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Behavioral transformation from the planktonic larval stage of some marine fishes reared in the laboratory Marliave, Jeffrey Burton

Abstract

This study tested the hypothesis that larvae of benthic species settle only on a substrate which is typical of the adult habitat and that those which fail to encounter a suitable substrate will not settle and will die before completion of metamorphosis. A similar hypothesis, that larvae of schooling species will fail to metamorphose if they cannot express adult schooling behavior, was tested by rearing them in isolation through metamorphosis. The range of substrate preferences for settling larvae of intertidal species corresponded to the adult niche breadth. Adults of Artedius lateralis occur in most shallow water habitats on the open coast. Their larvae settled on any substrate offerred. Adult Xiphister atropurpureus live only among pebbles and small rocks on cobble beaches. Their larvae settled only in gravel. Pholis laeta preferred to settle in eelgrass, Bothragonus swani on gravel and Leptocottus armatus on sand. These species settled directly onto the substrates most characteristic of the adult habitats. Larval Gobiesox maeandricus. however, settled onto smooth seaweeds rather than onto the rocks inhabited by adults. Thus, the preferred substrate for larval settlement was always an element within the adult habitat, but not necessarily the same substrate preferred by the adults. This study demonstrated that metamorphosis does not coincide with the behavioral transformation. Larvae of schooling species adopted adult schooling behavior well before the onset of their gradual metamorphosis. Aulorhynchus flavidus left the plankton and started to school just after hatching. In intertidal species, particular metamorphic changes occurred before the settlement period, which took place more or less abruptly. Immediately after settlement, metamorphic pigmentation occurred rapidly, followed by allometric growth. One benthic species which does not tend to be sedentary as an adult (Blepsias cirrhosus), metamorphosed completely before settling onto kelp. Considerable variability in the pattern of behavioral transformation and metamorphosis was witnessed between species. Prevention of schooling (by isolation) did not hinder metamorphosis of Clupea harengus pallasi or Aulorhynchus flavidus. Only in those benthic species having abrupt settlement and narrow substrate preferences was there a tendency for high mortality rates at transformation. Such mortalities took place only when suitable substrates were unavailable for settlement. The positive phototactic tendencies of larvae changed during settlement only in species which settled into a shaded substrate (Xiphister atropurpureus). Physical characteristics of substrates resulted in the settlement preferences, which are probably based on tactile cues and properties of light transmission. Observations of two species under varying current velocities indicated that species-specific preferences for flow rates also influenced settlement. A subtidal benthic species, Gilbertidia sigalutes. alternated between larval and adult behavior for a long period, during which their vertical migrations became increasingly nocturnal. This repeated settlement and reentry to the planktonic drift would maximize the effectiveness of sampling the bottom to locate the rare adult habitat (protected rocky crevices). Species in which the larval and adult habits differ to a great extent usually have an abrupt behavioral transformation which immediately precedes final stages of metamorphosis. Transformation of these species occurs only upon encounter with the adult habitat; the probability of such encounter reflects the mortality risk for this stage. Exceptions to these generalizations, such as G. sigalutes or A. flavidus. tend to have either unusually extended or, at the other extreme, unusually brief larval stages.

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