Behavioral adaptations of the cubozoan medusa <Emphasis Type="Italic">Tripedalia cystophora</Emphasis> for feeding on copepod (<Emphasis Type="Italic">Dioithona oculata</Emphasis>) swarms

The cubozoan medusa Tripedalia cystophora preys on dense swarms of the copepod Dioithona oculata in the mangrove prop-root habitat of Puerto Rico. The copepod swarms form in shafts of sunlight that penetrate the mangrove canopy during the day. T. cystophora are found primarily within the same illuminated areas, feeding heavily on the dense swarms of copepods. Laboratory studies were conducted to examine the behavioral adaptations of T. cystophora that allow them to remain within these dense copepod swarms. In the presence of vertical light shafts, T. cystophora showed both a significant change in swimming speed and a significant increase in turning rate. T. cystophora turn abruptly when they leave a vertical shaft of light and quickly re-enter. The presence or absence of copepod prey, either in swarms or dispersed, had no significant effect on T. cystophora swimming speed or turning behavior. Feeding rates of T. cystophora on the copepod D. oculata increase rapidly with copepod density, and maximum rates are not achieved until the high densities associated with swarms are reached. It seems doubtful that these small medusae could survive without their ability to encounter and exploit these dense swarms of copepods.     

Mechanics of prey selection by ephyrae of the scyphomedusa Aurelia aurita

In situ feeding patterns of ephyrae of the jellyfish Aurelia aurita (Linnaeus) revealed the importance of relatively large (>1 mm) prey in the diet of these scyphozoan predators. These studies were carried out in Narragansett Bay, Rhode Island, USA in March and April, from 1993 through 1996. Rotifers were the only small prey ingested in quantity, and then only when they were unusually abundant in the plankton. Copepod nauplii, similar in size to rotifers and equally abundant, were rarely consumed. Since copepods evince rapid escape responses, this observation suggested a role for prey escape in determining prey vulnerability, while the predominance of large prey in the diet suggested a role for prey size. Using two dimensional video observations of free-swimming ephyrae and their prey in the laboratory we tested hypotheses about the mechanisms underlying these dietary patterns, comparing mechanisms for capture of large versus small prey and for prey of equal size but differing escape behaviors. Capture efficiencies of ephyrae feeding on large prey were 4 to 12 times greater than for small prey taxa. Capture efficiencies for prey of equal size also differed significantly, indicating that other factors influence the outcome of predator–prey interactions. Most prey captures occurred while the ephyrae were swimming and creating fluid flows that entrained prey into the subumbrellar region. Even copepod nauplii were frequently drawn into the subumbrella of swimming ephyrae despite average potential escape velocities (25.7 mm s⁻¹) that exceeded mean maximum velocity of fluid flows around the ephyrae (13.1 mm s⁻¹). Large prey were more likely than small prey to contact nematocyst-bearing surfaces both before and after entrainment in flow fields. With regard to behavior, prey escape speeds were not the only predictor of prey vulnerability. Prey that continued swimming after entrainment (rotifers and brine shrimp) were captured more often than prey of equal size that ceased normal swimming (copepod nauplii and barnacle nauplii). Copepod nauplii were the prey least likely to be captured because they either “played dead” and were expelled from the subumbrella of the ephyrae before contacting a surface, or they eventually escaped at high velocity. These observations indicate that size-selective predation by ephyrae of A. aurita can be influenced by a variety of behavioral responses of the prey. Received: 9 April 1997 / Accepted: 5 September 1997     

Contribution of salps to carbon flux of marginal ice zone of the Lazarev Sea, southern ocean

