Critical swimming speeds of late-stage coral reef fish larvae: variation within species,among species and between locations

The swimming abilities of larval fishes are important for their survival, potentially affecting their ability to avoid predators, obtain food and control dispersal patterns. Near settlement swimming abilities may also influence spatial and temporal patterns of recruitment. We examined Critical speed (U-crit) swimming ability in late stage larvae of 89 species of coral reef fishes from the Great Barrier Reef and the Caribbean. Coefficients of variation in U-crit calculated at the individual level were high (28.4%), and this was not explained by differences in size or condition factor of these same larvae. Among species U-crit ranged from 5.5 cm s⁻¹ to 100.8 cm s⁻¹ (mean=37.3 cm s⁻¹), with 95% of species able to swim faster than the average current speed around Lizard Island, suggesting that most species should be capable of influencing their spatial and temporal patterns of settlement. Inter-specific differences in swimming ability (at both the family and species levels) were significantly correlated with size and larval morphology. Correlations were found between swimming performance and propulsive area, fineness ratio and aspect ratio, and these morphological parameters may prove useful for predicting swimming ability in other taxa. Overall, the swimming speeds of larvae from the same families at the two locations were relatively similar, although the Lutjanidae and Acanthuridae from the Caribbean were significantly slower than those from the great barrier reef. Differences in swimming speed and body form among late stage larvae suggests that they will respond differently to factors influencing survival and transport during their pelagic phase, as well as habitat use following settlement.     

Experimental analysis of the contribution of swimming and drifting to the displacement of reef fish larvae

The extent to which behaviour affects the dispersal of pelagic larvae in reef fishes has been a topic of major discussion among marine ecologists. Here, we experimentally quantified the extent to which the displacement of late-stage larvae of Abudefduf saxatilis is due to active movement (i.e. swimming) and drifting. We consider drifting as the component of larval displacement accounted for by the current. Drifting was quantified by comparing larval displacement to the displacement of passive particles in an extended flow chamber that gave larvae the free choice of swimming (i.e. swim with or against the current or not swim at all). We also determine whether drifting results from currents exceeding larval swimming capabilities or from the behavioural choice of larvae of not to swim against adverse currents. To do this, we compare the speeds of larval swimming in the extended flow chamber to those obtained in a smaller chamber in which larvae are behaviourally forced to swim due to space constraints and a retaining fence (most available data on larval swimming is based on this sort of chamber). Within the extended chamber, larvae tended to face the current and swim slower than it. This resulted in a net displacement increasingly determined by drifting. We also found that in the extended chamber, larvae swam at speeds between one and six times slower than the speeds they achieved in the “behaviourally modifying” smaller chamber. This suggests that the net displacement in the extended chamber was in part due to the behavioural choice of the larvae of not to swim. The importance of this “behavioural drifting” is discussed in terms of energy savings required for successful completion of the larval period and post-settlement survival. The idea that larvae may modulate their swimming behaviour raises caution for the use of published data regarding swimming capabilities of reef fish larvae when assessing the extent to which these fish actively affect their dispersal.     

Metabolic rates of juvenile scalloped hammerhead sharks (Sphyrna lewini)

Oxygen consumption of juvenile scalloped hammerhead sharks, Sphyrna lewini, was measured in a Brett-type flume (volume=635 l) to quantify metabolic rates over a range of aerobic swimming speeds and water temperatures. Oxygen consumption (log transformed) increased at a linear rate with increases in tailbeat frequency and swimming speed. Estimates of standard metabolic rate ranged between 161 mg O₂ kg^-1 h^-1 at 21°C and 203 mg O₂ kg^-1 h^-1 at 29°C (mean-SD: 189ᆣ mg O₂ kg^-1 h^-1 at 26°C). Total metabolic rates ranged from 275 mg O₂ kg^-1 h^-1 at swimming speeds of 0.5 body lengths per second (L s^-1) to a maximum aerobic metabolic rate of 501 mg O₂ kg^-1 h^-1 at 1.4 L s^-1. Net cost of transport was highest at slower swimming speeds (0.5-0.6 L s^-1) and was lowest between 0.75 and 0.9 L s^-1. Therefore, these sharks are most energy efficient at swimming speeds between 0.75 and 0.9 L s^-1. These data indicate that tailbeat frequency and swimming speed can be used as predictors of metabolic rate of free-swimming juvenile hammerhead sharks.     

