Swimming Inhibition Research Articles

The action of selected isothiocyanates on bacterial biofilm prevention and control

The activity of two selected isothiocyanates (ITCs), allylisothiocyanate (AITC) and 2-phenylethy-lisothiocyanate (PEITC) was evaluated on the prevention and control of biofilms formed by Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Listeria monocytogenes. In addition, the effect of ITCs was also tested on planktonic cell susceptibility, bacterial motility and adhesion. Biofilm prevention and control were tested using a microtiter plate assay and the effect of ITCs was assessed on biofilm mass and metabolic activity. The minimum bactericidal concentration for E. coli and P. aeruginosa was 1000 μg mL−1 (AITC) and >1000 μg mL−1 (PEITC), for S. aureus and L. monocytogenes was >1000 μg mL−1 (for both ITCs). AITC caused total inhibition of swimming (P. aeruginosa) and swarming (E. coli) motilities. PEITC caused total inhibition of swimming (E. coli, P. aeruginosa and L. monocytogenes) and swarming (E. coli and P. aeruginosa) motilities. Colony spreading of S. aureus was completely inhibited with PEITC. Adhesion assessed in terms of free energy was less favorable when bacteria were exposed to AITC for E. coli and P. aeruginosa and PEITC for P. aeruginosa. Both ITCs had preventive action on biofilm formation and showed a higher potential to reduce the mass of biofilms formed by the Gram-negative bacteria. AITC and PEITC promoted reductions in biofilm activity higher than 60% for all the biofilms tested. The overall study emphasizes the potential of ITCs as emergent products to inhibit bacterial motility and prevent/control biofilms of important human pathogenic bacteria.

Development of the γ-amino butyric acid (GABA)-ergic signaling system and its role in larval swimming in sea urchin

The present study aimed to elucidate the development and γ-amino butyric acid (GABA)-ergic regulation of larval swimming in the sea urchin Hemicentrotus pulcherrimus by cloning glutamate decarboxylase (Hp-gad), GABAA receptor (Hp-gabrA) and GABAA receptor-associated protein (Hp-gabarap), and by performing immunohistochemistry. The regulation of larval swimming was increasingly dependent on the GABAergic system, which was active from the 2 days post-fertilization (d.p.f.) pluteus stage onwards. GABA-immunoreactive cells were detected as a subpopulation of secondary mesenchyme cells during gastrulation and eventually constituted the ciliary band and a subpopulation of blastocoelar cells during the pluteus stage. Hp-gad transcription was detected by RT-PCR during the period when Hp-Gad-positive cells were seen as a subpopulation of blastocoelar cells and on the apical side of the ciliary band from the 2 d.p.f. pluteus stage. Consistent with these observations, inhibition of GAD with 3-mercaptopropioninc acid inhibited GABA immunoreactivity and larval swimming dose dependently. Hp-gabrA amplimers were detected weakly in unfertilized eggs and 4 d.p.f. plutei but strongly from fertilized eggs to 2 d.p.f. plutei, and Hp-GabrA, together with GABA, was localized at the ciliary band in association with dopamine receptor D1 from the two-arm pluteus stage. Hp-gabarap transcription and protein expression were detected from the swimming blastula stage. Inhibition of the GABAA receptor by bicuculline inhibited larval swimming dose dependently. Inhibition of larval swimming by either 3-mercaptopropionic acid or bicuculline was more severe in older larvae (17 and 34 d.p.f. plutei) than in younger ones (1 d.p.f. prism larvae).

The activity of ferulic and gallic acids in biofilm prevention and control of pathogenic bacteria

