J.W.M. Osse

Priorities during early growth of fish larvae

Abstract The small and numerous eggs of most teleost fishes develop into small larvae with little yolk, which is depleted soon after hatching. They are the smallest self-supporting vertebrates. When these larvae complete their morpho-functional systems necessary to escape from predators and to obtain external food with priority, their further chances for survival will increase. As predation risks decrease with increasing size (and therefore, escape velocity), early positive allometric growth of the tail is expected. Similarly, early positive allometric growth of parts of the head constructions involved in suction feeding is expected. Can symmorphosis, due to strong interdependency between feeding and swimming be found? Measurements of growth in size and position of elements, as well as performance of function, confirm our prospects. The inflexion points of the growth curves show that after early acceleration, growth becomes more isometrical.

Size of flatfish larvae at transformation, functional demands and historical constraints.

Abstract At hatching the larvae of flatfish closely resemble the bilateral symmetric larvae of other teleosts, especially perciforms. Literature data show that transformation to asymmetric benthic juveniles normally occurs at body lengths between 10 and 25 mm. Unexpectedly, minimal size at its completion (including eye migration) can be 4.1 mm SL and maximal size is over 72 mm. In this paper we consider the functional requirements for a successful switch from a symmetric pelagic larva to a typical asymmetric juvenile benthic flatfish partly based on evidence from other teleosts. The unfavourable period of eye migration and transition to a benthic habitat requires some food reserves and rewiring and/or recalibration of vision and gravity-associated structures utilised previously by the still symmetric larvae for e.g. food detection. Binocular fixation of the prey probably occurs in that stage. Critical or sensitive periods occurring during development of fish larvae suggest that a completely functional symmetric stage of development must precede transformation. The normal size range in flatfish larvae at transformation seems to confirm our considerations. Recent data on temperature effects during development provide an explanation for metamorphosis at the minimal size. Some evidence for paedomorphic heterochrony in flatfish larvae is presented.

Adaptive radiation of Lake Tana's (Ethiopia) Labeobarbus species flock (Pisces, Cyprinidae)

Studying species flocks (e.g. Darwins finches, Caribbean anoline lizards, East African cichlid fishes) has proven to be highly successful in understanding the forces driving speciation. The only known, intact species flock of cyprinid fishes, the 15 Labeobarbus species in Lake Tana (Ethiopia), includes eight piscivorous species. Piscivory is a rare specialisation among the highly successful (>2000 species) but mostly benthivorous Cyprinidae. The extent and mechanisms of diversification of this remarkable Labeobarbus species flock, particularly among the unexpected piscivorous species, are still largely unknown. In the present study we demonstrate that all 15 Labeobarbus species are segregated to a great extent along spatial, trophic and/or temporal dimensions. The spatial distribution, diet (prey species but not prey size), time of active feeding and predation techniques differed significantly among the eight piscivores. Lake Tanas cyprinids displayed their retained potential for ecological diversification and speciation, including the uncommon specialisation of piscivory. The latter is probably a result of the absence of common African specialist piscivores in Lake Tana. We suggest that the evolution of Lake Tanas Labeobarbus species flock at this stage is predominantly structured by ecological selection models. The labeobarbs most likely underwent sequential stages of radiation and speciation: habitat divergence followed by trophic divergence.

In Lake Tana, a unique fish fauna needs protection.

tion of biodiversity usually focus on terrestrial habitats, especially on rain forest ecosystems (Myers 1979, Simberloff 1984, Wilson 1989). Because of this bias, the value of aquatic communities, less accessible for direct observations, is often not fully appreciated. The purpose of this article is to draw attention to a unique freshwater systemLake Tana-and especially to its fishes. Thus we want to urge the international scientific community to increase knowledge on the origin and state of the current biodiver-

Structure, development and function of the branchial sieve of the common bream, Abramis brama, white bream, Blicca bjoerkna and roach, Rutilus rutilus.

