Reproductive Seasonality in Teleosts

Environmental Influences

Angus D. Munro, Alexander P. Scott, T. J. Lam

• Publisher: CRC Press
• Year: 2019
• eBook ISBN: 9781000722451
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This important publication provides, for the first time, a comprehensive review of knowledge of reproductive seasonality in teleosts. It addresses why a particular species should show such seasonality, and how environmental cues act as regulators to ensure that reproductive maturation and breeding occur at the optimum time. The book considers the ultimate factors responsible for the evolution of reproductive seasonality in fish. It reviews salient concepts of reproductive seasonality in mammals. This volume also includes a review of accumulated knowledge of the control mechanisms of salmonids, gasterosteids, temperate cyprinids, cyprinodonts and other brackish-water forms, and marine and tropical freshwater teleosts. This is a work of value to research scientists in the field of environmental physiology, reproductive biology, and comparative neuroendocrinology and endocrinology. In addition, it is relevant for institutions involved with aquaculture and fisheries management. It is useful for post-graduate as well as undergraduate courses in fish biology and various related subjects.

Chapter 1

GENERAL INTRODUCTION

A. D. Munro

TABLE OF CONTENTS

I. Teleost Modes of Reproduction

II. Environmental Influences on Reproductive Cycles

III. The Theoretical Basis for Reproductive Strategies

IV. Ultimate Factors

A. Water Quality

1. Oxygen Availability

2. Temperature

3. Other Aspects of Water Quality

B. Predation

C. Food Availability

D. General

V. Proximate Control Factors

A. Predictive Cues

B. Synchronizing Cues

C. Terminating Cues

D. Modifying Factors

E. General

References

I. Teleost Modes of Reproduction

In terms of reproductive strategies, as in many other aspects of their biology, teleost fish are the most diversified of vertebrates. Balon^1,2 has identified three types of behavioral strategies, depending upon the degree and type of the species’ parental involvement after fertilization (together with a suite of other adaptations — anatomical, physiological, etc.).

The most primitive strategy is the nonguarding one, where there is no parental care of the zygotes. Balon¹ recognized two main types of nonguarder strategy: egg scatterers (e.g., many marine fish, many cyprinids) and egg hiders (e.g., salmonids, many oviparous cyprinodonts, some cyprinids). All produce large numbers of small eggs, although there is evidence that egg hiders produce relatively fewer, larger eggs.^1,3

Parental care of the zygotes plays an important part in the other two broad categories of reproductive strategy.¹ This is associated with females producing fewer, larger eggs³ (or, more specifically, eggs with a greater yolk content).⁴ In the case of egg guarders,¹ the spawn is deposited either on a preselected substrate (e.g., certain cichlids) or in a prepared nest (e.g., sticklebacks), to be subsequently guarded by one or both parents.

The third strategy is egg bearing.^1,2 This may be either internal, where the eggs are retained within the reproductive system of the female after internal fertilization (i.e., ovoviviparous and viviparous species), or external, where the eggs are picked up by one or both parents and carried about for a period after fertilization (e.g., mouthbrooding cichlids).¹

II. Environmental Influences on Reproductive Cycles

Most habitats are subject to regular (i.e., at least moderately predictable) periodic changes, resulting from the superimposed actions of three types of astronomical cycles: diurnal, lunar, and annual. Within any environment, there usually are regional variations in the impact of these cycles.

Thus, another aspect of the reproductive strategies of many species is that breeding is generally restricted in time (e.g., a particular time of year) as well as in space (e.g., a suitable environment for the offspring). This leads to two questions which are relevant to the present volume.

First, why should each species breed only at a particular time and in a particular place? Each species should breed only when and where particular environmental factors (so-called ultimate factors)⁵ are likely to be optimal for the survival and growth of the progeny (as well as for the postbreeding survival of the parents in iteroparous species). Those genotypes which fail to concentrate their reproductive effort accordingly, in time and space, will leave few or no offspring and hence will be eliminated by natural selection. So, what are the ultimate factors which determine the species’ spawning periodicity?

This raises the second question: in species which do show a spawning periodicity, how may an individual be able to anticipate a suitable breeding period and thus undergo gonad growth, and how may it subsequently recognize the arrival of that suitable period, to respond by mating? Thus, what are the proximate⁵ environmental factors (“cues”) which allow an individual to correlate its reproductive cycles with environmental fluctuations?

III. The Theoretical Basis for Reproductive Strategies

There is evidence that an individual must reach a threshold size interval, or a particular age, before it is capable of initiating full gametogenesis in response to the appropriate environmental cues. Various factors suggest that an individual should postpone breeding for as long as possible. For example, both the number and the size of eggs produced by a female are positively correlated with her size (in terms of weight or length).⁶ In addition, large somatic size may enhance zygote production by being of positive sexual-selection value,^7,8 as well as increasing the resource-holding potential in, for example, territorial species.⁷ However, food availability, predation pressure, and other environmental variables can act as modifying influences, through their effects on growth rate and the pattern of juvenile vs. adult mortality. They will, thus, act in favor of those genotypes in a population (whether semelo- or iteroparous) which, by entering puberty at a particular smaller size and/or a younger age, maximize their life-span fecundity.^9,10,11,12 and ¹³

Once an individual reaches this critical size/age, there must be similar strong selection pressure to make the appropriate gonadal responses at the correct time(s) of the year in predictable environments. This is so because gonadal growth and reproductive behaviors are associated with a considerable energy investment at the expense of the soma (energy from the latter may be mobilized to provide material for gonad growth, particularly in short-lived species), together with the survival cost⁹ resulting from increased risk of subsequent mortality and a reduction in future fecundity.^13,14,15,16 and ¹⁷

Thus, even after puberty, natural selection should favor those genotypes which respond to the appropriate proximate factors in a manner which will permit the most efficient sequential distribution of resources between somatic and gonadal compartments. Ideally, somatic growth should be continued for as long as possible in order to maximize the potentially realizable fecundity for that season, before diverting energy to the gonads. Hence, if an individual diverts energy from somatic growth to gonadal investment too early, then it must either breed early (and, thus, jeopardize the survival of that brood) or await optimal breeding conditions (in which case its fecundity is likely to be less than if it had initiated the switch of resources to gonad development nearer the optimum time, after a longer period of somatic growth).

Conversely, an individual which postpones gonad growth will mature late in the spawning season. Subsequently, it must either undergo gonad regression, with recycling of gonadal material back to the soma (with associated energy losses); or try for a late (and probably unsuccessful) spawning. Thus, individuals which attempt to increase potential fecund...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• Editors
• Contributors
• Table of Contents
• Chapter 1 General Introduction
• Chapter 2 General Concepts of Seasonal Reproduction
• Chapter 3 Salmonids
• Chapter 4 Cyprinidae
• Chapter 5 Sticklebacks
• Chapter 6 Estuarine and Intertidal Teleosts
• Chapter 7 Temperate Marine Teleosts
• Chapter 8 Tropical Freshwater Fishes
• Index