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     

The oxygen microenvironment adjacent to the tissue of the scleractinian Dichocoenia stokesii and its effects on symbiont metabolism

The biology of symbiotic scleractinians is profoundly influenced by their intracellular zooxanthellae, and many studies have focused on the mechanistic basis of this influence. This has usually been accomplished by examining the metabolism of zooxanthellae under physical conditions measured in the open reef and assumed to be similar to conditions in hospite. Recent advances in the measurement of conditions near and within coral tissue suggests that this assumption may result in substantial errors. To address this possibility, the role of water flow in determining oxygen saturation adjacent to the tissue of Dichocoenia stokesii was investigated, and the effect of these measured oxygen saturations on the respiration and photosynthesis of zooxanthellae isolated from the same species was quantified. Using a microelectrode (700 μm diam), we measured oxygen saturations above (≤4 mm) the tissue in two flow speeds over 24 h periods in a flume receiving sunlight at in situ levels. The results were used as a proxy for ecologically relevant intracellular oxygen saturations, which were applied to zooxanthellae in vitro to assess their effect on symbiont metabolism. Microenvironment oxygen saturations (% air saturation) ranged from 74–159% in slow flow (2.7 cm s⁻¹) to 88–110% in faster flow (7.5 cm s⁻¹) over day–night cycles. Therefore, the metabolic rates of zooxanthellae were measured at 50 to 54% (hypoxia), 98 to 102% (normoxia) and 146 to 150% (hyperoxia) oxygen saturation. Oxygen saturation significantly affected the metabolism of zooxanthellae, with gross photosynthesis increasing 1.2-fold and dark respiration increasing 2-fold under hyperoxia compared to hypoxia. These results suggest that the metabolism of zooxanthellae in hospite is affected markedly by their microenvironment which, in turn, is influenced by flow-mediated mass transfer. Received: 13 July 1998 / Accepted: 30 April 1999     

Zooplankton responses to hypoxia: behavioral patterns and survival of three species of calanoid copepods

Seasonally recurrent and persistent hypoxic events in semi-enclosed coastal waters are characterized by bottom-water dissolved oxygen (d.o.) concentrations of < 2.0 ml l⁻¹. Shifts in the distribution patterns of zooplankters in association with these events have been documented, but the mechanisms responsible for these shifts have not been investigated. This study assessed interspecific differences in responses to hypoxia by several species of calanoid copepods common off Turkey Point, Florida, USA: Labidocera aestiva (Wheeler) (a summer/fall species), Acartia tonsa (Dana) (a ubiquitous year-round species), and Centropages hamatus (Lilljeborg) (a winter/spring species). Under conditions of moderate to severe hypoxia 24-h survival experiments were conducted for adults and nauplii of these species from August 1994 to October 1995. Experiments on adults used a flow-through system to maintain constant d.o. concentrations. Adults of A. tonsa showed no decline in survival with d.o. as low as 1.0 ml l⁻¹, sharp declines in survival at d.o. = 0.9 to 0.6 ml l⁻¹, and 100% mortality with d.o. = 0.5 ml l⁻¹. Adults of L. aestiva and C. hamatus were more sensitive to oxygen depletion: both species experienced significant decreases in survival for d.o. = 1.0 ml l⁻¹. Nauplii of L. aestiva and A. tonsa showed no significant mortality with d.o. = 1.1 to 1.5 ml␣l⁻¹ and d.o. = 0.24 to 0.5 ml l⁻¹, respectively. In addition, experiments investigating behavioral avoidance of moderate to severe hypoxia were carried out for adults of all three species. None of the three species effectively avoided either severely hypoxic (d.o. < 0.5 ml l⁻¹) or moderately hypoxic (d.o. ≈ 1.0 ml l⁻¹) bottom layers in stratified columns. These results suggest that in␣nearshore areas where development of zones of d.o. < 1.0 ml l⁻¹ may be sudden, widespread, or unpredictable, patterns of reduced copepod abundance in bottom waters may be due primarily to mortality rather than avoidance. Received: 31 August 1996 / Accepted: 24 September 1996     