SynopsisThe filter feeding organ of cyprinid fishes is the branchial sieve, which consists of a mesh formed by gill rakers and tiny channels on the gill arches. In order to establish its possible role during growth we measured the following morphological gill raker parameters over a range of sizes in three cyprinid fishes, bream, white bream and roach: inter raker distance, bony raker length, raker width, cushion length and channel width. At any given standard length common bream has the largest inter raker distance, roach the lowest and white bream is intermediate. In the ‘comb model’ of filter feeding the inter raker distance is considered to be a direct measure of the mesh size and retention ability (= minimal size of prey that can be retained) of a filter. For the three species under study there is a conflict between the comb model and experimental data on particle retention. Lammens et al. (1987) found that common bream has a large retention ability whereas roach and white bream have a much smaller one. A new model, the ‘channel model’ (Hoogenboezem et al. 1991) has been developed for common bream; in this model the lateral gill rakers can regulate the mesh size of the medial channels on the other side of the gill slit. The present data indicate that this model is not appropriate for white bream and roach. At any given standard length white bream and roach only reach 70% of the raker length of common bream, which means that in this model the gill slits should to be very narrow during filter feeding. The gill rakers consist of a bony raker and a fleshy cushion. The bony rakers have a rather long needle-like part outside the cushion in bream, but not in white bream and roach which have blunt gill rakers. Blunt gill rakers are not suited to reduce the diameter of the medial channels. The comb model seems more appropriate for white bream and roach, but doubts about the validity of this simple model remain. The sum of the areas of the medial channels is an approximation of the area through which water flows in the filter. This channel area therefore gives an impression of the capacity or flow rate of the filter. With this capacity estimation and an estimation of energy consumption we calculated an energy ratio of filter feeding. The energy ratio decreases with increasing standard length with an exponent close to the expected exponent of -0.40. The energy ratio is highest in bream, intermediate in white bream and lowest in roach.

Barbus tanapelagius, A New Species from Lake Tana (Ethiopia): its Morphology and Ecology

The endemic cyprinid species flock in Lake Tana consists of 15 species of large hexaploid barbs, eight of which are piscivorous. Previously, it was assumed that all piscivores preyed on the same small barb species, Barbus trispilopleura. In this paper we present a description of morphology and ecology of a new abundant small barb species, Barbus tanapelagius sp. nova (holotype RMNH 33731) from Lake Tana, Ethiopia, which appears to be the major prey species for the large pelagic piscivorous barbs. B. tanapelagius differs clearly in morphology from the other 3 small, diploid Barbus species known from Lake Tana, B. trispilopleura Boulenger, 1902, B. humilis Boulenger, 1902 and B. pleurogramma Boulenger, 1902. Conspicuous differences are its elongated body, large eye diameter, prominent and hooked lower jaw contour and colouration. Preliminary data suggest that B. tanapelagius also differs ecologically from the other small Barbus spp. by its pelagic, strictly zooplanktivorous feeding and its occurrence mainly in the deeper, offshore waters. The other small Barbus species are most probably largely benthic feeders and dominant in the shallow inshore waters. Previous views about the evolution of the present 8 endemic piscivorous large barb species therefore require reconsideration, as the present paper shows a more complex scenario including several prey species.

Zooplankton feeding in common bream (Abramis brama), white bream (Blicca bjoerkna) and roach (Rutilus rutilus): experiments, models and energy intake.

Three models of the sieving mechanism of the branchial sieve were used to predict the ability to retain zooplankton of three sympatric cyprinids: common bream, white bream and roach. The model predictions were tested with filter-feeding experiments, using three size classes of each species. Results of experiments in darkness corroborated closely with the reducible-channel model for common bream (retention in the medial channels on the gill arches; the diameter of these channels can be reduced with the lateral rakers), rather well with the unreducible-channel model for white bream (the channel diameter cannot be reduced) and possibly with the saw-tooth model for roach (retention on the gill slits). Common bream can adjust the mesh size of its branchial sieve, thus achieving a higher flexibility in food uptake than the other two species. In light experiments, roach and the small common and white bream switched to particulate intake, characterized by a lower retention ability and a higher filtering rate than during gulping. The retention ability was used to calculate the percentage of the available zooplankton energy that the three cyprinids can retain as a function of the fishs length. This retained energy percentage decreases sigmoidly with increasing fish length. At any length between 10-50 cm, common bream has the highest retained energy percentage, white bream the lowest and roach is intermediate. The population of common bream will therefore be at an advantage in the competition for food when zooplankton is a major food source, like in eutrophic lakes.