Acoustical identification of the concentration layer of a copepod species,Calanus euxinus

Swimming trajectories of Calanus euxinus Hulsemann in the Black Sea were studied using an echosounder at 120 and 200 kHz. C. euxinus were acoustically discriminated with respect to vertical migration and swimming speed, according to dissolved oxygen (DO) concentration and the timing of migrations. Species became torpid in water with DO values <0.5 mg l^у. The time spent swimming under DO conditions between 2 and 5 mg l^у was insignificant, and varied greatly from the 10% to 25% of total time spent swimming under normoxic conditions (5-10 mg l^у). C. euxinus formed a concentration layer in the water of 1-3 m thickness. Upward migration was completed in about 3.5 h, starting 2.5 h before and ending 1 h after sunset (average rate: 0.95 cm s^у) in summer. Species ascended discretely from the suboxic to the lower boundry of the cold intermediate layer (CIL) at 0.82 cm s^у, and passed up the CIL and thermocline fast (2.3 cm s^у). Downward migration took less time (2 h), starting ~1 h before and ending ~1 h after sunrise. Swimming speed within the thermocline and CIL was 2.7 cm s^у; copepods subsequently returned to daylight depth at a sinking speed of 0.57 cm s^у. Total time for C. euxinus to settle to their nocturnal depth layer was about 5 h.     

Some quantitative aspects of seagrass ecology in a coastal lagoon of Mauritius

Seagrass distribution was recorded by snorkel dives on a grid of stations in the waterfront of Club Méditerranée at Mon Choisy-Trou Aux Biches lagoon (NW Mauritius) and subsequently mapped using SURFER 6 computer software. Above-ground (AG) and below-ground (BG) standing biomass in terms of dry weight (DW) and ash-free dry weight (AFDW) as well as shoot density and shoot length were monitored monthly from June1997 to May1998 in a mixed stand of Halodule uninervis and Syringodium isoetifolium (dominant) at a shallow, nearshore station in the lagoon. Measurements of physical and chemical parameters [water temperature, current speed, salinity, pH, dissolved oxygen (DO), nitrate and phosphate concentrations] were made simultaneously, as well as at a reference station (ORE) outside the coral reef. The bottom sediment was analysed for grain size and type composition. Variation patterns were examined and statistical correlations drawn to relate plant performance to the environmental variables measured. The SURFER 6 programme generated a satisfactory contour map of seagrass distribution in the lagoon with a cover range of 0-60%. The densest patches occurred adjacent to the shoreline experiencing weaker water currents (3-13 cm s^-1) rather than near the reef (5-35 cm s^-1), where seagrasses were absent. Sand (0.063-2 mm grain size) constituted 97.2% and 77.6% of the nearshore and near-reef sediment, respectively. The dominant grain types were derived from corals (about 80%) and mollusc shells (about 14%). The recorded range of total standing biomass for H. uninervis was 243.1-468.2 g DW m^-2 (326.9ᇛ.7 g) or 71.7-141.2 g AFDW m^-2 (96.8ᆨ.1 g) and for S. isoetifolium it was 271.7-758 g DW m^-2 (460.4끯.1 g) or 119-220.5 g AFDW m^-2 (155.1ᆮ.5 g), with a maximum biomass increase during September-December in both species. AG:BG biomass ratios were generally <1 and approximated 1 during the warmest months of December-February only. Mean shoot density (1,077-4,364 shoots m^-2 in the overall range of 998-4,428 shoots m^-2) and mean shoot length (10.9-20.8 cm in the overall range of 7-31 cm) in S. isoetifolium were higher than in H. uninervis (1,732-4,137 shoots m^-2 in the overall range of 1,522-4,327 shoots m^-2 and 7.9-13.7 cm in the overall range of 6-20 cm, respectively). Temperature showed strong positive correlations with total AFDW biomass of both species (r=0.755, P<0.01 for H. uninervis; r=0.679, P<0.02 for S. isoetifolium) and with DO (r=0.925, P<0.01). High DO levels (10.7-11.2 mg l^-1) coincided with optimum standing biomass at 27.2°C. Correlations were also strong with shoot density (r=0.881, P<0.01 for H. uninervis; r=0.952, P<0.01 for S. isoetifolium) and shoot length (r=0.752, P<0.01 for H. uninervis; r=0.797, P<0.01 for S. isoetifolium). Under optimal environmental conditions, nutrient inputs from surface run-off or underground freshwater seepage in the lagoon due to heavy rainfall may boost up seagrass biomass, as suggested by positive significant correlations between phosphate levels and AG AFDW biomass (r=0.63, P<0.05 for H. uninervis; r=0.65, P<0.05 for S. isoetifolium) and shoot density (r=0.6, P<0.05 for H. uninervis; r=0.687, P<0.02 for S. isoetifolium). The results generated in this study suggest local seagrass standing biomass is comparable to that reported in monospecific stands from elsewhere. Anthropogenic activities increasingly draw down the resilience of the seagrass beds around Mauritius, and preventative measures are indispensable to achieve coastal ecological stability.     

