiances. The respiration of the host–symbiont system in darkness decreased the O₂ concentration at the shell surface to <70% of the oxygen content in the surrounding air-saturated water. The pH at the shell surface dropped to 7.9 in darkness. We measured a mean gross photosynthetic rate of 8.5 ± 4.0 nmol O₂ h⁻¹ foraminifer⁻¹. The net photosynthesis averaged 5.3 ± 2.7 nmol O₂ h⁻¹. In the light, the calculated respiration rates reached 3.9 ± 1.9 nmol O₂h⁻¹, whereas the dark respiration rates were significantly lower (1.7 ± 0.7 nmol O₂ h⁻¹). Experimental light–dark cycles demonstrated a very dynamic response of the symbionts to changing light conditions. Gross photosynthesis versus scalar irradiance curves (P vs E _o curves) showed light saturation irradiances (E _k) of 75 and 137 μmol photons m⁻² s⁻¹ in two O. universa specimens, respectively. No inhibition of photosynthesis was observed at irradiance levels up to 700 μmol photons m⁻² s⁻¹. The light compensation point of the symbiotic association was 50 μmol photons m⁻² s⁻¹. Radial profile measurements of scalar irradiance (E _o) inside the foraminifera showed a slight increase at the shell surface up to 105% of the incident irradiance (E _d). Received: 26 January 1998 / Accepted: 11 April 1998     

Symbiotic anemones can grow when starved: nitrogen budget for Anemonia viridis in ammonium-supplemented seawater

The ability of endosymbioses between anthozoans and dinoflagellate algae (zooxanthellae) to retain excretory nitrogen and take up ammonium from seawater has been well documented. However, the quantitative importance of these processes to the nitrogen budget of such symbioses is poorly understood. When starved symbiotic Anemonia viridis were incubated in a flow-through system in seawater supplemented with 20 μM ammonium for 91 d under a light regime of 12 h light at 150 μmol photons m⁻² s⁻¹ and 12 h darkness, they showed a mean net growth of 0.197% of their initial weight per day. Control anemones in unsupplemented seawater with an ammonium concentration of <1 μM lost weight by a mean of 0.263% of their initial weight per day. Attempts to construct a nitrogen budget showed that, over a 14 d period, ≃40% of the ammonium taken up could be accounted for by growth of zooxanthellae. It was assumed that the remainder was translocated from zooxanthellae to host. However, since the budget does not balance, only 60% of the growth of host tissue was accounted for by this translocation. The value for host excretory nitrogen which was recycled to the symbionts equalled that taken in by ammonium uptake from the supplemented seawater, indicating the importance of nitrogen retention to the symbiotic association. Received: 23 December 1997 / Accepted: 12 September 1998     

Amino acid synthesis in the symbiotic sea anemone Aiptasia pulchella

Symbiotic Aiptasia pulchella and freshly isolated zooxanthellae were incubated in NaH¹⁴CO₃ and NH₄Cl for 1 to 240 min, and samples were analysed by reverse-phase high-performance liquid chromatography (HPLC) and an online radiochemical detector. NH₄ ⁺ was first assimilated into ¹⁴C-glutamate and ¹⁴C-glutamine in the zooxanthellae residing in A. pulchella. The specific activities (dpm nmol⁻¹) of ¹⁴C-glutamate and ¹⁴C-glutamine in vivo, were far greater in the zooxanthellae than in the host tissue, indicating that NH₄ ⁺ was principally incorporated into the glutamate and glutamine pools of the zooxanthellae. ¹⁴C-α-ketoglutarate was taken up from the medium by intact A. pulchella and assimilated into a small amount of ¹⁴C-glutamate in the host tissue, but no ¹⁴C-glutamine was detected in the host fraction. The ¹⁴C-glutamate that was synthesized was most likely produced from transamination reactions as opposed to the direct assimilation of NH₄ ⁺. The free amino acid composition of the host tissue and zooxanthellae of A. pulchella was also measured. The results presented here demonstrate that NH₄ ⁺ was initially assimilated by the zooxanthellae of A. pulchella. Received: 3 February 1997 / Accepted: 24 October 1997     