The activity of two phenolic acids, gallic acid (GA) and ferulic acid (FA) at 1000 μg ml−1, was evaluated on the prevention and control of biofilms formed by Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Listeria monocytogenes. In addition, the effect of the two phenolic acids was tested on planktonic cell susceptibility, bacterial motility and adhesion. Biofilm prevention and control were tested using a microtiter plate assay and the effect of the phenolic acids was assessed on biofilm mass (crystal violet staining) and on the quantification of metabolic activity (alamar blue assay). The minimum bactericidal concentration for P. aeruginosa was 500 μg ml−1 (for both phenolic acids), whilst for E. coli it was 2500 μg ml−1 (FA) and 5000 μg ml−1 (GA), for L. monocytogenes it was >5000 μg ml−1 (for both phenolic acids), and for S. aureus it was 5000 μg ml−1 (FA) and >5000 μg ml−1 (GA). GA caused total inhibition of swimming (L. monocytogenes) and swarming (L. monocytogenes and E. coli) motilities. FA caused total inhibition of swimming (L. monocytogenes) and swarming (L. monocytogenes and E. coli) motilities. Colony spreading of S. aureus was completely inhibited by FA. The interference of GA and FA with bacterial adhesion was evaluated by the determination of the free energy of adhesion. Adhesion was less favorable when the bacteria were exposed to GA (P. aeruginosa, S. aureus and L. monocytogenes) and FA (P. aeruginosa and S. aureus). Both phenolics had preventive action on biofilm formation and showed a higher potential to reduce the mass of biofilms formed by the Gram-negative bacteria. GA and FA promoted reductions in biofilm activity >70% for all the biofilms tested. The two phenolic acids demonstrated the potential to inhibit bacterial motility and to prevent and control biofilms of four important human pathogenic bacteria. This study also emphasizes the potential of phytochemicals as an emergent source of biofilm control products.

Quorum Sensing Inhibition in Pseudomonas aeruginosa PAO1 by Antagonistic Compound Phenylacetic Acid

In Pseudomonas aeruginosa, quorum sensing (QS) autoinducer known as acyl homoserine lactone (AHL) acts as a key regulator in the expression of pathogenic characters. In this work, the efficiency of phenylacetic acid (PAA) in reducing the production of AHL-dependent factors in P. aeruginosa PAO1 was studied. PAA at a concentration of 200μgml(-1) displayed significant reduction in QS-dependent pyocyanin, exopolysaccharide, and protease and elastase production in PAO1. In swimming inhibition assay, PAA-treated PAO1 cells exhibited poor motility in swimming agar plate. In in vivo analysis, PAO1-preinfected Caenorhabditis elegans showed enhanced survival when treated with PAA. PAA at the QS inhibitory concentration showed no growth inhibitory activity on PAO1. Results of the present study revealed the potential of PAA as antipathogenic compound to prevent QS-dependent pathogenicity of P. aeruginosa.

2,5‐Piperazinedione inhibits quorum sensing‐dependent factor production in Pseudomonas aeruginosa PAO1

The effects of 2,5-piperazinedione in reducing the production of quorum sensing (QS)-dependent factors in Pseudomonas aeruginosa PAO1 were assessed both in vitro and in vivo. 2,5-Piperazinedione exhibited a 69% reduction in the azocasein-degrading proteolytic activity and a 48% reduction in the elastolytic activity of PAO1. Further, it showed 85% and 96% reduction in the production of pyocyanin and extracellular polymeric substances (EPS) of PAO1, respectively. In the swimming inhibition assay, 2,5-piperazinedione-treated PAO1 cells exhibited poor swimming motility in swim agar medium. In the in vivo analysis, an enhanced survival of PAO1-preinfected Caenorhabditis elegans was observed after treatment with 2,5-piperazinedione. Regarding the mode of action, in the molecular docking analysis, 2,5-piperazinedione interacts with the amino acid residue of the LasR receptor protein required for binding the natural ligand N -3-oxododecanoyl-l-homoserine lactone (3-oxo-C12-HSL). This demonstrates the probability of 2,5-piperazinedione to interfere with the binding process of 3-oxo-C12-HSL to its receptor protein. Thus, the findings of the present study reveal the potential of 2,5-piperazinedione in reducing the QS-dependent phenotypic features of PAO1.

A new inhibitory pathway in the jellyfishPolyorchis penicillatus

Contact of food with the manubrial lips in the genus Polyorchis A. Agassiz, 1862 evokes trains of electrical impulses (E potentials) that propagate to the margin. E potentials are also produced by food stimuli at the margin and tentacle bases. E potentials are shown to be associated with inhibitory postsynaptic potentials (ipsps) in the swimming motor neurons and contribute to the arrest of swimming during feeding. The conduction pathway for E potentials is a nerve plexus located in the endodermal walls of the stomach and radial and ring canals. We have explored the conducting properties of the system; the conduction velocity varies with stimulus frequency but is about 15 cm/s when stimuli are more than 50 s apart. Neurites belonging to the E system run around the margin adjacent to the inner nerve ring, where the swimming pacemaker neurons are located. We suggest that they may make inhibitory synapses on to the swimming motor neurons, but this has yet to be demonstrated anatomically. The reversal potential for ipsps, recorded intracellularly with potassium acetate micropipettes, was estimated to be about –69 mV. Swimming inhibition mediated by this endodermal pathway is distinct from that observed during protective “crumpling” behaviour and that associated with contractions of the radial muscles seen during feeding, though it may accompany the latter.