Settlement patterns of newly settled plaice (Pleuronectes platessa) in a non-tidal Swedish fjord in relation to larval supply and benthic predators

In demersal fish species with a pelagic larval stage, settlement patterns may be a consequence of variations in larval supply, habitat selection at settlement, and processes acting between the time of settlement and the time of benthic sampling. This study describes temporal (1994-1998) and spatial variation in plaice (Pleuronectes platessa L.) settlement densities in four semi-isolated nursery areas with similar habitat characteristics, in the non-tidal Gullmarsfjord on the west coast of Sweden. Juvenile abundance varied by a factor of ten, both among years and among nursery grounds. For the 3 years when larval sampling was undertaken (1994-1996) and all nursery areas, there was a significant positive relationship between larval supply and juvenile abundance (linear regression: r²=0.45, n=24, P<0.001). On the southern side of the fjord, a significant positive relationship between larval and juvenile abundance was found in one area (r²=0.62, n=6, P<0.05). The absolute mortality rate of plaice after settlement was related to the initial settlement density (r²=0.95, n=20, P<0.001), and to the abundance of predatory shrimps Crangon crangon (r²=0.44, n=20, P<0.01). Plaice otoliths were found in 6% of the shrimp stomachs analysed from an area with high density (13.3 m^-2) of newly settled plaice. The present study suggests that the density of juvenile plaice was limited by larval supply to the nursery grounds. Consistency in the relative abundance of juveniles among nursery grounds between years also suggested that some nursery areas may be in the settlement shadow of others. The irregular nature of the coastline in combination with larval depletion could thereby cause small-scale (10³-10⁴ m) variation in settlement densities of the same order of magnitude as the inter-annual variability in recruitment to individual nursery grounds.     

An experimental and theoretical analysis of the effect of added weight on the energetics and hydrostatic function of the swimbladder of European sea bass (Dicentrarchus labrax)

We conducted experiments to determine the effect of the increasing ultrasonic/radio transmitter weight on the routine metabolic rate of sea bass. We measured the oxygen consumption (MO₂) of fish tagged externally with a dummy transmitter made of a hollow pipe, the weight of which was adjusted with lead to represent in water 0, 1 and 4% (R_tf) of the animal weight. We then developed a theoretical model to estimate, for a given fish size, the range of added weight that fish can compensate for through swimbladder regulation. When R_tfБ%, MO₂ of untagged and tagged fish did not differ significantly. However, when R_tf reached 4%, fish that carried a tag incurred a significant elevation of oxygen consumption, which represented 28% of their total useable power (or metabolic scope). This result strongly supports the view that a high R_tf ratio contributes to a decrease in available metabolic energy by diverting energy from, e.g., growth or swimming performance. A comparison between the tagged fish and the theoretical model reinforced the hypothesis that, when R_tf attained 4%, the increase in metabolic rate reflected a supplementary and costly swimming effort necessary to maintain vertical position. In this condition, the swimbladder cannot regulate the buoyancy of tagged fish.     

