Swimming performance and endurance of the Norway lobster Nephrops norvegicus

The swimming behaviour of the Norway lobster, Nephrops norvegicus (L.), was studied in the laboratory. The lobsters were stimulated to swim repeatedly until they failed to respond to a single tactile stimulus. The position of the initiating stimulus on the body determined the height of the escape path above the bottom of the tank. A stimulus to the rostrum resulted in a low swimming trajectory, rarely exceeding a height of 0.1 m. A similar tactile stimulus to the abdomen produced higher mean swimming trajectories up to 0.5 m. Several parameters of swimming performance and endurance were measured for each swimming sequence, including distance, duration, mean and maximum velocities and the frequency and number of tail beats. Results showed no significant differences in the average swimming performance and endurance of males and females, but there was a sex difference in the relationship between swimming performance and N. norvegicus size.     

Locomotory behaviour and swimming performance of the Norway lobster,Nephrops norvegicus,in the presence of an acoustic tag

Observations have been made on the locomotory behaviour and swimming performances of the Norway lobster, Nephrops norvegicus (L.), fitted with an acoustic transmitter ventrally under the cephalothorax. The walking behaviour of adult males (44 mm carapace length) appeared to be unaffected, but the tag caused significant reductions in certain measures of tail-flip swimming performance such as swimming speed and endurance. Flume-tank experiments in low water currents suggested that the transmitter would increase hydrodynamic drag during swimming by 9 to 32%, depending on lobster size. Given the weight and dimensions of the acoustic transmitters currently available, it is considered advisable to confine acoustic tracking studies to relatively large N. norvegicus.Correspondence to: C. J. Chapman     

Survival and growth of the Norway lobster Nephrops norvegicus in relation to light-induced eye damage

 The long-term survival and growth of Norway lobsters, Nephrops norvegicus (L.), were examined in relation to light-induced retina breakdown. The incidence of eye damage was first assessed in tagged N. norvegicus released in Loch Torridon, on the west coast of Scotland between 1978 and 1983. Of 34 recaptures examined, all but four showed evidence of eye damage, varying in extent from 1 to 63% of the retina. Additional tagged N. norvegicus were released in Loch Torridon in 1984 and 1985 in order to compare recapture and growth rates in lobsters with and without eye damage. Three groups of N. norvegicus were released; normal sighted, partially sighted (median retina damage = 68%) and fully blinded (retina damage = 100%). From 1217 lobsters released, 235 were recovered by September 1992, when the experiment was concluded. The final proportions of N. norvegicus recaptured were found to be independent of eye-damage condition, and there was no evidence that growth rate was affected by eye damage. Combining all categories of releases, the overall proportion of females recaptured (0.244) was significantly greater than the proportion of males (0.145), suggesting better long-term survival in females than in males. At the time of release, ∼80% of the females were carrying recently spawned eggs. Although the proportion of berried females in the recaptures was slightly reduced in the two eye-damaged groups compared with the normal sighted group, the difference was not statistically significant. It is concluded that light-induced eye damage in N. norvegicus is irreversible, but such damage does not seem to influence their long-term survival, growth or reproduction. Received: 21 October 1998 / Accepted: 26 October 1999     

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     

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     

Effects of dusk and dawn on locomotor activity rhythms in the norway lobster Nephrops norvegicus

The endogenous locomotor activity rhythm of Nephrops norvegicus (L.) shows peaks during darkness, around 3 h after dusk. At the peaks, activity occurs mainly within the burrow, although the prawns repeatedly vacate the burrows during the same periods. The phase of the rhythm shifts with a change of light-dark regime and in response to changed timing of dusk when dawn is fixed. Activity is suppressed immediately at dawn, but recommences before complete darkness during gradually reduced light at simulated dusk. This difference in response to light at dusk and dawn indicates phase responsiveness which is characteristic of endogenous rhythms entrained by light. The light-entrained endogenous rhythm is complementary to the rhythm of emergence which, reflected in commercial catches, appears to be controlled by exogenous factors.     