In order to estimate the in situ grazing rates of Salpa thompsoni and their implications for the development of phytoplankton blooms and for the sequestration of biogenic carbon in the high Antarctic, a repeat-grid survey and drogue study were carried out in the Lazarev Sea during austral summer of 1994/1995 (December/January). Exceptionally high grazing rates were measured for S. thompsoni at the onset of a phytoplankton bloom (0.2 to 0.8 μg chlorophyll a l⁻¹) in December 1994, with up to ≃160 μg of plant pigments consumed by an individual salp of 7 to 10 cm length per day. Dense salp swarms extended throughout the marginal ice zone, consuming up to 108% of daily phytoplankton production and 21% of the total chlorophyll a stock. Due to the much faster sinking rates and higher carbon content of salp faecal pellets, the efficiency of downward carbon flux through salps is much higher than through the other major grazers, krill and copepods. S. thompsoni can thus export large amounts of biogenic carbon from the euphotic zone to the deep ocean. With the observed ingestion rates during December 1994, this flux could have attained levels of up to 88 mg C m⁻² d⁻¹, accounting for the bulk of the vertical transport of carbon in the Lazarev Sea. However, in January 1995, when phytoplankton concentrations exceeded a threshold level of 1.0 to 1.5 μg chlorophyll a l⁻¹, salps experienced a drastic reduction in their feeding efficiency, possibly as a result of clogging of their filtering apparatus. This triggered a dramatic reversal in the relationship, during which a dense phytoplankton bloom developed in conjunction with the collapse of the salp population. Increases in the biomass and geographic range of the tunicate S. thompsoni have occurred in several areas of the southern ocean, often in parallel with a rise in sea-surface temperature during sub-decadal periods of warming anomalies. Received: 10 August 1997 / Accepted: 21 October 1997     

Energy expenditure of swimming bottlenose dolphins (Tursiops truncatus)

The aim of our investigations was to determine, via oxygen and carbon-dioxide respirometry, how much energy dolphins (Tursiops truncatus) require when swimming at different speeds. Experiments were conducted on two female bottlenose dolphins (mean mass 162 kg) in the dolphinarium in Nuremberg Zoo, Germany, between March and August 1997. Animals were stationed in a respiration chamber for a minimum of 90 s after performing a variety of activities. We measured respiration frequency and oxygen requirements during (1) resting, (2) swimming at various velocities and (3) leaping to various heights. Resting metabolic rate of our bottlenose dolphins (2.15 W kg⁻¹) was comparable to previously published data. Metabolic rate in swimming dolphins increased to 2.47 W kg⁻¹ at 2 m s⁻¹, while leaps to 2.2 and 3 m height required a power input of 3.5 and 4 W kg⁻¹, respectively. Transport costs of swimming dolphins were lowest (1.16 J kg⁻¹ m⁻¹, corresponding to 0.12 J N⁻¹ m⁻¹) at a speed of 2.5 m s⁻¹, yielding an optimal range speed of between 1.9 and 3.2 m s⁻¹ (corresponding to minimum cost of transport ±10%). Breathing rates during all experiments correlated very well with oxygen consumption (r ² > 0.89) and could be used to derive metabolic rates in unencumbered dolphins at sea. Received: 18 December 1998 / Accepted: 27 April 1999     

Vulnerability of the copepod Acartia tonsa to predation by the scyphomedusa Chrysaora quinquecirrha : effect of prey size and behavior

Although scyphomedusae have received increased attention in recent years as important predators in coastal and estuarine environments, the factors affecting zooplankton prey vulnerability to these jellyfish remain poorly understood. Current models predicting feeding patterns of cruising entangling predators, such as Chrysaora quinquecirrha (Desor, 1948), fail to account for the selection of fast-escaping prey such as copepods. Nevertheless, our analysis of gastric contents of field-collected medusae showed that this scyphomedusa fed selectively on the calanoid copepod Acartia tonsa (Dana, 1846) and preferentially ingested adult over copepodite stages. We measured feeding rates in a planktonkreisel while simultaneously videotaping predator–prey interactions. C. quinquecirrha consumed adult A. tonsa ten times faster than copepodites. Differences in prey behavior, in the form of predator–prey encounter rates or post-encounter escape responses, could not account for the elevated feeding rates on adults. Prey size, however, had a dramatic impact on the vulnerability of copepods. In experiments using heat-killed prey, feeding rates on adults (1.5 times longer than copepodites) were 11 times higher than on copepodites. In comparison, medusae ingested heat-killed prey at only two to three times the rate of live prey. These results suggest that during scyphomedusan–copepod interactions, prey escape ability is important, but ultimately small size is a more effective refuge from predation. Received: 26 September 1997 / Accepted: 20 May 1998     

Environmental preferences of yellowfin tuna (Thunnus albacares) at the northern extent of its range