In situ swimming and settlement behaviour of larvae of an Indo-Pacific coral-reef fish, the coral trout Plectropomus leopardus (Pisces: Serranidae)

Late larvae of the serranid coral trout Plectropomus leopardus (Lacepède), captured in light traps, were released during the day both in open water and adjacent to two reefs, and their behaviour was observed by divers at Lizard Island, northern Great Barrier Reef. Coral trout larvae (n = 110) were present in light-trap catches from 18 November to 3 December 1997, including new moon (30 November). The swimming speed of larvae in open water or when swimming away from reefs was significantly greater (mean 17.9 cm s⁻¹) than the speed of larvae swimming towards or over reefs (mean 7.2 cm s⁻¹). Near reefs, larvae swam at average depths of 2.7 to 4.2 m, avoiding 0 to 2 m. In open water, swimming depth varied with location: larvae >1 km east of Lizard Island swam steeply downward to >20 m in 2 to 4 min; larvae >1 km west oscillated between 2.6 and 13 m; larvae 100 to 200 m east of Lizard Island oscillated between 0.8 and 15 m. Nearly all larvae swam directionally in open water and near reefs. In open water, the average swimming direction of all larvae was towards the island, and 80% (4 of 5) swam directionally (p < 0.05, Rayleigh's test). Larvae swam directionally over the reef while looking for settlement sites. The frequency of behaviours by larvae differed between two reefs of different exposure and morphology. Depending on site, 26 to 32% of larvae released adjacent to reefs swam to open water: of these, some initially swam towards or over the reef before swimming offshore. In some cases, offshore-swimming seemed to be due to the presence of predators, but usually no obvious cause was observed. Depending on the reef, 49 to 64% of the larvae settled. Non-predatory reef residents aggressively approached 19% of settlers. Between 5 and 17% of the larvae were eaten while approaching the reef or attempting to settle, primarily by lizardfishes but also by wrasses, groupers and snappers. A higher percentage of larvae settled in the second week of our study than in the first. Average time to settlement was short (138 s ± 33 SE), but some larvae took up to 15 min to settle. Average settlement depth was 7.5 to 9.9 m, and differed between locations. No settlement took place on reef flats or at depths <4.2 m. Larvae did not appear to be selective about settlement substrate, but settled most frequently on live and dead hard coral. Late-stage larvae of coral trout are capable swimmers with considerable control over speed, depth and direction. Habitat selection, avoidance of predators and settlement seem to rely on vision. Received: 7 July 1998 / Accepted: 26 January 1999     

Blue sprat <Emphasis Type="Italic">Spratelloides robustus</Emphasis> (Clupeidae: Dussumieriinae): a temperate clupeoid with a tropical life history strategy?

This article describes the life-history strategy of the blue sprat Spratelloides robustus in South Australia and compares the demographic traits observed with those of other clupeoids. Validation studies that involved marking the sagittae of captive fish with oxy-tetracycline suggested that growth increments are deposited daily. The oldest fish examined was 82 mm caudal fork length and 241 days old, which suggests S. robustus may live for less than 1 year. Growth rates were high during larval stages (0.34 mm day^у) and remained high throughout juvenile (0.33 mm day^у) and adult stages (0.19 mm day^у). S. robustus reached 50% maturity at approximately 60 mm caudal fork length after approximately 135 days. Spawning occurred from October to February (spring to late summer) and larvae were found mainly in Spencer Gulf, Gulf St Vincent, and Investigator Strait. Females spawned multiple batches of demersal eggs every 1-2 days. Batch fecundities were low (mean=756, SD=341) and increased linearly with length and weight. The life history of S. robustus is dissimilar to other small to medium-sized temperate clupeoids, but similar to those of many small sub-tropical and tropical clupeoids, including other Spratelloides species. Gulf St Vincent and Spencer Gulf may be considered to be "seasonally subtropical systems" in an otherwise temperate region that support a suite of species, including S. robustus, that have life-history strategies similar to those of sub-topical and tropical taxa.     