Environmental hypoxia and haemocyanin between-individual variability in Norway lobsters Nephrops norvegicus (L.)

The extent to which exposure to environmental hypoxia (PO₂=8, 6, and 4 kPa, approximately 40%, 30%, and 20% saturation, respectively) resulted in an increase in the concentration of the respiratory pigment haemocyanin ([Hc]) in individuals of the Norway lobster Nephrops norvegicus depended crucially on the initial [Hc] as well as the intensity of the hypoxia itself. While mean [Hc] did increase with decreasing PO₂ (and the variation decreased), for individuals with relatively high initial [Hc] there was no change or even a decrease observed. The greatest hypoxia-related increases in [Hc] were observed in the individuals with the lowest initial [Hc]. Consequently the notion of an 'optimum' [Hc] for a given level of oxygenation was postulated. The changes in [Hc] recorded took place over a considerably shorter time scale (hours rather than days) than has been observed in previous studies. It was not possible to correlate environmental PO₂ and median [Hc] in the field. However, it was interesting that the [Hc] frequency distribution curve for lobsters collected from a site that had been markedly hypoxic (PO₂=8.0 kPa, approximately 40% saturation) until a few days before sampling was almost exactly the same shape (i.e. leptokurtic) and position (i.e. similar median) to that obtained when lobsters were exposed to PO₂=8.0 kPa in the laboratory. Although it would have been desirable to investigate the effect of hypoxia on the frequency distribution of a physiological trait, in this case [Hc], it was not possible due to insufficient sample size, even though the sample sizes employed were considerably larger than those normally used in physiological investigations.     

Observations on the burrows and burrowing behaviour of two mud-dwelling decapod crustaceans,Nephrops norvegicus and Goneplax rhomboides

The burrows of the Norway lobster Nephrops norvegicus (L.) and of the crab Goneplax rhomboides (L.) were studied in Loch Torridon, Scotland. Polyester resin casts of burrows in the sea were made by divers to reveal their subsurface form. Tunnels made by N. norvegicus were usually simple, with two or more openings on the mud surface, and penetrated to a depth of about 30 cm. G. rhomboides burrows did not descend more than about 15 cm beneath the surface, but were usually more complex than the lobster burrows and had several openings. The methods of burrow construction used by the two crustaceans are described from aquarium observations. Neither N. norvegicus nor G. rhomboides show obvious morphological adaptations for burrowing, and it is suggested that the fossorial habit was adopted very early by decapods. The burrows of N. norvegicus do not seem to have assumed any functions in addition to the original one of providing refuge from predators. There is not sufficient known of the biology of the crab to indicate whether the same is true in its case.     

Effects of hypoxia on the respiratory and circulatory regulation of Nephrops norvegicus

The burrowing decapod Nephrops norvegicus (L.) was kept under various degrees of hypoxia in order to measure respiration, heart rate, scaphognathite rate, haemolymph oxygen content and pH. An emergence reaction to hypoxia occurred only in dim light (<10^-2 m-c) or darkness, but after 10 d of moderate hypoxia the decapods showed no emergence response at all. The weight specific respiration of quiescent individuals was relatively low and increased only slightly in hypoxia (P_wO₂=40 torr). Heart rate, about 50 beats min^-1, changed little during hypoxia, down to P_wO₂=40 torr, whereas scaphognathite rates rose from about 60 beats min^-1 at normoxia to peak at 120 beats min^-1 at P_wO₂=40 torr. The oxygen extraction efficiency (E) remained at 20 to 30% during the first hour of hypoxia then rose gradually to maximum values of 30 to 40%. A small respiratory alkalosis of the blood became evident only after 4h of hypoxia (P_wO₂=50 torr). Normoxic postbranchial O₂ tensions (P_aO₂) were low (25–30 torr) and showed only a small decline during hypoxia. Over 10 to 13 d in moderate hypoxia an effective biosynthesis of 0.024 mM haemocyanin individual^-1 d^-1 occurred in fed decapods, whereas controls (normoxic) showed no significant change in pigment levels. A linear relationship between oxygen carrying capacity and haemocyanin concentration was found. It is contended that N. norvegicus is better able to cope with periodic exposure to hypoxia when food of sufficient quantity and quality is available.     