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     

Respiration of the eggs of the giant cuttlefish Sepia apama

On the roofs of subtidal crevices, the giant cuttlefish (Sepia apama) of southern Australia lays clutches of lemon-shaped eggs which hatch after 3 to 5 mo. Diffusion of oxygen through the capsule and chorion membrane to the perivitelline fluid and embryo was modelled using the equation V˙ _O2 = G _O2(P _O2out−P _O2in), where V˙ _O2 = rate of oxygen consumption, G _O2 = oxygen conductance of the capsule, and P _O2 values = oxygen partial pressures across the capsule. During development, V˙ _O2 rose exponentially as the embryo grew, reaching 5.5 μl h⁻¹ at hatching. Throughout development, the capsule dimensions enlarged by absorption of water into the perivitelline space, increasing G _O2 by a combination of increasing surface area, and decreasing thickness of the capsule. These processes maintained P _O2in high enough to allow unrestricted V˙ _O2 until shortly before hatching. Diffusion limitation of respiration in hatching-stage embryos was demonstrated by (1) increased embryonic V˙ _O2 when P _O2out was experimentally raised, (2) greater V˙ _O2 of resting individuals immediately after hatching, and (3) reduced V˙ _O2 of hatchlings at experimental P _O2 levels higher than P _O2in before hatching. Thus, low P _O2in may be the stimulus to hatch. Potential problems of diffusive gas-exchange are mitigated by the relatively low incubation temperature (12 °C), which may be a factor limiting the distribution of the species to cool, southern waters. Received: 14 August 1999 / Accepted: 24 January 2000     

Studies on the causes of mortality of the estuarine bivalve Macoma balthica under conditions of (near) anoxia

Survival of the bivalve Macoma balthica in (near) anoxic seawater was studied in a static system and a flow-through system and compared with emersed exposure to air and N₂. In the static system, a decrease in pH and exponential accumulation of sulphide in the incubation medium were observed, indicating excessive growth of (sulphate-reducing) bacteria. These changes in the chemical environment were prevented by the use of a flow-through system. However, this treatment hardly affected survival time. Median mortality times were 8.3 and 9.0 days for the static and flow-through incubation, respectively. Addition of the antibiotic chloramphenicol strongly increased survival time in both systems with corresponding values of 17.9 and 23.0 days. A similar value was obtained for survival in air (LT₅₀= 21.7 days). In a second experiment (1 year later), we obtained much lower values for anoxic survival in a static system, although laboratory conditions, season and temperature were similar. The pH values were adjusted to 6.5, 7.2 and 8.2 by buffering the media (25 mM Tris-HCl), and the corresponding LT₅₀ values were 5.5, 5.7 and 4.7 days, respectively. In the presence of chloramphenicol the values were 10.8, 10.9 and 9.5 days, respectively. These values show that a slightly acidic medium increased survival time. Exposure to an atmosphere of N₂ resulted in a survival time close to that in anoxic seawater without chloramphenicol (LT₅₀= 6.4 days). Overall the results indicate that proliferation of anaerobic bacteria associated with the bivalves was the main cause of death. Since chloramphenicol also displayed a strong positive effect in the flow-through system, which prevented the accumulation of released waste products and a decrease of pH, bacterial damage must have been by injury of the tissues of the clams and not by the release of noxious compounds to the medium. Bacterial outbreaks are a part of every anoxic event (eutrophication), and therefore, in their habitats, direct bacterial infection may also be the cause of clam mortality. It is concluded that laboratory studies on anoxic tolerance, or impact of sulphide, may produce artefacts when no precautions are taken to suppress bacterial proliferation. Received: 4 July 2000 / Accepted: 6 December 2000     

Effect of Ca2+ on survival and development of metamorphosing bonefish (Albula sp.) leptocephali