We used acoustic telemetry to examine the small-scale movement patterns of yellowfin tuna (Thunnus albacares) in the California Bight at the northern extent of their range. Oceanographic profiles of temperature, oxygen, currents and fluorometry were used to determine the relationship between movements and environmental features. Three yellowfin tuna (8 to 16 kg) were tracked for 2 to 3 d. All three fish spent the majority of their time above the thermocline (18 to 45 m in depth) in water temperatures >17.5 °C. In the California Bight, yellowfin tuna have a limited vertical distribution due to the restriction imposed by temperature. The three fish made periodic short dives below the thermocline (60 to 80 m), encountering cooler temperatures (>11 °C). When swimming in northern latitudes, the depth of the mixed layer largely defines the spatial distribution of yellowfin tuna within the water column. Yellowfin prefer to spend most of their time just above the top of the thermocline. Oxygen profiles indicated that the tunas encountered oceanic water masses that ranged most often from 6.8 to 8.6 mg O₂ l⁻¹, indicating no limitation due to oxygen concentrations. The yellowfin tuna traveled at speeds ranging from 0.46 to 0.90 m s⁻¹ (0.9 to 1.8 knots h⁻¹) and frequently exhibited an oscillatory diving pattern previously suggested to be a possible strategy for conserving energy during swimming. Received: 14 February 1997 / Accepted: 14 April 1997     

Physiological and behavioural responses of the intertidal scavenging gastropod Nassarius festivus to salinity changes

Physiology (oxygen consumption and ammonia excretion) and behaviour (feeding and activity) of the intertidal gastropod Nassarius festivus (Powys) at five different salinities [15, 20, 25 (control), 30 and 35‰] were studied for 4 weeks. Oxygen consumption and ammonia excretion rates were reduced immediately after salinity was either elevated or reduced. Subsequently, both rates were increased while the O:N ratio was decreased at all salinities, including the control, in the first 2 weeks and then levelled off. Such changes were probably attributed to osmotic adjustment and reproductive activity. Activity and feeding were reduced at low salinity, particularly in the first week. Reproductive output, in terms of the total number of egg capsules and the mean number of eggs per capsule, was also lowered at reduced salinities. Nevertheless, individuals at all salinities are able to maintain a positive energy balance. Results are discussed with respect to the distribution of N. festivus in Hong Kong waters. Received: 14 April 1997 / Accepted: 9 May 1997     

Physiological effects of the detergent linear alkylbenzene sulphonate on blue mussel larvae (Mytilus edulis) in laboratory and mesocosm experiments

A series of laboratory (short-term exposure in small beakers) studies and a 19 d mesocosm (6 m³ polyethylene bags filled with fjord water) study were conducted on blue mussel, Mytilus edulis, larvae and plantigrades exposed to a concentration gradient of the detergent linear alkylbenzene sulphonate (LAS, 0 to 39 mg l⁻¹). LAS is increasingly found in nearshore environments receiving wastewater from urban treatment plants. The aims were to observe physiological effects on swimming, grazing and growth in the laboratory and effects on settling and population development at in situ conditions (in field mesocosms) in order to evaluate the damages on ciliated meroplankton caused by LAS. In the laboratory the larvae showed a 50% mortality at 3.8 mg LAS l⁻¹ after 96 h exposure whether or not food was provided. Additionally the swimming behaviour was affected at 0.8 mg LAS l⁻¹ (i.e. a more compact swimming track, a smaller diameter of the swimming tracks, and reduced swimming speed). The larval particle grazing was reduced 50% at 1.4 mg LAS l⁻¹. The specific growth rate of the larvae was reduced to half at 0.82 mg LAS l⁻¹ over 9 d. During the mesocosm experiment, the larval population showed a dramatic decrease in abundance within 2 d at concentrations as low as 0.08 mg LAS l⁻¹, both due to a significantly increased mortality, but also due to settling. The settling success was reduced at the same LAS concentration as that at which mortality was observed to increase significantly. In addition to reduced settling rate, the larvae showed delayed metamorphosis and reduced shell growth as a response to LAS. Our hypothesis that the larval ciliary apparatus, crucial for normal swimming, orientation, and settling behaviours and for particle uptake, was damaged due to LAS exposure is supported by our results. This is confirmed by the physiological data (grazing, growth) and in the direct video-based observations of larval performance (swimming) and provides a reasonable explanation for what was observed in the bags (abundance, settling, mortality). These physiological effects on blue mussel larvae/plantigrades occurred at LAS concentrations reported to occur in estuarine waters. Received: 15 January 1997 / Accepted: 12 February 1997     