Diel movement patterns of the Hawaiian stingray,<Emphasis Type="Italic"> Dasyatis lata</Emphasis>: implications for ecological interactions between sympatric elasmobranch species

The Hawaiian stingray, Dasyatis lata, is a common benthic elasmobranch in nearshore Hawaiian waters. Acoustic telemetry was used to track the movements of seven rays in Kaneohe Bay, Oahu, Hawaii. Rays were tracked continuously over 31-74 h periods. Geographical movements were analyzed to determine space utilization and rate of movement. Rays were found to utilize significantly larger activity spaces at night (0.83ǂ.70 km²) (mean-SD) than during the day (0.12ǂ.15 km²). Mean total activity space for rays tracked was 1.32ǂ.75 km². Rates of movement were also significantly higher at night (0.34ǂ.30 km h^-1) than during the day (0.15ǂ.22 km h^-1). Average straight-line swimming speed was 0.64ǂ.16 km h^-1, with a maximum observed swimming speed of 1.9 km h^-1. Tidal stage had no effect on rate of movement. Comparison with previously published data on juvenile scalloped hammerhead sharks, Sphyrna lewini, in Kaneohe Bay revealed a high degree of overlap in habitat use and time of activity, suggesting possible ecological interactions between these two species.     

Swimming behaviour of the saucer scallop Amusium balloti (Mollusca: Pectinidae)

The swimming performance of the saucer scallop Amusium balloti (Bernardi) was recorded from tests conducted in a natural environment in Shark Bay, Western Australia, in June, July and November 1984 and June and September 1985. Unlike all other scallops described in the literature, the swimming performance (both speed and distance) of A. balloti increases with size. Maximum distance swum in a single swimming event was 23.1 m, while the maximum cumulative distance swum (four swimming events) was 30.8 m. Swimming speeds for larger scallops were generally between 0.8 and 1.0 m s^-1 (1.6 and 2.0 knots), with a maximum speed of 1.6 m s^-1 (3.1 knots). Variations in swimming performance and response times with size and season are probably the major cause of variations in the scallop's vulnerability to fishing gear.     

Swimming behavior of juvenile anchovies (<Emphasis Type="Italic">Anchoa</Emphasis> spp.) in an episodically hypoxic estuary: implications for individual energetics and trophic dynamics

Diel swimming behaviors of juvenile anchovies (Anchoa spp.) were observed using stationary hydroacoustics and synoptic physicochemical and zooplankton profiles during four unique water quality scenarios in the Neuse River Estuary, NC, USA. Vertical distribution of fish was restricted to waters with DO greater than 2.5 mg O₂ l⁻¹, except when greater than 70% of the water column was hypoxic and a subset of fish were occupying water with 1 mg O₂ l⁻¹. We made the prediction that an individual fish would select a swim speed that would maximize net energy gain given the abundance and availability of prey in the normoxic waters. During the day, fish adopted swim speeds between 7 and 8.8 bl s⁻¹ that were near the theoretical optimum speeds between 7.0 and 8.0 bl s⁻¹. An exception was found during severe hypoxia, when fish were swimming at 60% above the optimum speed (observed speed = 10.6 bl s⁻¹, expected = 6.4 bl s⁻¹). The anchovy is a visual planktivore; therefore, we expected a diel activity pattern characteristic of a diurnal species, with quiescence at night to minimize energetic costs. Under stratified and hypoxic conditions with high fish density coupled with limited prey availability, anchovies sustained high swimming speeds at night. The sustained nighttime activity resulted in estimated daily energy expenditure over 20% greater than fish that adopted a diurnal activity pattern. We provide evidence that the sustained nighttime activity patterns are a result of foraging at night due to a lower ration achieved during the day. During severe hypoxic events, we also observed individual fish making brief forays into the hypoxic hypolimnion. These bottom waters generally contained higher prey (copepod) concentrations than the surface waters. The bay anchovy, a facultative particle forager, adopts a range of behaviors to compensate for the effects of increased conspecific density and reduced prey availability in the presence of stratification-induced hypoxia.     