Effects of dissolved ammonium addition and host feeding with Artemia salina on photoacclimation of the hermatypic coral Stylophora pistillata

 Effects of nutrient treatments on photoacclimation of the hermatypic coral Stylophora pistillata (Esper) were studied. Studies on photoacclimation of colonies from different light regimes in the field were evaluated and used to design laboratory experiments. Coral colonies were collected in the Gulf of Eilat (Israel) from January to March 1993. Exterior branches of colonies from different depths (1 to 40 m) displayed different trends in production characteristics at reduced and very low levels of illumination. From 24 ± 3% to 12 ± 2% of incident surface photosynthetic active radiation (PAR_o), zooxanthella population density and chlorophyll a+c per 10⁶ zooxanthellae increased, a trend seen in the range of light levels optimal for coral growth (90 to 30% PAR_o). The P _max of CO₂ per 10⁶ zooxanthellae decreased, while P _max of CO₂ per 10³ polyps increased, indicating an increase in zooxanthella population density at low light levels. Proliferous zooxanthella frequency (PZF, a measure of zooxanthella division) declined significantly at light levels <18 ± 3% PAR_o. At the lowest levels of illumination (<5% PAR_o), zooxanthella population density decreased, as did the PZF; chl a+c per 10⁶ zooxanthellae was unchanged. In 28-d experiments, exterior coral branches from the upper surfaces of colonies from 3 m depth (65 ± 4% PAR_o) were incubated in aquaria under bright (80 to 90% PAR_o), reduced (20 to 30% PAR_o), and extremely low (2 to 4% PAR_o) light intensities. At each light intensity, the corals were maintained in three feeding treatments: sea water (SW); ammonium enriched SW (SW + N); SW with Artemia salina nauplii (SW + A). An increase in P _max of CO₂ per 10³ polyps was found in corals acclimated to reduced light (20 to 30% PAR_o) in nutrient-enriched SW, while in SW, where the increase in zooxanthella population density was smaller, it did not occur. Nutrient enrichments (SW + N at 2 to 4% PAR_o and SW + A at 20 to 30% PAR_o) increased zooxanthella population density, but had no effect on chl a+c per 10⁶ zooxanthellae. Acclimation for 14 d to reduced (10 to 20% PAR_o) and extremely low (1 to 3% PAR_o) light intensities shifted ¹⁴C photoassimilation into glycerol and other compounds (probably glycerides), rather than sugars. Both ammonium addition and feeding with Artemia salina nauplii resulted in an increase in photosynthetic assimilation of ¹⁴C into amino acids. We conclude that acclimation to reduced light consists of two processes: an increase in photosynthetic pigments and in zooxanthella population density. Both processes require nitrogen, the increase in zooxanthella population density needing more; this adaptation is therefore limited in nitrogen-poor sea water. Received: 19 June 1998 / Accepted: 13 June 2000     

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

 The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O₂, CO₂, pH, Ca²⁺ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O₂ up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO₂ fixation decreased the CO₂ at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O₂ level to 91% air saturation and the CO₂ concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O₂, CO₂ and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O₂ cm⁻³ s⁻¹) than in stagnant water (1.6 nmol O₂ cm ⁻³ s⁻¹), whereas net photosynthesis rates were unaffected by flow conditions. The Ca²⁺ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca²⁺ gradients at the shell surface showed total Ca²⁺ uptake rates of 0.6 to 4.2 nmol cm⁻² h⁻¹ in A. lobifera and 1.7 to 3.6 nmol cm⁻² h⁻¹ in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis. Received: 6 July 1999 / Accepted: 28 April 2000     

Does oxygen deficiency modify the functional response of Saduria entomon (Isopoda) to Bathyporeia pilosa (Amphipoda)?