Zooplankton growth rates: the influence of size in nauplii of tropical marine copepods

Growth rates and development times were determined for nauplii of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanus and Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high local temperatures (∼28 °C), total naupliar development time was short: 3 to 4 d inshore and 4 to 5 d offshore. Mean instantaneous growth rates (g) ranged from as high as 0.90 d⁻¹ for Parvocalanuscrassirostris to as low as 0.41 d⁻¹ for Corycaeus spp. In general, nauplii of cyclopoid copepods appeared to grow more slowly than those of calanoids of the same size. Naupliar growth rates were significantly related to body size (r ²= 0.43 to 0.50), but were unrelated to chlorophyll concentration in any measured size-fraction. This suggests that nauplii are generally not limited by resources, but are growing at their maximum temperature and size-dependent rates. Received: 30 May 1997 / Accepted: 13 May 1998     

Daytime surface swarms of the euphausiid Thysanoessa inermis off the west coast of Hokkaido,northern Japan

Biological characteristics of the daytime swarming of the euphausiid Thysanoessa inermis (Krøyer) were examined off the west coast of Hokkaido in the northeast Sea of Japan between 1982 and 1987. Swarms were composed exclusively of fully mature individuals; males possessed spermatophores at the ejaculatory ducts and females had attached spermatophores at the thelycum. Females predominated in most swarms, while males or nearly balanced swarms were infrequently encountered. Fully mature females occurred in the late swarming season, usually mid-April. Examination of stomachs indicated that feeding activity was low during the daytime surface swarming behavior. The body sizes of swarming T. inermis occurring around Yagishiri Island were significantly larger than those found simultaneously off the Shakotan Peninsula. This suggests that the euphausiid populations of the two areas are different.     

Effects of prey escape ability,flow speed,and predator feeding mode on zooplankton capture by barnacles

Experimental studies of feeding on zooplankton often involve the use of non-evasive Artemia spp. to represent zooplanktonic prey. Some zooplankton, however, such as copepods, are potentially evasive due to possession of effective predator-avoidance mechanisms such as high-speed escape swimming. In the present study, we compared the efficiencies with which non-evasive (A. salina) and evasive (copepods) zooplankton were captured by a sessile, suspension feeder, the coral-inhabiting barnacle Nobia grandis (Crustacea, Cirripedia). N. grandis specimens and zooplankton used in the present study were collected near Eilat, Israel in 1993. The effect of different flow speeds (from 0 to 14 cm s^-1) on captures of the two preys was also investigated. Additionally, we examined the effect of a flow-induced barnacle behavioral switch from active to passive suspension feeding, on zooplankton capture. Two video cameras were used to make close-up, three dimensional recordings of predator-prey encounters in a computer-controlled flow tank. Frame-by-frame video analysis revealed a highly significant difference (P< 0.001) in the efficiency with which A. salina and copepods were caught (A. salina being much more readily captured than copepods). After an encounter with cirri of feeding barnacles, copepods were usually able to swim out of the barnacles capture zone within one video frame (40 ms), by accelerating from a slow swimming speed (approximately 1.85 cm s^-1) to a mean escape swimming speed of 18.11 cm s^-1 (ca. 360 body lengths s^-1). This was not the case for A. salina nauplii, which usually remained in contact with cirri before being transferred to the mouth and ingested. Thus, experimental studies addressing the methodology of organisms feeding on zooplankton should consider that slow-swimming prey like Artemia sp. nauplii may only represent the non-evasive fraction of natural mesozooplankton assemblages.     