Bonefish (Albula sp.) larvae (leptocephali) from the Gulf of California complete metamorphosis in ˜10 d in natural seawater (35‰S; Ca²⁺ conc = 10.5 mM). The increase in ossification that occurs near the end of the non-feeding metamorphic period, in addition to the ability of larvae to complete metamorphosis in dilute seawater (8‰ S) prompted the present study, where the effects of varying the external calcium ion concentration, [Ca²⁺]_e, of artificial seawater (ASW) on the survival, development and internal (whole-body) calcium ion content, (Ca²⁺)_i, of unfed metamorphosing larvae were investigated. Early-metamorphosing larvae placed in␣ASW, where [Ca²⁺]_e = 10.1 mM, survived for up to 10 d and developed normally without exogenous nutrients. In shorter-term experiments (4 to 5 d), no differences in survival were found for larvae in ASW with [Ca²⁺]_e rang-ing from 1.5 to 10.1 mM. However, in Ca²⁺-free ASW, most larvae died within 27 h and no larvae survived more than 42 h; the median lethal time (LT₅₀), and its 95% confidence limits, were 14.5 (10.0 to 20.9) h. High mortality (81% after 20 h) also occurred in 1.0 mM Ca²⁺ ASW, but 2 of 16 larvae tested survived for 96 h. The 96 h median tolerance limit (TL_M), corrected for control mortality, was 1.2 mM Ca²⁺. In natural seawater, larval (Ca²⁺)_i remained relatively constant ( = 0.419 mg larva⁻¹)␣in early- and intermediate-metamorphosing larvae, and then increased to a mean value of 0.739 mg larva⁻¹ in advanced larvae, indicating that Ca²⁺ was␣taken up from the medium at this stage; the increase in (Ca²⁺)_i corresponded to the period of ossification of the vertebral column. Internal (whole-body) magnesium ion content (Mg²⁺)_i showed no significant change during metamorphosis ( = 0.089 mg larva⁻¹). No significant differences in (Ca²⁺)_i were found in advanced larvae in natural seawater and those in ASW, with [Ca²⁺]_e ranging from 2.0 to 10.1 mM. However, clearing and staining revealed that ossification of the vertebral column had not yet occurred in advanced larvae from 2.0 to 10.1 mM Ca²⁺ ASW. Also, low [Ca²⁺]_e (1.0 to 2.0 mM) usually produced deformed larvae that swam erratically, at times showing “whirling” behavior. Received: 21 August 1996 / Accepted: 26 August 1996     

Effects of ambient ammonia levels on blood ammonia, ammonia excretion and heart and scaphognathite rates of Nephrops norvegicus

During commercial handling of Nephropsnorvegicus (L.) there are a number of situations when the prawns may be exposed to very high ambient ammonia levels. These experiments evaluated the effects of increased levels of ambient total ammonia (TA = NH₃ + NH₄ ⁺) on␣blood ammonia, ammonia efflux rates and on the cardio-ventilatory performance of N. norvegicus. When prawns were taken from <1 to 2000 μmol TA l⁻¹ medium, blood TA concentrations increased rapidly for the first 2 h but tended to drop thereafter. Original blood TA levels were restored 6 h after the prawns were transferred back from seawater containing 2000 to <1 μmol TA l⁻¹. Sudden exposure to 500, 1000, 2000 or 4000 μmol TA l⁻¹ medium induced blood TA concentrations to increase respectively to 50, 30, 33 and 36% of external concentrations (normally, internal TA values are much higher than external levels). Immediately after transfer back to seawater with low ammonia concentration ( <1 μmol TA l⁻¹), excretion rates were higher than those of control prawns, and the absolute amounts of TA excreted were considerably higher than those calculated to have accumulated in the haemolymph. Heart rate (HR) and scaphognathite rate (SR) were not altered when prawns were subjected to sudden alterations in ambient ammonia ( <1 to 2000 to <1 μmol TA l⁻¹). When water ammonia concentrations were altered more gradually, both rates increased, but only at 4000 μmol TA l⁻¹. These results show that N. norvegicus is able to remove ammonia from the haemolymph and/or transform ammonia into some other substance when subjected to increased levels of ambient ammonia. Possible mechanisms involved (e.g. active transport across the gills; storage in some other tissue; glutamate synthe sis) are discussed. Received: 20 May 1996 / Accepted: 1 July 1996     