Production of Nephrops norvegicus larvae in the Irish Sea

Plankton survey data from 1982, 1985 and 1995 were used to estimate the annual production of Nephrops norvegicus (L.) larvae in the Irish Sea. The larval abundance data from each set of surveys were analysed using the same temperature-to-stage-duration relationships. The production of larvae was similar in all 3?yr at approx. 360?×?10⁹ Stage I larvae per year. The estimated production in the eastern Irish Sea was 5% of the value for the western Irish Sea. This corresponded well to the sizes of the fisheries and the relative areas of the mud patches. Larvae in both areas were found to hatch over 5½?wk, centred on the first week in May. The time lag between onset of production of each stage reflected the estimated stage durations, but the lag between maximum production of each stage was much shorter than expected.     

Field observations on the emergence rhythm of the Norway lobsterNephrops norvegicus,using different methods

The emergence ofNephrops norvegicus (L.) from their burrows was studied on several fishing grounds around Scotland at 28 to 62 m depth. Various methods were used: sampling by trawl and baited creels, observations by television and time-lapse cameras. Peak emergence generally occurred around dusk and dawn. In shallow water (30 m) there was also a significant level of activity during the intervening night period. Day/night variations in the size composition of catches suggest that relatively more time is spent out of the burrow with increasing size. There was good agreement between the various methods employed, suggesting that trawl and camera avoidance has little influence on the pattern of emergence observed. The use of bait in some experiments confirmed that emergence is essentially a feeding activity.     

The eye and some effects of light on locomotor activity in Nephrops norvegicus

The compound eye of Nephrops norvegicus (L.) is of the superposition type, well-adapted to the low levels of light prevailing at the sea bed during the activity periods of the species. Only the proximal retinal shielding pigment responds to light, the distal retinal shielding pigment being in the dark-adapted position at all times. The response of the proximal pigment appears to vary seasonally. Field observations compared light intensity at the sea bed with the numbers of N. norvegicus caught by trawl at various times of day in the Irish Sea in summer and winter. Laboratory experiments were combined with these field data to indicate that light is an important modulator of locomotor activity in this species.     

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     

Age and growth in Nephrops norvegicus (Decapoda: Nephropidae)

The growth of the lobster Nephrops norvegicus (L.) in laboratory tanks was studied. The moult increments of males and females were not significantly different. The growth per moult was of the progressive geometric type throughout the life of the lobster, including the larval instars. Moulting in both the laboratory and the natural population occurred throughout the year in males, but there was a peak of moulting activity in females from June to August. There was no indication of a terminal anecdysis. Size-frequency histograms from monthly samples taken 10 to 20 miles south-west of the Isle of Man over a period of 21/2 years provided data on the growth of the year classes. It is possible to recognise the year classes in females at least, until the lobsters are 3 to 4 years old. Instar determination based on the numbers of aesthetasc-bearing segments of the antennule and on the numbers of lamellae in the endocuticle was unsuccessful. Estimates of the numbers of moults occurring in each year, and the expected mean sizes of the year classes are given.     