Swimming inhibition by elevated pCO2 in ephyrae of the scyphozoan jellyfish, Aurelia

Ephyrae of the scyphozoan jellyfish, Aurelia, were exposed to hypercapnic seawater ( pCO2 5,000 to 50,000 matm) for 96 h, to study the impacts of potential CO2 seepage from a geological storage site beneath the ocean floor. Geological CO2 storage has been proposed as a mitigation measure against global warming but ecological consequences in the case of seepage are largely un- known. No mortality occurred within the pCO2 range used in the present study. Swimming arm pulsation was significantly depressed in animals exposed to 5,000 matm pCO2 compared to control animals, and immediately ceased in animals exposed to �30,000 matm. When returned to normocapnic seawater (pCO2 380 matm) after 96 h exposure to 50,000 matm pCO2, some ephyrae showed strong arm inversion. These results indicate that even though Aurelia is able to survive short-term exposure to pCO2 of up to 50,000 matm, the strong inhibition of swimming activities under these conditions would reduce the environmental fitness of affected animals.

Nerves in the endodermal canals of hydromedusae and their role in swimming inhibition

N eoturris breviconis (Anthomedusae) has a nerve plexus in the walls of its endodermal canals. The plexus is distinct from the ectodermal nerve plexuses supplying the radial and circular muscles in the ectoderm and no connections have been observed between them. Stimulation of the endodermal plexus evokes electrical events recorded extracellularly as "E" potentials. These propagate through all areas where the plexus has been shown by immunohistology to exist and nowhere else. When Neoturris is ingesting food, trains of "E" potentials propagate down the radial canals to the margin and cause inhibition of swimming. This response is distinct from the inhibition of swimming associated with contractions of the radial muscles but both may play a part in feeding and involve chemoreceptors. Preliminary observations suggest that the "E" system occurs in other medusae including Aglantha digitale (Trachymedusae) where the conduction pathway was previously thought to be an excitable epithelium.

Control of swimming in the hydrozoan jellyfishAequorea victoria: subumbrellar organization and local inhibition

The subumbrella of the hydrozoan jellyfish Aequorea victoria (previously classified as Aequorea aequorea) is divided by numerous radial canals and attached gonads, so the subumbrellar musculature is partitioned into subumbrellar segments. The ectoderm of each segment includes two types of muscle: smooth muscle with a radial orientation, used for local (feeding and righting) and widespread (protective) radial responses, and striated muscle with a circular orientation which produces swim contractions. Two subumbrellar nerve nets were found, one of which stained with a commercial antibody produced against the bioactive peptide FMRFamide. Circular muscle cells produce a single, long-duration action potential with each swim, triggered by a single junctional potential. In addition, the circular cells are electrically coupled so full contractions require both electrotonic depolarization from adjacent cells and synaptic input from a subumbrellar nerve net. The radial cells, which form a layer superficial to the circular cells, are also activated by a subumbrellar nerve net, and produce short-duration action potentials. The radial muscle cells are electrically coupled to one another. No coupling exists between the two muscle layers. Spread of excitation between adjacent segments is decremental, and nerve net-activated junctional potentials disappear during local inhibition of swimming (such as with a radial response). Variable swim contractions are controlled by a combination of synaptic input from the motor network of the inner nerve ring, synaptic input from a subumbrellar nerve net, and electrotonic depolarization from adjacent, active muscle cells.

Neuroendocrine and behavioral responses to weak electric fields in adult sea lampreys ( Petromyzon marinus)

We characterized the behavioral and neuroendocrine responses of adult sea lampreys ( Petromyzon marinus) to weak electric fields. Adult sea lampreys, captured during upstream spawning migration, exhibited limited active behaviors during exposure to weak electric fields and spent the most time attached to the wall of the testing arena near the cathode (−). For adult male sea lampreys, exposure to weak electric fields resulted in increased lamprey (l) GnRH-I mRNA expression but decreased lGnRH-I immunoreactivities in the forebrain, and decreased Jun (a neuronal activation marker) mRNA levels in the brain stem. Similar effects were not observed in the brains of female sea lampreys after weak electric field stimulation. The influence of electroreception on forebrain lGnRH suggests that electroreception may modulate the reproductive systems in adult male sea lampreys. The changes in Jun expression may be associated with swimming inhibition during weak electric field stimulation. The results for adult sea lampreys are the opposite of those obtained using parasitic-stage sea lampreys, which displayed increased activity during and after cathodal stimulation. Our results demonstrate that adult sea lampreys are sensitive to weak electric fields, which may play a role in reproduction. They also suggest that electrical stimuli mediate different behaviors in feeding-stage and spawning-stage sea lampreys.