The functional response of the predatory isopod Saduria entomon to the prey amphipod Bathyporeia pilosa was measured in normoxia (95% O₂ saturation), moderate hypoxia (45% O₂ saturation) and hypoxia (35% O₂ saturation) in aquarium experiments. The prey densities tested ranged from 400 to 8000 ind m⁻². Prey density influenced consumption rates of S. entomon in normoxia and 45% O₂ saturation, but there was no difference between consumption rates at these two oxygen levels. Nevertheless the form of functional response differed. In normoxia S. entomon showed a positively density-dependent functional response to B. pilosa, indicating a potentially stabilizing effect on the prey population. In moderate hypoxia the variance in consumption increased, decreasing the statistical power to distinguish between response models. The functional response of S. entomon in moderate hypoxia was best described with a density-independent response, characterized as destabilizing for the prey population. In hypoxia (35% O₂) predation by S. entomon did not respond to increasing prey density, as almost no amphipods were eaten at this oxygen level. The results are discussed in terms of the usability of theoretical models to examine predator–prey relationships in stressful environments. Received: 26 April 1997 / Accepted: 20 May 1997     

Fecal discharge of zooxanthellae in the giant clam Tridacna derasa, with reference to their in situ growth rate

The population dynamics of zooxanthellae living in the mantle of a giant clam, Tridacna derasa, was studied. The giant clams with shell lengths of 5 to 6 cm which had been reared in the Palau Mariculture Demonstration Center, in the Republic of Palau, were transferred to aquaria on deck of the R.V. “Sohgen-maru” and kept in running sea water at 29 to 30 °C. Two clams were removed from the aquaria, and zooxanthellae in the mantle were isolated every 2 h for 24 h. Numbers of the zooxanthellae in or not in the cell division stage were counted for calculations of the zooxanthellae population in the mantle and their mitotic index (MI). The MI increased after sunset and reached the maximum values of 6.1 to 11.5% at 03:00 to 05:00 hrs. The specific growth rate, μ, estimated from the MI was 0.083 to 0.14 d⁻¹. Five clams were kept in each of 2 Plexiglas containers in the aquarium for collection of the discharged feces every 3 to 4 h. The discharged zooxanthellae in the feces were counted. The zooxanthellae discharged in 24 h were 0.38 to 1.46% of the total zooxanthella population in the mantle, and 2.7 to 16.9% of the newly formed zooxanthella population in a day. Increase of zooxanthella population in the mantle was estimated from clam shell growth rate and from the correlation between zooxanthella population and clam shell size. Daily increase of zooxanthella population in the mantle was estimated to be approximately 7.6 to 19% of the newly formed zooxanthella population. Therefore, the sum of zooxanthellae populations accounting for daily increase in the mantle and discharge in the feces was 11 to 36% of the newly formed population. About 64 to 89% of the newly formed cells were missing; some of these may have been digested by the clam. Received: 14 July 1996 / Accepted: 19 August 1996     

Modification of light utilization for skeletal growth by water flow in the scleractinian coral <Emphasis Type="Italic">Galaxea fascicularis</Emphasis>

In this study, we tested the hypothesis that the importance of water flow for skeletal growth (rate) becomes higher with increasing irradiance levels (i.e. a synergistic effect) and that such effect is mediated by a water flow modulated effect on net photosynthesis. Four series of nine nubbins of G. fascicularis were grown at either high (600 μE m⁻² s⁻¹) or intermediate (300 μE m⁻² s⁻¹) irradiance in combination with either high (15–25 cm s⁻¹) or low (5–10 cm s⁻¹) flow. Growth was measured as buoyant weight and surface area. Photosynthetic rates were measured at each coral’s specific experimental irradiance and flow speed. Additionally, the instantaneous effect of water flow on net photosynthetic rate was determined in short-term incubations in a respirometric flowcell. A significant interaction was found between irradiance and water flow for the increase in buoyant weight, the increase in surface area, and specific skeletal growth rate, indicating that flow velocity becomes more important for coral growth with increasing irradiance levels. Enhancement of coral growth with increasing water flow can be explained by increased net photosynthetic rates. Additionally, the need for costly photo-protective mechanisms at low flow regimes could explain the differences in growth with flow.     

