Vulnerability of late larval and early juvenile Atlantic herring, Clupea harengus, to predation by whiting,Merlangius merlangus

Laboratory experiments investigated changes in the vulnerability of herring to predation by whiting during growth from <34 to >80mm in length, Premetamorphic herring (<50mm) failed to react to c . 50% of attacks by the predator, but this was reduced to c . 20% in postmetamorphic fish. Premetamorphic herring failing to react had c. 30% probability of survival due to unforced errors by the predator (unsuccessful attacks that did not elicit a reaction by the prey), but these errors did not occur after herring metamorphosis. The loss of the advantage of lower conspicuousness following metamorphosis was counterbalanced by increased reactivity. Antipredator benefits should increase with schooling behaviour (first evident in experiments at c . 50 mm), so predation mortality should decrease in postmetamorphic herring in the sea. Metamorphosis itself would be associated with high predation risk since conspicuousness is enhanced but reactivity and schooling behaviour are still not fully developed.     

Ontogeny of the Digestive and Feeding Systems in the Anemonefish Amphiprion Melanopus

Ontogenetic growth and development in the anemonefish Amphiprion melanopus (Pomacentridae) is very rapid when compared to other tropical and temperate fish species. A. melanopus hatched with a highly differentiated digestive tract and the ability to capture and ingest prey items. Their alimentary tract changes rapidly throughout the larval period. Concurrent with yolksac absorption at three days after hatching was the development of the stomach followed by calcification of the jaw structures. This period of acute structural change may be a critical period in their development. Metamorphosis coincided with settlement at 8 days after hatching and was marked by calcification of fins and acquirement of adult coloration. The rapid development found in this species may be a specialisation to enhance the return of larvae to a patchy and highly specific settlement habitat.     

Effects of temperature on larval fish swimming performance: the importance of physics to physiology

Temperature influences both the physiology offish larvae and the physics of the flow conditions under which they swim. For small larvae in low Reynolds number (Re) hydrodynamic environments dominated by frictional drag, temperature‐induced changes in the physics of water flow have the greatest effect on swimming performance. For larger larvae, in higher Re environments, temperature‐induced changes in physiology become more important as larvae swim faster and changes in swimming patterns and mechanics occur. Physiological rates at different temperatures have been quantified using Q₁₀s with the assumption that temperature only affected physiological variables. Consequently, Q₁₀s that did not consider temperature‐induced changes in viscosity overestimated the effect of temperature on physiology by 58% and 56% in cold‐water herring and cod larvae respectively. In contrast, in warm‐water Danube bleak larvae, Q₁₀s overestimated temperature‐induced effects on physiology by only 5–7%. This may be because in warm water, temperature‐induced changes affect viscosity to a smaller degree than in cold water. Temperature also affects muscle contractility and efficiency and at high swimming velocities, efficiency decreases more rapidly in cold‐exposed than in warm‐exposed muscle fibres. Further experiments are needed to determine whether temperature acts differently on swimming metabolism in different thermal environments. While hydrodynamic factors appear to be very important to larval fish swimming performance in cold water, they appear to lose importance in warm water where temperature effects on physiology dominate. This may suggest that major differences exist among locomotory capacities of larval fish that inhabit cold, temperate waters compared to those that live in warm tropical waters. It is possible that fish larvae may have developed strategies that affect dispersal and recruitment in different aquatic habitats in order to cope not only with temperature‐induced physiological challenges, but physical challenges as well.     

Patterns of prey size and taxonomic composition in larval fish: are there general size-dependent models?

The allometric relationship of maxilla length to larval fish weight approached a slope of 1/3 for 11 species from Conception Bay, Newfoundland. In 10 of these species, mean prey size increased with length of the maxilla but not constantly. In six species, niche breadth increased with length of maxilla. The rate of change in niche breadth was inversely related to the rate of change in mean prey size. Nauplii and copepodites of calanoids were selected positively, and cyclopoids negatively for almost all species of larval fish.     