Effects of chemical cues on orientation of blue crab, <Emphasis Type="Italic">Callinectes</Emphasis><Emphasis Type="Italic">sapidus</Emphasis>, megalopae in flow: implications for location of nursery areas

Megalopae (postlarvae) of the blue crab Callinectes sapidus Rathbun use flood-tide transport (FTT) for movement into and up estuaries. Since they settle around the time of slack water at the end of flood tide during FTT, it was predicted that orientation toward primary nursery areas of aquatic vegetation occurs at this time. This study tested the hypotheses that megalopae locate nursery areas by swimming upstream in the presence of chemical odors from potential nursery areas and avoid adverse microhabitats by swimming downstream when predator or adverse environmental odors are present. Megalopae were tested in a flume where they were exposed to the sequence of cues mediating FTT (i.e. 2 psu increase in salinity followed by an increase and a decrease in current speed and turbulence). The flume contained odor water either from the developmental area (offshore water), nursery area vegetation (seagrass, Zostera marina; salt marsh cord grass, Spartina alterniflora), predators (fiddler crab, Uca pugilator; mud crab, Panopeus herbstii; grass shrimp, Palaemonetes pugio), or chemicals associated with adverse environments (ammonium). Vertical positions of premolt and intermolt megalopae were similar in water devoid of estuarine chemical cues (offshore water) and water containing seagrass odor. Upstream swimming behavior (orientation) of intermolt megalopae was also similar in these waters. However, there was an ontogenetic behavioral change, as the proportion of premolt megalopae oriented upstream generally increased as the concentration of seagrass and salt marsh cord grass odor increased and as current speed decreased. Upstream orientation of premolt megalopae in response to seagrass odor decreased significantly (i.e. downstream swimming increased) in the presence of odor from U. pugilator, P. pugio, and ammonium, but not from P. herbstii. Thus, the hypothesis was supported. These results suggest premolt megalopae orient toward nursery areas by swimming upstream in response to odors from aquatic vegetation as current speeds decrease at the end of nocturnal flood tides. Moreover, these results also indicate that megalopae may discriminate among microhabitats and avoid adverse settlement habitat, as orientation toward nursery areas is reversed by predator odors and ammonium.     

A comparison of catches of fishes and invertebrates by two light trap designs, in tropical NW Australia

Light traps were deployed in two sampling programs. In the first, small and large traps were released to drift with the current at stations along a cross-shelf transect on the NW Shelf off the coast of Western Australia. In the second program, pairs of small and large traps were deployed on moorings 150 m off the coastline. The composition and size-frequency distributions of catches of fishes in small and large traps were similar for both modes of deployment. In drifting traps, nearly 78% of this catch was composed of reef fishes, and these were collected in significantly greater numbers by the small design than by large traps (9.51 vs. 5.84 individuals h^-1, respectively). Nine taxa (amphipods, mysids, crab megalopae, copepods, cumaceans, isopods, caridean shrimps, polychaetes and the euphausiid, Pseudeuphausia latifrons) accounted for 99% of the total catch of invertebrates by drifting traps. Of these, catches of amphipods, copepods, cumaceans and P. latifrons were greater in large traps than in small traps (3,134 vs. 1,687 h^-1, 1,018 vs. 214 h^-1, 551 vs. 165 h^-1 and 74 vs. 9 individuals h^-1, respectively). In contrast, crab megalopae were more abundant in catches by small traps than by large traps (3,134 vs. 1,687 individuals h^-1, respectively). The catch rate of fishes in moored traps was higher than in drifting traps (105 vs. 20 fishes h^-1) and was dominated by baitfishes (86% of total catch). Reef fishes were also captured in greater numbers by small traps than by the large design (10.17 vs. 4.4 individuals h^-1) in this mode of deployment. Despite these differences in catch rates, multivariate analysis showed that cross-shelf patterns in catches of fishes and invertebrates were mapped equally well by both trap designs. Variation in the efficiency of trap designs thus appears to be small when compared to changes in the composition and abundance of zooplankton assemblages that occur at scales of tens of kilometers.     