Physiological traits associated with a reproductive-resting stage in Coullana canadensis (Copepoda: Harpacticoida)

We investigated physiological traits associated with a reproductive-resting stage in adult female harpacticoid copepods, Coullana canadensis (Willey). Our hypothesis was this stage represents a life-history strategy that could increase fitness by improving winter survivorship and future reproductive success in spring. To test if physiological rates are suppressed in this stage, we compared gut-cell morphology, nitrogen excretion rates, enzyme activities, and phytoplankton grazing rates of reproductive-resting and reproductive females reared in the laboratory under high food conditions. Copepods came from laboratory cultures originating from individuals collected in Maine and Maryland, USA in 1990. Reproductive-resting females had lower physiological rates, and the surface area of gut-cells was reduced compared to reproductive females. Distinct morphological differences in the distribution of lipids between reproductive and reproductive-resting females were observed under light and electron microscopy, the latter having a diffuse accumulation of lipid in the area normally occupied by the ovaries. Differences in lipid composition were also found. Reproductive copepods had a significantly higher ratio (%) of polar lipids to total lipid, and a lower proportion of triacylglycerols compared to reproductive-resting copepods. These laboratory findings are consistent with the hypothesis that the reproductive-resting stage in C. canadensis is an adaptive response to increase winter survival. Received: 29 May 1996 / Accepted: 3 December 1997     

Zooplankton growth rates: the influence of size and resources in tropical marine copepodites

Growth rates were determined for copepodites of the genera: Acartia, Centropages, Corycaeus, Oithona, Paracalanus, Parvocalanus and Temora in nearshore waters of Jamaica from in situ microcosm incubations. At these high local temperatures (∼28 °C), total copepodite development time was as short as 4 to 5 d. Mean instantaneous growth rates (g) ranged from as high as 1.2 d⁻¹ to as low as 0.1 d⁻¹. In general, cyclopoid copepods appeared to grow more slowly than calanoids of the same size. Enhancement of resources by nutrient addition caused a 32% increase in growth rates in experiments from a mesotrophic site, but only a 17% increase at a more eutrophic site. Additionally, copepodites at both sites showed faster development and generally larger size at stage in response to nutrient addition. Growth rates were positively related to chlorophyll concentration in the >2 μm size-fraction. A significant relationship of growth rate to body size (r ² = 0.45) emerged across a wide range of trophic status, but it was confounded with resource availability. It appears that growth in tropical copepod copepodites may be frequently limited by resources in a size-dependent manner. Received: 30 May 1997 / Accepted: 13 May 1998     

Feeding and predation impact of two chaetognath species, Eukrohnia hamata and Sagitta gazellae, in the vicinity of Marion Island (southern ocean)

The predation impact of the two chaetognaths Eukrohnia hamata and Sagitta gazellae on mesozooplankton standing stock were investigated in three depth layers during two 24 h stations occupied in the vicinity of Marion Island in late austral summer (April/May) 1986. The zooplankton community at both stations was dominated by small copepods (Oithona spp., Microcalanus spp.), which accounted for >95% of total zooplankton abundance. Chaetognaths comprised <2% of total zooplankton abundance. E. hamata constituted >95% of the total chaetognath stock. The general trend in both species was decreasing abundance with increasing depth, which appeared to be correlated to the distribution of copepods (r ² = 0.45; P <0.05). Gut-content analysis showed that copepods (mainly Oithona spp., Calanus spp. and Rhincalanus gigas) and ostracods were the main prey of both species, accounting for 87 and 61% of the total number of prey in E. hamata and S.␣gazellae stomachs, respectively. In the guts of S.␣gazellae, pteropods (Limacina spp.) and chaetognaths were also well represented. The mean number of prey items (NPC) for E. hamata ranged from 0.02 to 0.06 prey individual⁻¹ which corresponds to an individual feeding rate (Fr) of between 0.05 and 0.12 prey d⁻¹. For S.␣gazellae, the NPC values were higher, varying between 0.04␣and 0.20 prey individual⁻¹, or between 0.15 and 0.76 prey d⁻¹. The daily predation impact of the two chaetognaths was estimated at between 0.3 and 1.2% of the copepod standing stock or between 7 and 16% of the daily copepod production. Predation by S. gazellae on chaetognaths accounted for up to 1.6% of the chaetognath standing stock per day. Received: 26 November 1996 / Accepted: 31 October 1997     

Role of protein in larval swimming and metamorphosis of Bugula neritina (Bryozoa: Cheilostomatida)