Feeding ecology of larval and juvenile black seabream (Acanthopagrus schlegeli) and Japanese seaperch (Lateolabrax japonicus) in Tolo Harbour, Hong Kong

Black seabream, Acanthopagrus schlegeli, and Japanese seaperch, Lateolabrax japonicus, are important commercial species in the coastal waters of western Pacific Ocean, including Japan, Korea and China. In Hong Kong, larvae and juveniles of these two species occur in bays and estuaries during late winter and spring. This study reports on the ontogenetic changes in food habits in larvae and juveniles of these species in an artificial rocky shore area. Copepods and cladocerans were the most numerous food items for black seabream. There was a shift to larger and benthic prey as the fishes grew. Japanese seaperch <2.1cm fed predominantly on copepods and cladocerans, while larger prey were added as fish size increased. Japanese seaperch >6.0cm were piscivorous. Maximum prey width increased with fish standard length and mouth gape width in both species. Overall, black seabream showed greater diet breadth than did Japanese seaperch. In black seabream, diet breadth increased with fish size. In Japanese seaperch, diet breadth increased with size for fishes <4.0cm, then decreased as the fishes became piscivorous. Prey selectivity in black seabream was determined using information on prey availability in plankton samples. In general, preference was stronger for cypris larvae, Penilia avirostris and decapod larvae than for copepods and podonids. In recent years, overfishing and environmental degradation have led to the decline of fish populations in Tolo Harbour. Absence of fishes with empty gut indicates that inner Tolo Harbour is still an important nursery area for these two commercial species.     

Trilobite larvae and larval ecology

Silicified trilobite faunas yield a well preserved and abundant record of early development in many taxa. Although their morphologies are well known, protaspid larvae are poorly understood in terms of trilobite life history and developmental biology. Two distinct morphologic types are recognized among the great diversity of protaspides studied; these are termed adult‐like and nonadult‐tike according to gross similarity to later developmental stages. Transitions between nonadult‐like and adult‐like morphologies are abrupt, occurring between successsive instars, and, thereby, constitute metamorphoses.

By analogy with developmental patterns among modern marine arthropod taxa, metamorphoses during early trilobite ontogeny correspond to radical modifications in life mode and ecology. Adult‐like trilobite protaspides possess a dorso‐ventrally flattened tergum which displays a coarsely featured prosopon and surface‐parallel spines; these are interpreted as benthic larvae. In contrast, nonadult‐like protaspid larvae display an ovoid to spheroidal shape with spine pairs that project at right angles to one another; these are considered to have been planktonic. Protaspid morphologies are compared and discussed in light of these inferred modes of life.     

Distribution, abundance, behavior and metabolism of Periphylla periphylla, a mesopelagic coronate medusa in a Norwegian fjord

The distribution, behavior and metabolism of the mesopelagic jellyfish, Periphylla periphylla (Péron & Lesueur), were investigated in Lurefjorden, Norway. Field studies, conducted in 1998–1999 with plankton nets and a remotely operated vehicle, indicated that 80-90% of the dense (up to 2.5 m^–3) population migrated 200–400 m vertically each day throughout the year. In situ observations with red light revealed that swimming rates and feeding activity varied with age and time of day. Detection of turbulence and contact with surfaces caused this medusa to conceal one or all of its tentacles in the stomach or to shed nematocyst-laden tissue from the tentacles. Stomachs of medusae collected with nets were often full of prey entangled with the sloughed tissue. Stomachs of medusae captured individually with ROV samplers were empty or contained only a few prey in their stomachs (typically, 1–4 copepods Calanus spp. or chaetognaths Eukrohnia hamata Möbius per medusa). Low rates (0.4–5.6 l O₂ mg C^–1 h^–1) of oxygen consumption of P. periphylla suggested that this species was sustained by relatively few (1–34) prey d^–1.     

Water temperature, light intensity and zooplankton density and the feeding activity of juvenile brook charr (Salvelinus fontinalis)