Ammonium, but not nitrate, stimulates an increase in glutamine concentration in the haemolymph of Tridacna gigas

The concentration of glutamine in Tridacna gigas haemolymph increased >35-fold following exposure to sea water supplemented with ammonium (20 μM), but no increase was observed with nitrate (20 μM). Lack of a diel cycle, no decrease in haemolymph glucose levels, the expression patterns of glutamine synthetase in zooxanthellae and host, and the lack of glutamine release in response to nitrate supplementation all support the proposition that the increase in haemolymph glutamine is a product of the host and not the zooxanthellae. Unlike ammonium, nitrate accumulates rapidly in the haemolymph. It has no effect on the concentration of glutamine in the haemolymph, but there is an increase in arginine, histidine and lysine in the haemolymph, suggesting the release of these essential amino acids from zooxanthellae. Glutamine synthetase (GS) activity decreased markedly in the gill and less so in the mantle over a period of 6 d exposure to elevated ammonium (20 μM). In contrast, GS activity in zooxan- thellae doubled. The response of zooxanthellae in situ was confirmed by incubating freshly isolated zooxanthellae for 4 d in ammonium, which resulted in a ten-fold increase in GS activity. Comparison of the in situ response of zooxanthellae with that obtained in vitro indicates that the symbionts are likely to be exposed to ammonium concentrations lower than that found in the haemolymph. Received: 14 November 1997 / Accepted: 28 April 1998     

Photosynthetic utilisation of carbon and light by two tropical seagrass species as measured in situ

In situ measurements of seagrass photosynthesis in relation to inorganic carbon (Ci) availability, increased pH and an inhibitor of extracellular carbonic anhydrase were made using an underwater pulse amplitude modulated (PAM) fluorometer. By combining the instrument with a specially designed Perspex chamber, we were able to alter the water surrounding a leaf without removing it from the growing plant. Responses to Ci within the chamber showed that subtidal plants of the seagrasses Cymodocea serrulata and Halophila ovalis had photosynthetic rates that were limited by the ambient Ci concentration depending on the irradiance that was available during short-term photosynthesis–irradiance trials. Relative electron transport rates (RETRs) at light saturation (up to 500 μ mol photons m⁻² s⁻¹) increased by 66–100% when the Ci concentration was increased from ca. 2.2 to 6.2 mM. On the other hand, intertidal plants of the same species exhibited a much lesser limitation of photosynthesis by Ci at any irradiance (up to 1500 μ mol photons m⁻² s⁻¹). Both species were able to use HCO⁻ ₃ efficiently, and there was stronger evidence for direct uptake of HCO⁻ ₃ rather than extracellular dehydration of HCO⁻ ₃ to CO₂ prior to Ci uptake. Subtidally, H. ovalis and C. serrulata grew to 10 and 12 m, respectively, where ambient irradiances were approximately 16 and 11% of those at the surface. Maximum RETRs (at light saturation) were lower for these deep-growing plants than for the intertidally growing ones. For both species, the onset of light saturation of photosynthesis (E _k) occurred at approximately 100 μ mol photons m⁻² s⁻¹ for the deep water populations, which was four and two times lower than for the shallow populations of C. serrulata and H. ovalis, respectively. This, and the differences in maximal photosynthetic rates (RETR _max), reflects an acclimation of the deep-growing populations to the lower light environment. The results presented here show that photosynthesis, as measured in situ, was limited by the availability of Ci for the deeper growing plants in Zanzibar, while the intertidally growing plants photosynthesised at close to Ci saturation. The latter result is contrary to previous conclusions regarding Ci limitations for these intertidal plants, and, in general, our findings highlight the need for performing similar experiments in situ rather than under laboratory conditions. Received: 4 April 2000 / Accepted: 31 August 2000     

Physiological and behavioral aspects of Calanus euxinus females (Copepoda: Calanoida) during vertical migration across temperature and oxygen gradients