Serotonergic Neural System not only Activates Swimming but also Inhibits Competing Neural Centers in a Pteropod Mollusc1

Initiation of a particular behavior requires not only activation of the neural center directly involved in its control but also inhibition of the neural networks controlling competing behaviors. In the pteropod mollusc, Clione limacina, many identified serotonergic neurons activate or modulate different elements of the swimming system resulting in the initiation or acceleration of the swimming behavior. Cerebral serotonergic neurons are described here, which produce excitatory inputs to the swimming system as well as inhibitory inputs to the neural centers that control competing behaviors. Whole-body withdrawal behavior is incompatible with swimming activity in Clione. The main characteristic of whole-body withdrawal is complete inhibition of swimming. Cerebral serotonergic neurons were found to produce a prominent inhibition of the pleural neurons that control whole-body withdrawal behavior. By inhibiting pleural withdrawal cells, serotonergic neurons eliminate its inhibitory influence on the swimming system and thus favor increased swimming speed. Serotonergic neurons also produce a prominent inhibition of the Pleural White Cell, which is presumably involved in reproductive or egg-laying behavior. Thus the serotonergic system directly activates swimming system and, at the same time, alters a variety of other neural systems preventing simultaneous initiation of incompatible behaviors.

Antisettlement and Narcotic Action of Analogues of Diterpene Marine Natural Product Antifoulants from Octocorals.

: Prevoius studies have determined that the octocorals Renilla reniformis and Leptogorgia virgulata contain diterpenes that are potent inhibitors of barnacle settlement. These antifoulants-the renillafoulins and pukalide-are, however, comparatively complex and thus are not amenable to commercial exploitation. The present study examined 19 analogues, based on the functional groups of lactone and furan rings in the parent molecules, for antisettlement activity and toxicity. The latter parameters are presented as EC(50) values for inhibition of cypris settlement and naupliar swimming, respectively. Assays of a subset of the analogues indicated that they were active in solution rather than when bound to a surface and that at relatively high concentrations they had a narcotic action. The mechanism whereby some of the analogues were able to inhibit settlement at nontoxic concentrations has yet to be explained but suggests that there is merit in the present approach to antifoulant development.

Increased aggressiveness in gravid American lobsters, Homarus americanus

Reports in the literature suggest that gravid female lobsters (Homarus americanus) are more aggressive than non-gravid female lobsters. In an earlier study [Cromarty SI et al. 1998. Biol Bull 194:63–71], we showed that, with one exception, gravid females did not respond to an unfamiliar stimulus with escape swimming. Here we present quantitative evidence showing that gravid female lobsters are more aggressive than non-gravid females. Gravid and non-gravid females were allowed to fight against larger, non-gravid female opponents in an enclosed arena. The videotaped behaviors were ranked in a behavioral hierarchy (Rank of Aggression) according to their degree of aggressiveness and intensity. The behaviors were classified and ranked as follows: (1) aggression toward opponent, (2) redirected aggressive behavior (toward arena wall), (3) redirected defensive behavior, (4) defensive behavior against opponent, and (5) avoidance behavior. Gravid lobsters performed more behaviors and more aggressive behaviors than did non-gravid controls. The behaviors of gravid females ranked higher on the Rank of Aggression scale than did non-gravid females. The opponents of gravid females performed more aggressive and more intensely aggressive acts than the opponents of non-gravid females; they also performed more defensive acts (i.e., those used to ward off an opponent's attack) than did their non-gravid opponents, although there were no differences in intensity of defensive behaviors between gravid and non-gravid animals. There were no significant differences in the number of avoidance behaviors among the four types of combatants (gravid, non-gravid, gravid opponent, and non-gravid opponent), and none in avoidance tailflipping. However, gravid females were more likely to perform aggressive tailflips and performed significantly more aggressive tailflips than non-gravid animals. Together, this suggests that any physiological inhibition of escape swimming may have been overcome by the heightened intensity of gravid fights and the increased aggressiveness of gravid animals. Aggr. Behav. 25:451–472, 1999. © 1999 Wiley-Liss, Inc.