Biochemical composition of Nephrops norvegicus : changes associated with ovary maturation

Changes in the biochemical composition of the ovary of Nephrops norvegicus (L.) were examined throughout maturation at stations with fast and slow growing individuals in Scottish waters and in the Mediterranean. Ovary index (ovary wt:total body wt) increased significantly with maturation. Lipid, protein and water content of the ovary increased with maturation, while carbohydrate content decreased. Similar changes were also identified in the hepatopancreas. Significant spatial differences in ovary and hepatopancreas composition were identified, which may be related to whole-body growth rate. The ovary maturation scale commonly used for N. norvegicus was shown to reflect biochemically distinct stages in ovary development. Received: 7 February 1997 / Accepted: 25 February 1997     

A bilateral gynandromorph of Nephrops norvegicus (Decapoda: Nephropidae)

A bilateral gynandromorph specimen of Nephrops norvegicus (L.) from the Irish Sea is described. Previous records of gynandromorphs within the Nephropidae are discussed, together with other types of sexual abnormality found in N. norvegicus. It is suggested that the bilateral asymmetry is due to a chromosomal aberration in the first embryonic cleavage division, and that further division is determinate. In addition, it appears that the androgenic gland in N. norvegicus, Homarus gammarus and H. americanus does not have the same mode of action as that reported for Orchestia gammarella and O. cavimana.     

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     

The influence of social experience on cooperative behaviour of rats (Rattus norvegicus): direct vs generalised reciprocity

Cooperation among non-kin has been attributed sometimes to reciprocal altruism: Two or more individuals exchange behaviour that benefits the respective partner. According to direct reciprocity, cooperation is based on past behaviour of a known partner. In contrast, in generalised reciprocity, cooperation is based on anonymous social experience where the identity of the partner is irrelevant. In a previous study, female Norway rats (Rattus norvegicus) were found to cooperate according to a generalised reciprocity mechanism. In this study, we tested whether Norway rats would also cooperate as predicted by a direct reciprocity mechanism and whether direct reciprocity would cause a higher propensity to cooperate than generalised reciprocity. Focal animals were experimentally manipulated to receive social experience from known or unknown, helpful or defecting partners in an instrumental cooperative task. Our first experiment shows that rats are more helpful towards a partner from which they had received help before than towards a partner that had not helped (i.e. direct reciprocity). Our second experiment revealed that after receiving help by others, rats were more helpful towards a partner from which they had received help before than towards a new partner (i.e. direct reciprocity generated a higher cooperation propensity than generalised reciprocity). We conclude that in female Norway rats, the tendency to cooperate is influenced by partner-specific information. To our knowledge, this is the first study to demonstrate direct reciprocity in rodents, and it is the first study testing direct vs generalised reciprocity in animals.     

Inter-individual and within-brood variability in the fatty acid profiles of Norway lobster, <Emphasis Type="Italic">Nephrops norvegicus</Emphasis> (L.) embryos

The present study investigated the existence of inter-individual and within-brood variability in the fatty acid (FA) profile of developing embryos of Nephrops norvegicus. In all surveyed females (n = 5), the quantitatively most important FAs were as follows: 22:6n-3 (20.8 ± 3.9% average of total FA ± standard error), 18:1n-9 (19.5 ± 2.0%), 16:0 (15.2 ± 3.4%), 20:5n-3 (10.2 ± 1.4%), 16:1n-7 (8.9 ± 1.6%), and 18:1n-7 (5.7 ± 1.3%). Differences in FA profiles of embryos in the same clutch were assessed using brooding chamber side (left and right) and pleopod (1st and 2nd, 3rd and 4th, and 5th) as predictive factors. There were no significant differences in the FA composition of embryos sampled from both sides of the brooding chamber in 4 of the 5 surveyed females. However, all females exhibited significant differences in the FA profiles of embryos sampled from different pleopods. Both saturated FA (SFA) and highly unsaturated FA (HUFA) present in developing embryos exhibited marked differences along the breeding chamber. Overall, FA reserves appeared to vary significantly within broods, which can ultimately be reflected on early larval survival. A potential cause for the within-brood variation recorded in the FA profile of developing embryos include (1) differential female investment during ovarian maturation, mainly due to variation in food quality/availability; (2) differential lipid catabolism during the incubation period of developing embryos, as a consequence of embryos position within the female’s brooding chamber; or (3) differential female investment during ovarian maturation amplified by differential lipid catabolism during the incubation period.