Larvae of the marine cheilostomatid bryozoan Bugula neritina (L.) were prevented from settling for 1, 4 and 8 h by mechanical agitation, following which settlement and metamorphosis success were examined. Settlement rates were significantly affected by swimming time, which decreased from 100% after 2 h to 93.7 ± 4.3% after 8 h. Similarly, metamorphosis to the feeding ancestrula was significantly impaired following a swimming time of 8 h, declining from 93.7 ± 4.3% after 1 h to 65.9 ± 7.0% after 8 h. The resultant colonies grew well for the first 3 wk, following which time, growth patterns became erratic. Growth rate was in all cases highly variable, and did not correlate with enforced swimming times. Larval protein composition was examined after 1, 4 and 8 h swimming time, and post-larval composition 1, 2, 5, 24 and 48 h after settlement using sodium-dodecyl-sulphate polyacrylamide-gel electrophoresis (SDS-PAGE). Individual protein content was measured using a densitometer. Larvae did not consume protein during swimming, however a protein measuring 170 kdaltons was consumed during metamorphosis. These results are discussed in the context of larval settlement and energetics. Received: 19 July 1998 / Accepted: 3 December 1998     

Fitting fertilisation kinetics models for free-spawning marine invertebrates

 To determine how fertilisation varied with sperm concentration for two species of scallop, Chlamys (Equichlamys) bifrons (Lamarck) and C. asperrima (Lamarck), we performed a simple series of sperm dilution experiments, and measured egg size and sperm swimming speeds. C. bifrons eggs were much larger (average diam=116.5 μm), and sperm swimming speeds faster (209.8 μm s⁻¹), than C. asperrima (71.2 μm, 166.0 μm s⁻¹). In both species, maximum fertilisation occurred at an ambient sperm concentration of around 100 sperm μl⁻¹; the maximum proportion of eggs fertilised was less than 0.70 in the C. bifrons experiments, but nearer 1.0 with C. asperrima. At high sperm concentrations (>100 sperm μl⁻¹), fertilisation decreased (presumably due to polyspermy) with increasing sperm concentration, but decreased more rapidly in C. bifrons than C. asperrima. A polyspermy-adjusted fertilisation kinetics model could be fitted to the experimental data, but unique parameter estimates could not be determined. Received: 7 October 1999 / Accepted: 8 July 2000     

Feeding and bacteriolytic responses of the deposit-feeder Abarenicola pacifica (Polychaeta: Arenicolidae) to changes in temperature and sediment food concentration

Deposit-feeders can respond to seasonal fluctuations in food concentration both functionally (e.g. by adjusting feeding rates) and physiologically (e.g. by changing the concentration of bacteriolytic agents in gut fluids). Laboratory feeding experiments were carried out (11 to 21 July 1997) with the arenicolid polychaete worm Abarenicola pacifica (Healy and Wells). Objectives were to test for separate and interactive effects of sediment food concentration and temperature (6, 11, and 16 °C) on deposit-feeder functional (feeding rates) and physiological (bacteriolytic activity of gut fluids) responses. Food concentration was varied experimentally using sieved (1 mm) natural sediments (Md φ=2.00; 0.6% organic) mixed with combusted (500 °C, 8 h) sediments for final concentrations of 25, 50, and 100% natural sediment. Sediment food quality was measured as: (1) bioavailable amino acids (EHAA), (2) chlorophyll a (chl a), and (3) bacterial abundance. Feeding rates were inferred from egestion rates (ER, g h⁻¹) and analyzed with respect to worm size. Bacteriolytic activity of midgut fluids was assayed turbidimetrically against two bacterial isolates, after worms had fed on experimental sediments for 15 d. Temperature and food concentration both significantly affected feeding rates, with maxima occurring at 50 and 100% natural sediment mixtures, and at high (16 °C) temperature. ER was positively, but not significantly correlated with EHAA and chl a; a positive, significant correlation was detected between ER and sediment bacterial abundance. Overall, functional responses agreed with earlier compensatory intake models for deposit-feeders. However, the size and direction of these responses was temperature-sensitive, suggesting that these models need to be adjusted for changes in absorption rates. No effects of ambient temperature or food concentration on bacteriolytic rates were observed, possibly due to compensatory mechanisms or the presence of multiple bacteriolytic agents in gut fluids. Received: 28 June 1999 / Accepted: 14 March 2000     

Variation in foraging effort by lactating Antarctic fur seals: response to simulated increased foraging costs