The ‘small barbs’ Barbus humilis and B. trispilopleura of Lake Tana (Ethiopia): Are they Ecotypes of the Same Species?

Four species of ‘small barbs’ (Barbus, subgenus Enteromius Cope, 1869) are known from Lake Tana, isolated in the Ethiopian highlands: B. humilis, B. trispilopleura, B. pleurogramma (all Boulenger, 1902) and B. tanapelagius de Graaf, 2000. However, only three species appear valid from cluster analysis using 32 morphometric characters and taking specimens from different locations in the southern Gulf of Lake Tana during August–October 1999. B. humilis and B. trispilopleura significantly differ from B. tanapelagius and B. pleurogramma in up to 36 characters. However, B. humilis and B. trispilopleura cannot be distinguished from each other by morphometric analysis or by gut contents. Specimens from clear, shallow rocky areas with vegetation have a darker back, will be more susceptible to birds, have significantly higher infection by cestodes, smaller size at first reproduction, lower fecundity, and correspond most to the B. trispilopleura phenotype. Specimens in turbid deeper water without vegetation are most similar to the B. humilis phenotype. We conclude that both species actually are extremes (ecotypes) of a continuum, belonging to a single biological species. The observed variation may well be induced by habitat-dependent predation pressure by birds. The high frequency (57%) of spot numbers intermediate between Boulenger’s number for B. trispilopleura (3) and for B. humilis (0) demonstrates the continuum best. Pigment spots and colour change in response to aquarium conditions and are in this case no valid taxonomic characters. Both characters may reduce the risk of predation. It is concluded that B. trispilopleura is a synonym of B. humilis. For future research we recommend to use the most appropriate name, B. humilis, for both types.

Body Size and Swimming Types in Carp Larvae; Effects of Being Small

The viscosity of water is a major hydrodynamic force and especially important for tiny objects (on or below a mm scale) moving at low speeds. Fish larvae between 4.8 and 8.1 mm total length (TL) are of intermediate size. During their growth the flow while swimming becomes dominated by inertial instead of viscosity forces. The present paper presents changes in beat frequency, wave speed, absolute speed, distance covered per swimming cycle and the distribution of body curvature along the trunk during larval growth. Just hatched larvae show only one type of swimming but further types develop during growth. Early wriggling (resistive) swimming is rapidly replaced by beating with tail and tail fin suggesting the increasing dominance of inertial hydrodynamic forces. Similar movements as in the 4.8 mm larvae are found in the first body wave of bigger larvae thus strengthening the hypothesis of the changing relationship between viscosity effects and swimming motion with size.

Structure and function of the median finfold in larval teleosts

SUMMARY This paper offers a structural and mechanical analysis of the median finfold in larval teleosts. The median finfold is strengthened by bundles of collagen fibres, known as actinotrichia. We demonstrate that these structures contribute to increase the mass of backward accelerated water during swimming. The amount, dimensions, orientation and growth of actinotrichia were measured at various locations along the finfold in several developmental stages of common carp (Cyprinus carpio) and zebrafish (Danio rerio). Actinotrichia morphology, using light microscopy (e.g. diameter, orientation) and electron microscopy (which revealed their anchoring at proximal and distal ends), correlated with expected lateral forces exerted on the water during swimming. An analytical model is proposed that predicts the extent of camber from the oblique arrangement of the actinotrichia and curvature of the body. Camber of the finfold during swimming was measured from high-speed video recordings and used to evaluate the model predictions. Based on structural requirements for swimming and strain limits for collagen, the model also predicts optimal orientations of actinotrichia. Experimental data confirm the predictions of the model.