 In the Black Sea, during summer stratification, Calanus euxinus (Hulsemann) undertakes diel vertical migrations with an amplitude of about 117 m from oxygenated, warm (18 °C) surface layers to hypoxic (∼0.8 mg O₂ l⁻¹) zones with lower temperature (7.9 °C). When such changes in temperature and oxygen concentration are reproduced in the laboratory, total metabolism, basal metabolism and scope of activity of copepods decrease 7.2, 7.8 and 6.7 times, respectively, while the frequency of locomotory acts and mechanical power decline 3.4- and 9.5-fold, respectively. These changes allowed the copepods to conserve a significant portion of food consumed near the surface for transformation to lipid reserves. Diel respiratory oxygen consumption of migrating individuals, calculated so as to include actual duration of residence in layers with different temperature and oxygen concentrations, is estimated at 17.87 μg O₂ ind⁻¹. The net energy cost of vertical migration made up only 11.6% of the total. Copepods expend 78.6% of diel energy losses during approximately 10 h in the surface layers, while about 5.4% is required during about 9 h at depth. Hypoxia is shown to have a significant metabolic advantage during diel vertical migrations of C. euxinus in the Black Sea. Received: 1 October 1999 / Accepted: 11 July 2000     

Growth of zooxanthellae in culture with two nitrogen sources

Physiological characteristics of zooxanthellae were examined under nutrient-saturated conditions created by mixing ammonium (¹⁵NH₄) with nitrate (¹⁵NO₃) to give 0.88 mM total nitrogen. Growth rate varied with the form of nitrogen provided. Ammonium alone resulted in the lowest C:N and C:chl-a ratios. Although zooxanthellae took up nitrate in the absence of ammonium, ammonium assimilation was 1.3 times higher than nitrate assimilation. Ammonium strongly inhibited nitrate assimilation. While high-ammonium treatments resulted in the highest ¹⁴C incorporation into intermediate compounds, high nitrate levels resulted in the highest ¹⁴C incorporation into protein, suggesting that the intermediate compounds are produced prior to the subsequent production of protein when ammonium is the dominant N source. The enhanced production of intermediate compounds at the expense of carbon directed to protein synthesis in the presence of ammonium might be analogous to the “host factor” observed in zooxanthellae–host symbioses, since growth rate is depressed due to low production of protein. Received: 16 March 2000 / Accepted: 26 August 2000     

Free amino acids and energy metabolism in eggs and larvae of seabass, Lates calcarifer

Contents of free amino acids (FAA), protein and ammonium ions together with rates of ammonia excretion and oxygen consumption were measured in order to study the role of FAA as an energy substrate in developing eggs and larvae of seabass (Lates calcarifer) maintained in seawater (30 ppt) at 28 °C without feeding. Initially eggs contained 25.3 nmol ind⁻¹ of FAA of which 21.5 nmol was rapidly utilised by the developing eggs and larvae during the period up to 40 h post spawning (PS) when nearly all the yolk had been resorbed. During the same period, a net increase in protein content of 1.7 μg ind⁻¹ was observed, indicating that the major part of the amino acids lost from the free pool had been polymerised into body proteins. Assuming that the balance of the FAA after protein synthesis was used entirely for energy metabolism, FAA appeared to be an important energy substrate during the embryonic stages (2 to 16 h PS); after hatching, the contribution of FAA to energy metabolism was less significant. From 50 h PS until the end of the study period at 100 h PS, amino acids derived from somatic protein were used for energy metabolism. For the overall period from just after spawning up to 100 h PS, the data indicate that ca. 14% of the total aerobic energy metabolism was derived from amino acid catabolism. Received: 26 September 1997 / Accepted: 1 April 1998     

Heat production in Saduria entomon (Isopoda) from the Gulf of Gdańsk during an experimental exposure to anoxic conditions

The benthic crustacean Saduria entomon occurs frequently in deeper parts of the Gulf of Gdańsk. It is one of a few species able to survive oxygen deficiencies in its natural environment. The anaerobic heat production of S. entomon during 40 h of anoxia was determined. Additionally, the effects of size, sex and activity of the organism on its heat production were investigated. Average heat production of S. entomon was 0.25 ± 0.16 mJ g⁻¹ wet wt s⁻¹ (n=55, avg. length 39 ± 6 mm). The amount of heat produced decreased with increasing body size. The heat production of S. entomon males was greater than that of females ( p<0.05). Specimens kept in chambers with glass beads and water were less active and had lower metabolic rates than those placed in chambers containing water only (more active). During 40 h of anoxia S. entomon gradually decreased its heat production to 5–16% of aerobic level, demonstrating the high adaptation of this species to changeable oxygen conditions in the Baltic Sea. Received: 31 July 1997 / Accepted: 21 January 1998