SUMMARY 1. The objective of this study was to evaluate the effects of zooplankton biomass (as a measure of density), fish biomass, light intensity and water temperature on the attack rate and swimming characteristics (i.e. swimming speed and angle of turn) of juvenile (1+) brook charr (Salvelinus fontinalis) in field enclosures. We used a portable underwater camera system in a series of pelagic enclosures to quantify the feeding behaviour of brook charr over a gradient of natural conditions. 2. In simple linear or non‐linear regression models we found (i) that attack rate and angle of turn were positively related to water temperature, (ii) that attack rate and swimming speed were positively related to zooplankton biomass and light intensity and (iii) that attack rate was positively related to swimming speed. In multiple regression models, fish biomass, light intensity and variance of the angle of turn accounted for 87% of the variation in attack rate. Light intensity and water temperature accounted for 86% of the variation in swimming speed. Fish gut fullness and attack rate accounted for 83% of the variation in the variance of the angle of turn executed by fish. 3. The increase in the number of attacks as zooplankton biomass increases conforms to the general positive functional response observed in other fish species. Our results also support the hypothesis that swimming speed increases with prey biomass. We did not observe a plateau in attack rate as zooplankton biomass increased. As our experiments were performed under natural biotic and abiotic conditions, factors other than zooplankton biomass might affect or limit this response, such as water temperature and light intensity. 4. Because zooplankton biomass was correlated with water temperature and light intensity, it was not possible to evaluate the independent contribution of these factors on the attack rate and swimming characteristics (swimming speeds and angle of turn) of brook charr. However, this study highlighted the impact of these factors on the feeding behaviour of juvenile brook charr when feeding in the pelagic habitat under natural conditions, and their importance in future models of optimal foraging and fish habitat quality.     

Feeding of larval blue whiting and Atlantic mackerel: a comparison of foraging strategies

Fish larvae employ different feeding strategies depending on area and season of spawning and hatching of larvae. Feeding and growth of larvae of blue whiting Micromesistius poutassou and mackerel Scomber scombrus from Porcupine Bank and the Celtic Shelf Break, west of Ireland, were compared based on prey concentrations in the environment and larval feeding behaviour. Both species were adapted to different environmental conditions. The mesopelagic blue whiting spawned in oceanic water that was well mixed. It was characterized by low production and low prey densities with minimum prey densities <1.0 organism 1⁻¹. Larvae of the Atlantic mackerel hatched later in the season in more productive water that was well stratified. Prey densities in the mackerel environment reached up to 1001⁻¹. Blue whiting larvae displayed a rather random distribution in the water column. Mackerel larvae <7 mm standard length ( L _s) were concentrated above the thermocline, while larvae >7 mm traversed the thermocline into deeper layers. Mackerel larvae >5mm L _s displayed marked cannibalism, exceeding 70%. Daily ration calculated on the basis of gut contents was rather low in both species: between 2.6 and 5.0% in blue whiting, but only 0.6 to 5.4% in mackerel. The results are discussed in relation to the respective environment both species encounter during their early larval life.     

Comparative limb bone scaling in turtles: Phylogenetic transitions with changes in functional demands?

Several terrestrial vertebrate clades include lineages that have evolved nearly exclusive use of aquatic habitats. In many cases, such transitions are associated with the evolution of flattened limbs that are used to swim via dorsoventral flapping. Such changes in shape may have been facilitated by changes in limb bone loading in novel aquatic environments. Studies on limb bone loading in turtles found that torsion is high relative to bending loads on land, but reduced compared to bending during aquatic rowing. Release from torsion among rowers could have facilitated the evolution of hydrodynamically advantageous flattened limbs among aquatic species. Because rowing is regarded as an intermediate locomotor stage between walking and flapping, rowing species might show limb bone flattening intermediate between the tubular shapes of walkers and the flattened shapes of flappers. We collected measurements of humeri and femora from specimens representing four functionally divergent turtle clades: sea turtles (marine flappers), softshells (specialized freshwater rowers), emydids (generalist semiaquatic rowers), and tortoises (terrestrial walkers). Patterns of limb bone scaling with size were compared across lineages using phylogenetic comparative methods. Although rowing taxa did not show the intermediate scaling patterns we predicted, our data provide other functional insights. For example, flattening of sea turtle humeri was associated with positive allometry (relative to body mass) for the limb bone diameter perpendicular to the flexion-extension plane of the elbow. Moreover, softshell limb bones exhibit positive allometry of femoral diameters relative to body mass, potentially helping them maintain their typical benthic position in water by providing additional weight to compensate for shell reduction. Tortoise limb bones showed positive allometry of diameters, as well as long humeri, relative to body mass, potentially reflecting specializations for resisting loads associated with digging. Overall, scaling patterns of many turtle lineages appear to correlate with distinctive behaviors or locomotor habits.