Gender- and oxygen-related irrigation behaviour of the decapod Nephrops norvegicus

The Norway lobster Nephrops norvegicus (L.) inhabits burrows in muddy clay sediments (e.g. on the Swedish west coast), where an autumnal oxygen deficiency in the bottom water can occur. Our experiments investigated whether the irrigation of the burrows would reflect a behavioural adaptation to hypoxia, and whether any gender differences of such behaviour exist. Irrigation is performed by the pleopods which may compensate for a decreasing oxygen tension. Pleopod activity (total number of strokes per sampling time), associated with oxygen concentration and gender, was studied in N. norvegicus kept in artificial burrows resembling their natural habitat. Male and female lobsters were separately exposed to either normoxia (70% oxygen saturation) or hypoxia (30% oxygen saturation). A sexual difference in behaviour was found, where females irrigated the burrow less than males during normoxia. Females showed a significant increase of pleopod activity in hypoxia compared with normoxic conditions, which was not displayed by the males probably due to the degree of individual variation found. However, when only males were studied during progressive hypoxia (from 60 to 5% oxygen saturation), following any changes of irrigational behaviour, a significant increase of accumulated pleopod activity occurred. A major increase of pleopod activity appeared between 60 and 50% oxygen saturation, below which the activity remained high until a critical point (<10% saturation, 11 °C, 33 psu) where irrigation dropped to a level close to that of normoxic values. Activity sessions during hypoxia were longer and had a higher stroke rate than during normoxia. Received: 22 October 1997 / Accepted: 26 February 1998     

Ultrastructure and anaerobic metabolism of mitochondria in the marine oligochaete Tubificoides benedii : effects of hypoxia and sulfide

It has often been suggested that ultrastructural properties of mitochondria are correlated with oxygen and sulfide levels from the environment, although careful analyses of this question are rare. In this study the ultrastructure and distribution of mitochondria in Tubificoides benedii, a marine oligochaete from sulfide-rich sediments, were investigated after a series of oxic, hypoxic and hypoxic–sulfidic (200 μM H₂S) incubations up to 24 h. Succinate, one of the key endproducts of an anaerobic metabolism, was used as an indicator of mitochondrial anaerobiosis. Consistent differences in mitochondrial ultrastructure were not observed in any of the incubations, even after 24 h. Stereological parameters of mitochondria (volume density, surface density of the outer mitochondrial membrane, and specific surface) in epidermal and intestinal tissues of T. benedii were not affected by hypoxia or sulfide either. On the other hand, succinate concentrations increased significantly within 24 h under hypoxic and hypoxic–sulfidic conditions. Thus, experimental hypoxia and sulfide clearly caused mitochondrial anaerobiosis without affecting ultrastructure or distribution of mitochondria in T. benedii. Distinct differences in ultrastructural and stereological parameters were common between different tissues and between individuals, showing that different forms of mitochondria can occur within one species. Our results imply that a mitochondrial ultrastructure specific to thiobiotic animals does not appear to exist. Received: 4 August 1996 / Accepted: 20 September 1996     

Effects of contrasting modes of larval development on the genetic structures of populations of three species of prosobranch gastropods