Gut characteristics and assimilation efficiencies in two species of herbivorous damselfishes (Pomacentridae: Stegastes dorsopunicans and S. planifrons)

The morphological and physiological mechanisms by which marine herbivores assimilate energy and nutrients from primary producers and transfer them to higher trophic levels of reef ecosystems are poorly understood. Two wide-ranging Caribbean fishes, the dusky damselfish, Stegastes dorsopunicans, and the threespot damselfish, S. planifrons, defend territories on patch reefs in the Archipelago de San Blas, Republic of Panama. We examined how relative intestine length and retention time influence digestion and absorption of energy and nutrients in these fishes. The dusky damselfish has a relative intestine length (RIL=intestine length/standard length) of 1.2 and a Zihler index {ZI=intestine length (mm)/10[mass(g)^1/3]} of 3.4. These values are significantly lower (P_RIL=P_ZI<0.0001) than those for the threespot damselfish (3.0 and 8.2, respectively). Both RIL and ZI for both species fall well below previously published values for other herbivorous pomacentrids, and may reflect their primary food resource at San Blas (diatoms). Energy-rich diatoms may be easier to digest than refractory macroalgae characteristic of diets of many herbivorous fishes (RIL range: 2-20). Despite differences in RIL and ZI between these two species, gut retention time is the same (P>0.05) for both dusky (6.6 h) and threespot damselfish (6.5 h). Thus, food travels the length of the threespot damselfish intestine ~2.5 times faster than it does in the dusky damselfish intestine. Levels of protein, carbohydrate, and lipid are significantly (0.003<P<0.030) higher in the feces of dusky damselfish than in the feces of threespot damselfish, when both species were fed a natural diet of benthic diatoms collected from damselfish territories. This indicates threespot damselfish have a greater nutrient-specific and total assimilation efficiency than do dusky damselfish. Furthermore, when fed an artificial pellet diet, protein absorption efficiency differed significantly (P=0.014) between species; threespot damselfish absorbed 98.3% of dietary protein, whereas dusky damselfish absorbed 96.4% of dietary protein.     

Alternative feeding mechanisms in megalopae of the blue crab Callinectes sapidus

The apparent mismatch between the energy requirements for planktotrophic growth and prey availability has long been paradoxical. One hypothesis to explain this paradox is that planktotrophic larvae display plasticity in feeding mechanisms in response to variable prey types and concentrations. This hypothesis was tested by videotaping megalopae of the brachyuran crab Callinectes sapidus Rathbun feeding on various-sized prey. Frame-by-frame analysis of the videotapes indicated that C. sapidus megalopae used both raptorial and suspension feeding to capture prey while in the water column. Raptorial feeding was used to capture macro-zooplankton, including copepods. The swimming form of suspension feeding was based on a modified fling-and-clap mechanism using the chelipeds. Suspension feeding while at rest utilized a weak current generated by the mouthparts to direct prey to the mouth. Both suspension-feeding mechanisms resulted in the efficient capture of rotifer-sized particles. The energy/handling time ratios for all three feeding mechanisms are very similar (E/H range 0.016-0.019 µg C s^-1) for the natural prey tested. These results support the hypothesis that feeding in brachyuran larvae is plastic and includes mechanisms of both raptorial and suspension feeding. The ability to suspension feed at rest is adaptive, since megalopae use selective tidal transport to re-invade an estuary and may spend up to 18 h day^-1 clinging to a benthic substrate. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00227-002-0781-1.     

Energy budgets and feeding rates of Coryphaenoides acrolepis and C. armatus

This study develops energy budgets and estimates feeding rates for two macrourid fishes, Coryphaenoides acrolepis, dominant in the bathyal eastern North Pacific, and the abyssal cosmopolitan species, Coryphaenoides armatus. Daily energy expenditure by C. acrolepis was nearly twice that of C. armatus. C. acrolepis allocated nearly equal amounts of energy to metabolism and growth. Once sexual maturity was reached reproduction became the dominant energetic cost. Either these costs are necessary to retain adequate numbers of eggs and larvae on the continental slopes, or this fish does not reproduce on an annual basis and the calculated costs are an overestimate. C. armatus allocated relatively more energy to metabolism than growth. It may be semelparous, and this strategy would be of great energetic savings in its food-poor but stable environment. Individual daily ration for C. acrolepis decreased from 0.31% to 0.07% of body weight (BW) and for C. armatus from 0.12% to 0.02% BW with increasing fish length. These rates are substantially lower than those for fishes living in cold waters on the continental shelves. The population feeding rates for C. acrolepis ranged from 0.8 to 15 kg km^-2 day^-1 and for C. armatus from 5 to 2,800 g km^-2 day^-1. The scavenging behaviour of C. acrolepis was used to investigate the role of carrion as a food supply to the deep-sea benthos. It was estimated that the carrion eaten by C. acrolepis is equivalent to 0.04 mg C m^-2 day^-1 or only 0.2-0.4% of the average small particulate flux. Carrion consumption is important for scavengers like C. acrolepis, but it is not an important component of the carbon flux into the deep-sea benthic environment.     