Increased aggressiveness in gravid American lobsters,Homarus americanus

Reports in the literature suggest that gravid female lobsters (Homarus americanus) are more aggressive than non-gravid female lobsters. In an earlier study [Cromarty SI et al. 1998. Biol Bull 194:63–71], we showed that, with one exception, gravid females did not respond to an unfamiliar stimulus with escape swimming. Here we present quantitative evidence showing that gravid female lobsters are more aggressive than non-gravid females. Gravid and non-gravid females were allowed to fight against larger, non-gravid female opponents in an enclosed arena. The videotaped behaviors were ranked in a behavioral hierarchy (Rank of Aggression) according to their degree of aggressiveness and intensity. The behaviors were classified and ranked as follows: (1) aggression toward opponent, (2) redirected aggressive behavior (toward arena wall), (3) redirected defensive behavior, (4) defensive behavior against opponent, and (5) avoidance behavior. Gravid lobsters performed more behaviors and more aggressive behaviors than did non-gravid controls. The behaviors of gravid females ranked higher on the Rank of Aggression scale than did non-gravid females. The opponents of gravid females performed more aggressive and more intensely aggressive acts than the opponents of non-gravid females; they also performed more defensive acts (i.e., those used to ward off an opponent's attack) than did their non-gravid opponents, although there were no differences in intensity of defensive behaviors between gravid and non-gravid animals. There were no significant differences in the number of avoidance behaviors among the four types of combatants (gravid, non-gravid, gravid opponent, and non-gravid opponent), and none in avoidance tailflipping. However, gravid females were more likely to perform aggressive tailflips and performed significantly more aggressive tailflips than non-gravid animals. Together, this suggests that any physiological inhibition of escape swimming may have been overcome by the heightened intensity of gravid fights and the increased aggressiveness of gravid animals. Aggr. Behav. 25:451–472, 1999. © 1999 Wiley-Liss, Inc.

An action spectrum for the light-dependent inhibition of swimming behavior in newly hatched white sturgeon, Acipenser transmontanus.

Newly hatched white sturgeon (Acipenser transmontanus) yolk-sac larvae are negatively phototactic, photokinetic and geotactic--seeking cover in the substrate during daylight hours. If cover is unavailable and the light intensity is low, they move into the water column and alternate periods of rapid, upward swimming with periods of quiescence during which they sink. In the dark they swim almost continuously. A flash of light inhibits swimming activity totally for a period dependent on the light intensity. Using this observation, we obtained an action spectrum for swimming inhibition for 30 newly hatched larvae. This action spectrum is best fit by a single visual pigment with absorbance maximum at 537 nm. The lack of red sensitivity fits well with previous work showing that red-sensitive photoreceptor cells are absent in young sturgeon. The possible significance of red insensitivity is discussed from both ecological and practical standpoints.

Morphology of wing mechanoreceptors involved in the wing retraction reflex in the pteropod molluscClione limacina

Tactile stimulation of the wings (parapodia) of actively swimming Clione limacina results in inhibition of swimming and retraction of the wings. Electrophysiological evidence suggests that wing mechanoreceptors have central cell bodies and wide innervation fields in the ipsilateral wing. Scanning electron microscopy of expanded wings reveals ciliary cone processes arranged in a pattern that is similar to the electrophysiologically-determined innervation fields of wing mechanoreceptors. Transmission electron microscopy suggests that the ciliary cone structures are terminal processes of neuron-like cells. Three-dimensional reconstructions of serially-sectioned terminal processes indicate that cell bodies are not found in the wing epithelium or immediately under the epithelium, further supporting the notion that the wing mechanoreceptors have central cell bodies.