 In south-eastern Australia, the prosobranch gastropods Morula marginalba (Blainville), Cominella lineolata (Lamarck) and Bedeva hanleyi (Angas) have similar fine-scale distributions, but appear to possess very different dispersal capabilities due to contrasting modes of larval development. M.marginalba produce planktonic larvae, whereas C. lineolata and B. hanleyi undergo direct development in benthic egg capsules and emerge as crawling juveniles. To test for possible effects of contrasting life histories on levels of genetic variation within and among populations, a survey was conducted of allozyme variation at six polymorphic loci in 8 to 9 local populations of each species. Collections of snails were made between June 1992 and November 1993. Sampling ranges spanned between 162 and 180 km of coast. Regardless of larval type, proportions of single-locus genotypes in each collection were consistent with the recruitment of offspring which had been generated through random mating. However, genotypic diversity was lower in those species that undergo direct development. Loci surveyed in C. lineolata and B. hanleyi were polymorphic (i.e. frequency of most common allele <95%) in fewer populations than those examined for M.␣marginalba (P <0.001) and, where polymorphisms occurred, also possessed significantly fewer alleles (P <0.001). Consequently, average levels of expected heterozygosity were greater in populations of M. marginalba than in those of either of the other species (P <0.001). Genetic variation among populations, expressed as the standardised variance in allele frequencies (F _ST ), was inversely related to expected larval dispersal capability. The nine collections of M. marginalba showed little overall differentiation (F _ST  = 0.017; P <0.001), reflecting the ability of planktonic larvae to interconnect local populations, and so limit divergence due to drift and natural selection. In contrast, there were high levels of allelic heterogeneity among the nine collections of C. lineolata (F _ST  = 0.523; P <0.001) and eight collections of B. hanleyi (F _ST  = 0.140; P <0.001). These data imply that for species which undergo direct development, local populations are effectively closed and evolve largely independent of one another. Received: 3 May 1996 / Accepted: 12 July 1996     

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     

Group foraging, patch exploitation time and the finder's advantage

 We use a combination of the marginal value theorem (MVT) of Charnov (1976), and a group foraging model featuring information sharing to address patch residence in an environment where food occurs in discrete patches. We shall show that among equal competitors the optimal patch time for the individual that finds the food patch is shorter than that for the non-finder among equal competitors, T _E < T _N. This is the case if the patch-finder commences food harvesting in the patch earlier and manages to monopolise a fraction of the prey items (finder's advantage) before the other individuals come to take their benefit. When individuals differ in their food-searching abilities so that some of them (producers) contribute proportionally more to food-searching than others (scroungers), and differ in ability to compete for the food found, a difference emerges between producer and scrounger individuals in the optimal patch time. Within a patch we always have the finder's advantage (T _E < T _N) regardless of phenotype. Between patches a suite of optimal patch times for encountering individuals emerges depending on the performance of producers and scroungers when changing from solitary feeding to feeding in a group. The optimal patch time for individuals that are affected more severely by competition is shorter than that for individuals of the phenotype with better competitive ability. When both phenotypes are affected similarly no difference in optimal patch times emerges. Received: 13 February 1996 / Accepted after revision: 28 September 1996     

Blood oxygen-binding characteristics of bigeye tuna (Thunnus obesus), a high-energy-demand teleost that is tolerant of low ambient oxygen

 We found blood from bigeye tuna (Thunnus obesus) to have a significantly higher O₂ affinity than blood from other tunas. Its P₅₀ (partial pressure of oxygen, P_O2 required to reach 50% saturation) was 1.6 to 2.0 kPa (12 to 15 mmHg) when equilibrated with 0.5% CO₂. Previous studies employing similar methodologies found blood from yellowfin tuna (T. albacares), skipjack tuna (Katsuwonus pelamis), and kawakawa (Euthynnus affinis) to have a P₅₀ of 2.8 to 3.1 kPa (21 to 23 mmHg). These observations suggest that bigeye tuna are more tolerant of low ambient oxygen than other tuna species, and support similar conclusions derived from laboratory whole-animal studies, depth-of-capture data, and directly-recorded vertical movements of fish in the open ocean. We also found the O₂ affinity of bigeye tuna blood to be essentially unaffected by a 10 C° open-system temperature change (as is the blood of all tuna species studied to date). The O₂ affinity of bigeye tuna blood was, however, more affected by a 10 C° closed-system temperature change than the blood of any tuna species yet examined. In other words, bigeye tuna blood displayed a significantly enhanced Bohr effect (change in log P₅₀ per unit change in plasma pH at P₅₀) when subjected to the inevitable changes in partial pressure of carbon dioxide (P_CO2) and plasma pH that accompany closed-system temperature shifts, than when subjected to changes in plasma pH accomplished by changing P_CO2 alone. In vivo, the resultant large decrease in O₂ affinity (i.e. the increase in P₅₀) that occurs as the blood of bigeye tuna is warmed during its passage through the vascular counter-current heat exchangers ensures adequate rates of O₂ off-loading in the swimming muscles of this high-energy-demand teleost. Received: 12 March 1999 / Accepted: 18 December 1999     