Seasonally breeding predators, which are limited in the time available for provisioning young at a central location, and by the fasting abilities of the young, are likely to maximize energy delivery to the young by maximizing the rate of energy delivery averaged over the whole period of investment. Reduction in food availability or increased foraging costs will alter the optimal behavior of individuals. This study examined the behavioral adaptations of a diving predator, the Antarctic fur seal, to increased foraging costs during lactation. One group of mothers (n=5, treatment) was fitted with additional drag to increase the cost of transport in comparison with a control group (n=8). At the scales of the individual dives, the treatment group made more shorter, shallower (< 30 m) dives. Compensation for slower swimming speeds was achieved by diving at a steeper angle. Overall, diving behavior conformed to several specific theoretical predictions but there were also departures from theory, particularly concerning swimming speed during diving. Diving behavior appears to be adjusted to maximize the proportion of time spent at the bottom of dives. At the scale of diving bouts, no difference was observed between the treatment and control groups in terms of the frequency and duration of bouts and there was also no difference between the two groups in terms of the proportion of time spent diving. At the scale of complete foraging cycles, time taken to return to the pup was significantly longer in the treatment group but there was no difference in the rate of delivery of energy (measured from pup growth rate) to the pups in each group. Since mothers in the treatment group did not use significantly more body reserves, we conclude that behavioral adjustments at the scale of individual dives allowed mothers in the treatment group to compensate for the additional foraging costs. Pup growth rate appears to be less sensitive to the foraging conditions experienced by mothers than foraging trip duration. Received: 14 June 1996 / Accepted after revision: 16 November 1996     

Effect of amino acids on the swimming activity of newly hatched turbot larvae (Scophthalmus maximus)

The swimming behaviour of newly hatched turbot (Scophthalmus maximus L.) larvae was observed in artificial seawater (ASW) and in solutions of 21 l-amino acids at a concentration of 10⁻⁵  M. The behaviour of 20 larvae was analysed in each solution. Each larva was observed for 1 min. Individual movements were recorded on video and analysed using a computer-assisted program. The larvae swam in convoluted, randomised three-dimensional paths, rested and started swimming again. There were large variations in the swimming behaviour of turbot larvae during ontogeny. In ASW the mean frequency of trajectories longer than a body length of 4 mm larva⁻¹ min⁻¹ increased from 1.2 at Day 1, to 10 at Day 4. Analysing the data (Dunnett's method) revealed that the frequency of swimming trajectories increased in the presence of glycine, histidine and glutamine, and decreased in the presence of proline. The total distance swum increased for glycine but decreased for proline. The threshold concentration for glycine detected by turbot larvae was 10⁻⁵  M. The straightness index did not change in the presence of the amino acids. The possible role of these changes in behaviour is discussed. Received: 12 June 1997 / Accepted: 13 January 1998     

Effects of ultraviolet-B enhancement on marine trophic levels in a stratified coastal system

We examined the effects of enhanced UV-B radiation (relative to ambient) on marine trophic levels inhabiting a stratified coastal ecosystem, using living models (13 000 liter marine enclosures) of a temperate estuarine water column. The experiment was carried out in June and July 1994 on a plankton community drawn from lower Narragansett Bay, Rhode Island, USA. The effects of altered UV-B radiation (elevated 50% over ambient, tenfold DNA-weighted) on three trophic strata: the primary producers (photosynthetic algae), primary herbivores (copepods), and fish eggs and larvae (Anchoa mitchilli Cuvier and Valenciennes) were examined. The goal was to determine if UV-B–induced alterations at the base of the food chain had impacts on other elements of the trophic web. Phytoplankton abundance (P=0.02) and biomass (P=0.007) were significantly reduced in UV-B–enhanced treatments, above but not below the thermocline (2.25 m), during the second week of the study. Copepod nauplii were significantly less abundant in UV-B–enhanced mesocosms than in control treatments during the third and fourth weeks of the study (P=0.01). A portion of the impact on nauplii may be a result of alterations at the base of the food web. The greater mortality of nauplii in UV-B–enhanced systems did not translate to reduced abundance of copepodite (P=0.83) or adult (P=0.29) copepods. No significant effects were observed for microzooplankton (P=0.15). Neither the mortality rates nor the growth rates of larval anchovy were significantly affected by the experimental increase in UV-B (P>0.05). Despite the tenfold increase in biologically damaging UV-B, effects were not seen at higher trophic levels, most likely because of the rapid extinction of UV-B in the highly colored coastal water. Received: 13 November 1996 / Accepted: 18 July 1997