Modulation of swimming in Tritonia: excitatory and inhibitory effects of serotonin

1. In the mollusc Tritonia escape swimming is produced by a network of central pattern generator (CPG) neurons. The purpose of this study was to determine which neurotransmitters might be involved in the swim system. 2. Injection of serotonin (5HT) into whole animals elicited swimming followed by a long-lasting inhibition of swimming. In isolated brain preparations, bath-applied 5HT elicited a swim pattern at short latency and also caused a long-lasting inhibition of the swim pattern. The activation of swimming by 5HT was associated with a tonic depolarization of cerebral cell 2 (C2) and the dorsal swim interneurons (DSI) which form part of the swim CPG network. 3. In isolated brain preparations, bath applied glycine, histamine, proctolin, and FMFRamide had no effect on the swim motor pattern elicited by electrical stimulation of a peripheral nerve. Aspartate, carbacol, dopamine, glutamate, octopamine, pilocarpine, and small cardioactive peptide-B (SCPB) inhibited the activation of swimming by nerve stimulation. 4. The 5HT antagonists cyproheptidine, tryptamine, and 7-methyltryptamine had no effect on swimming, but methysergide and fenfluramine inhibited swimming to both normal sensory stimuli and exogenously applied 5HT. 5. Staining with a polyclonal antibody indicated that one class of CPG neurons, the dorsal swim interneurons (DSI), was immunoreactive for 5HT.

THE SHADOW RESPONSE OF A. HYDROMEDUSAN (POLYORCHIS PENICILLATUS): BEHAVIORAL MECHANISMS CONTROLLING DIEL AND ONTOGENIC VERTICAL MIGRATION.

A variety of photic stimuli were presented to the hydromedusan Polyorchis penicillatus under controlled conditions to characterize the photic responses of various sizes of P. penicillatus and to determine the role of these responses in its behavior. "Treadmill" experiments showed that for all but very small hydromedusae, swimming frequencies at different constant light intensities did not differ. Swimming frequency of P. penicillatus was, however, directly proportional to rates of decrease in light intensity. Slowly increasing light intensity caused an inhibition of swimming and "crumpling." Rapid, 100% shadows of various absolute magnitudes usually caused only a single swimming contraction. The maximal response to rapid shadows of monochromatic light occurred around 450-550 nm. These results suggest that the shadow response of P. penicillatus does not function in predator avoidance, but more likely contributes to nighttime upward movement in the water column. The inhibition of swimming during increasing light intensity may initiate dawn sinking. Most of the photic responses of P. penicillatus show size-(age) related differences which may result in ontogenic changes in distribution and feeding behavior.

EFFECTS OF K+ AND Ca2+ IONS ON MOTILITY AND PHOTOSENSORY RESPONSES OF STENTOR COERULEUS

Extracellular K+ ions above a critical concentration induce ciliary reversal in unstimulated Stentor coeruleus and suppress step‐up photophobic response. This threshold concentration of K+ ions depends on the extracellular Ca2+ concentration, and the subsequent backward gyration and light‐sensitivity suppression seem to depend on the relative concentrations of K+ and Ca2+. The concentration of Ca2+ necessary to overcome K+‐mediated inhibition of phobic response and backward swimming increases non‐linearly with increasing K+ concentration. The Ca2+‐blocking agent. D‐600, selectively inhibits photophobic responses of Stentor, thus further confirming the role of Ca2+ ions in photosensory transduction of this ciliate.

Influence of temperature and salinity on survival of the freshwater mullet, Rhinomugil corsula (Hamilton)

The upper incipient lethal temperatures of the freshwater mullet, Rhinomugil corsula, acclimated to 15, 20, 25, 30 and 35°C in fresh water, were 32.4, 34.1, 36.0, 36.2 and 36.5°C respectively, and the corresponding lower lethal temperatures were 10.5, 11.5, 13.2, 15.8 and 19.5°C. The mullet has a total tolerance (area of thermal polygon) of 569°C with an upper and lower thermal tolerance of 253 and 316°C 2. Likewise, the total resistance of the mullet was 391°C 2, with upper and lower resistance zones of 181 and 210°C respectively. The upper critical temperatures of swimming inhibition of R. corsula (17.2 cm; acclimation 30°C), determined in a swimming tunnel, were 35.2, 34.6 and 34.2 for water current velocities of 38, 62 and 77 cm s −1 respectively. The corresponding lower critical temperatures were 26.2, 27.5 and 28.1°C. These results indicated the stenothermal nature of the mullet by comparison with other fishes, e.g. Tilapia mossambica. In tests on the influence of ambient salinity on thermal resistance, R. corsula survived longest at 7‰ (iso-osmotic salinity). At salinities above and below this point, survival times were shorter at any lethal temperature. In a tentative scheme for quantification of stress due to temperature and salinity at death (after acclimation to 30°C and tested at 37°C), the hypo-osmotic and hyper-osmotic stress were estimated to be 50 and 31% of the thermal stress (100%) respectively.