Carbon-dioxide production and metabolic parameters in the ophiurid Ophiothrix fragilis

As part of the evaluation of fluxes between the water column and a rich benthic community of the Dover Strait (Eastern English Channel), laboratory measurements of oxygen consumption were carried out on a common ophiurid, Ophiothrix fragilis (Abildgaard), from February 1993 to February 1995. The mean O₂-consumption rate was evaluated at 0.31 mg O₂ g⁻¹ h⁻¹ (ash-free dry weight). Simultaneous measurements of O₂ consumption and CO₂ production using the pH-alkalinity method revealed an average respiratory quotient of 0.69 proved suitable for converting oxygen demand to carbon flux. A seasonal trend in respiration data was demonstrated by sinusoidal curves fitted to O₂-uptake and CO₂-release data as a function of time. The influence on respiration rate of two seasonal parameters (temperature and food availability) is discussed; linear regression indicated a highly significant relationship between O₂ consumption (or CO₂ production) and temperature; both O₂-consumption and CO₂-production rates decreased with starvation. The average O:N ratio was estimated at 8.46, close to the theoretical value when proteins constitute the catabolic substrate. The annual carbon respired by the O. fragilis community examined and the estimated annual primary production by phytoplankton indicate that the respiration of the O. fragilis community could supply 35% of phytoplankton carbon requirements. Received: 1 August 1996 / Accepted: 4 September 1996     

A sensitive method for determining nitrogen-15 isotope in urea

A method is presented in which the ¹⁵N at.% of urea is determined with high precision on liquid samples containing as little as 10 nmol of urea. The method involves removal of interference from NH₄ ⁺ initially present in the sample by cation exchange. Urea in the sample is subsequently hydrolyzed to NH₄ ⁺ by the enzyme urease. Liberated NH₄ ⁺ is separated from the alkaline sample by diffusion as NH₃ through a helium gas phase where it is finally oxidized to N₂ by reaction with hypobromite iodine. The isotopically labeled N₂ thus formed is mixed with the N₂ initially present in the sample, and the ¹⁵N at.% of urea is calculated from the relative amounts of ¹⁴N¹⁵N and ¹⁵N¹⁵N by means of the isotope pairing principle. The presented method for ¹⁵N-urea analysis proved to be precise (SE < 0.4 at.%, n = 5), when applied on both marine and freshwater samples with an ¹⁵N enrichment >1 at.%, and interference was found only from volatile methyl amines. Received: 19 September 1996 / Accepted: 25 October 1996     

Haemolymph constituent levels and ammonia efflux rates of Nephrops norvegicus during emersion

Nephropsnorvegicus (L.) were subjected to 8 h of emersion, either between layers of seawater-soaked hessian with periodical (20 min) flushes of seawater (high humidity, HH) or to unprotected emersion (low humidity, LH). Blood ammonia levels rose during emersion in both groups but reached higher levels under LH conditions. Ammonia efflux rates after re-immersion were higher than those of control prawns, and amounts of ammonia excreted at such times were considerably higher than those calculated to have accumulated in the blood during emersion. Possible explanations for such differences are discussed. C _aO2 and C _vO2 decreased rapidly to ca. 10% normoxia values within 2 h of HH and LH emersion and remained low throughout the remaining emersion time. Emersion-induced tissue hypoxia increased blood concentrations of glucose and lactate. Lactate accumulation was higher during LH emersion, compared with HH emersion. Blood pH dropped ca. 0.40 units but increased again after 2 h of re-immersion. Acidosis was probably related more to respiratory difficulties (CO₂ accumulation) than to lactate accumulation, as blood lactate values remained high after 2 h of re-immersion. The ability of N. norvegicus to cope with emersion appears to be little influenced by high humidity conditions. Received: 26 June 1996 / Accepted: 5 August 1996     