Arm regeneration in the field in Ophiocoma echinata (Echinodermata: Ophiuroidea): effects on body composition and its potential role in a reef food web

Eighteen percent and 47% of two populations of Ophiocoma echinata in the Florida Keys were undergoing regeneration. An individual would take approximately 720 days to completely regenerate three arms. Regeneration of three arms had a greater effect on gonad production in females than in males, and reduced storage material in the stomach of both. Regeneration in O. echinata was estimated to be 0.07 kJ m^-2 day^-1, which could provide the equivalent of 0.07% of primary production on a reef to higher trophic levels per day.     

Effects of diet and laboratory rearing on demography of Dinophilus gyrociliatus (Polychaeta: Dinophilidae)

Life table response experiments were performed to evaluate the demographic consequences of: (1) the dietary regimes and (2) the length of laboratory rearing in strains of Dinophilus gyrociliatus, a small infaunal polychaete. The first experiment was performed using animals recently collected from the natural environment and fed either on spinach or on Tetramin (artificial fish food with high caloric content). Starting from this original group, two distinct laboratory strains were established: the first raised only with spinach, the second only with Tetramin. In the first experiment, the group fed on Tetramin exhibited greater population growth rate (5), shorter generation time (T) and reduced expectation of life (e⁰) with respect to the animals fed on spinach. The second experiment took place 2 years later to evaluate the difference in life history traits between these two laboratory strains. In the case of the group fed on Tetramin, population parameters exhibited marked variations; in fact, 5 and the net reproductive rate (R₀) were significantly higher and T and e⁰ were shorter than the corresponding parameters observed in the first experiment. Conversely, the demographic variations induced by laboratory rearing on a spinach diet were limited to a reduction in the expectation of life. The decomposition analysis showed that the reduction in generation time and the increase in fecundity occurring during the first 4 weeks of life accounted for nearly all the differences in 5. During the long breeding period at constant temperature, photoperiod and salinity, a continuous selection of the most precocious and fecund individuals may have taken place as a consequence of the abundance of resources and the lack of predation.     

Effect of oxygen concentration on metabolism and locomotory activity of <Emphasis Type="Italic">Moina</Emphasis><Emphasis Type="Italic">micrura</Emphasis> (Cladocera) cultured under hypo- and normoxia

The influence of oxygen concentration on total and basal metabolism, scope of activity, drag force and duration of jerks, time spent swimming and energy cost of locomotion in Moina micrura Hellich females cultured under hypo- and normoxia was investigated. Scope of activity (Q_l) of hemoglobin-rich red individuals (M_a) acclimated to hypoxia depended less upon oxygen concentration than that of non-acclimated, pale individuals (M_na). Within the range 10-0.3 mg O₂ l^-1 Q_l decreased 4.4-fold in M_a and 62.5-fold in M_na. In both M_a and M_na the integral drag force of antenna fell from 0.22ǂ.07 to 0.12ǂ.04 dyn (1 dyn=1·10^-5 N), the duration of jerks increased from 0.06ǂ.01 to 0.1ǂ.02 s in the range from ~2.0 mg O₂ l^-1 to sublethal oxygen concentrations. At 0.7-0.8 mg O₂ l^-1 M_na stopped filtration and increased time spent swimming. In contrast, even under more severe hypoxia (~0.2 mg O₂ l^-1), M_a maintained their filtering activity using energy (up to 80% of total metabolism) achieved due to increased oxygen capacity of the blood. Separating locomotion and feeding functions, M. micrura can spend less energy for swimming and use its energy budget more plastically under changing environmental conditions.