Survival, growth and reproduction of the salt-marsh amphipod Uhlorchestia spartinophila reared on natural diets of senescent and dead Spartina alterniflora leaves

The talitrid amphipod Uhlorchestia spartinophila lives in close association with standing-dead leaves of the smooth cordgrass Spartina alterniflora Loisel in salt marshes along the Atlantic coast of North America. This study probed the strength of the trophic link between the amphipod population and the decomposition process in this detrital-based ecosystem. We measured survival, growth and reproductive output in groups of amphipods reared for 6 wk on five diets derived from sheath and blade portions of S. alterniflora leaves just prior to (senescent) and during (dead) decomposition. In unfed treatments, the daily specific mortality rate was 0.391 and starved amphipods survived no longer than 11 d. Among the fed treatments, a diet of senescent sheaths resulted in the lowest survival (20%) and yielded no offspring. Groups fed senescent blades, dead sheaths, dead blades and unwashed dead sheaths had survival rates of 56 to 84% and produced 5.0 to 12.5 offspring replicate⁻¹. Sex ratio usually favored females, but approached unity in treatments with high overall survival, suggesting that quality of available food resources may influence sex ratio in this species. Mean specific growth rates (mm mm⁻¹ d⁻¹) ranged from 0.013 to 0.016, and matched previous estimates of growth from field populations. Overall ecological performance (survival + growth + reproduction) was similar for all food treatments, except senescent sheaths, which yielded a final mean (±SD) dry biomass (0.4 ± 0.42 mg replicate⁻¹) of amphipods significantly lower than that of other diets (1.7 ± 0.81 to 2.6 ± 0.69 mg replicate⁻¹). Natural diets derived from decomposing cordgrass leaves can fulfill the nutritional requirements of U. spartinophila populations, but variation in initial amounts of living fungal biomass among the five experimental diets only partially explained the responses of amphipods in our experiment. Structural characteristics and variation in rates of fungal occupation within different portions of cordgrass leaves may affect the amphipod's ability to access plant production made available by decomposers. Received: 12 December 1996 / Accepted: 18 December 1996     

Disease resistance: a benefit of sociality in the dampwood termite Zootermopsis angusticollis (Isoptera: Termopsidae)

The benefit of sociality in relation to disease susceptibility was studied in the dampwood termite Zootermopsis angusticollis. Although contact with high concentrations of fungal conidia is lethal, the survivorship of nymphs exposed to spore suspensions ranging from 6 × 10⁶ to 2 × 10⁸ spores/ml of the fungus Metarhizium anisopliae increased with group size. The survivorship (measured as LT₅₀) of isolated individuals ranged from 3.0 to 4.8 days, but infected nymphs living in groups of 10 and 25 individuals survived significantly longer (5.6–8.3 and 5.6–9.1 days, respectively). In most cases, there were no significant differences in the survival distributions of the 10- and 25-termite groups. When nymphs were infected with concentrations of 7 × 10¹–7 × 10⁴ spores/ml and allowed to interact with healthy nestmates, fungal infections were not contracted by the unexposed termites. Moreover, infected termites benefitted from social contact with unexposed nestmates: their survival rates were significantly higher than those of infected termites living with similarly infected nestmates. Allogrooming, which increased in frequency during and after exposure to conidia, appeared to remove potentially infectious spores from the cuticle, thus increasing termite survivorship. These results suggest that allogrooming plays a crucial role in the control of disease and its death hazard in termites. The infection-reducing advantage of group living may have been significant in the evolution of social behavior in the Isoptera. Received: 18 March 1998 / Accepted after revision: 31 May 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