Reproduction Study Guide: FISH 4500

5 Developmental Stages of a Fish

Embryonic, larval, juvenile, adult, senescence

Embryonic Stage

Cleavage, embryo, and free embryo phases; development depends on yolk

Larval Stage

Begins with exogenous feeding and ends when the axial skeleton forms and median fin-fold disappears

Juvenile Stage

Miniature adult with fully formed organs; ends when gonads mature

Adult Stage

Mature gonads, active spawning behavior, and secondary sex characteristics

Senescence

Growth ceases and gonads become non-functional

Embryonic Sub-phases

Cleavage, embryo, and free embryo

Cleavage

Initial cellular divisions after fertilization

Embryo Phase

Developing structure before hatching

Free Embryo

Hatched individual still completely dependent on yolk reserves

Larval Stage Start

Onset of exogenous feeding

Larval Stage End

Complete axial skeleton formation and disappearance of median fin-fold

Metamorphosis

Physical transition from larva to juvenile

Saltational Theory

Development occurs gradually with abrupt transition points or leaps

Chorion

Protective membrane surrounding the developing embryo inside the egg

Gonochoristic

Individual remains biologically male or female throughout life

Differentiated Gonochorist

Undifferentiated gonad develops directly into either a testis or ovary

Undifferentiated Gonochorist

Gonad initially develops as an ovary, then some tissue re-differentiates into testes

Labile Period

Period when sex is not fixed and can be influenced or manipulated

Hermaphroditism

Reproductive system in which an individual has male and/or female reproductive function

Protandrous

Born male; some individuals later change to female

Protogynous

Born female; some individuals later change to male

Simultaneous Hermaphroditism

Functional male and female reproductive tissue present simultaneously

Heterogametic Sex Determination

Sex determined by distinct sex chromosomes

XY Sex Determination

Males are XY and females are XX

ZW Sex Determination

Males are ZZ and females are ZW

Polygenic Sex Determination

Sex controlled by multiple genes without distinct sex chromosomes

Environmental Sex Determination

Environmental factors such as temperature influence sex

Social Sex Control

Behavior or pheromones influence sex changes in some fish

Oviparous

Eggs are laid and hatch outside the female

Viviparous

Live birth; embryos receive all nutrition directly from the mother

Ovoviviparous

Eggs hatch internally; embryos receive nutrition only from yolk

Semelparous

Spawns once during its lifetime and dies afterward

Iteroparous

Spawns multiple times during its lifetime

Synchronous Iteroparity

Oocytes mature and are released together as one discrete batch

Asynchronous Iteroparity

Oocytes develop at different stages, allowing multiple spawning events

External Fertilization

Gametes are released into the water where fertilization occurs

Internal Fertilization

Sperm is deposited directly into the female reproductive tract

Primary Sex Characteristics

Gonads that directly produce gametes, such as testes and ovaries

Secondary Sex Characteristics

External physical or behavioral traits distinguishing sexes

Sexual Dimorphism

Physical or structural differences between males and females

Dichromatism

Sex-specific differences in coloration

Intromittant Organs

Male structures used to transfer sperm during internal fertilization

Claspers

Modified pelvic fins of elasmobranchs used for internal fertilization

Gonopodium

Modified anal fin of livebearing teleosts used for internal fertilization

Ovipositor

Tubular female structure used to deposit eggs into substrates or hosts

Tubercules

Raised contact organs on male fish used during courtship and spawning

Mesentery

Double fold of peritoneal membrane supporting and anchoring internal organs

Moi li'i

Hawaiian term for immature or juvenile moi; gonads are thin and undeveloped

Mana moi

Hawaiian term for mature male moi; gonads are large and milky

Moi

Mature female moi; gonads are large and contain visible eggs

Oocyte

Individual egg cell within a supporting ovarian follicle

Follicle

Supporting cellular structure surrounding an oocyte

Oogenesis

Process of egg formation and development

Vitellogen

Precursor protein from which yolk is derived

Vitellogenesis

Synthesis and accumulation of yolk within the oocyte

Perinucleolar Oocyte

Nucleoli are located around the periphery of the germinal vesicle

Cortical Alveolar Oocyte

Cytoplasm contains amphophilic cortical alveoli

Early Vitellogenic Oocyte

Fine pale pink and dark red yolk granules appear centrally

Late Vitellogenic Oocyte

Yolk granules nearly fill the ooplasm while the nucleus remains unmigrated

Mature/Spawning Oocyte

Yolk granules reach maximum size and the nucleus is no longer evident

Ovary Stage 0

Entirely perinucleolar oocytes; undeveloped

Ovary Stage 1

Predominantly perinucleolar to cortical alveolar oocytes

Ovary Stage 2

Approximately 50% early vitellogenic oocytes

Ovary Stage 3

Majority of oocytes are late vitellogenic

Ovary Stage 4

Late vitellogenic and mature/spawning oocytes; largest follicles

Ovary Stage 5

Post-spawning ovary; predominantly spent follicles

Ovulation

Release of mature oocytes from follicles into the ovarian lumen or body cavity

Spermatogenesis

Entire process of sperm cell formation

Spermatocyst

Functional testicular cyst where sperm cells develop

Spermiogenesis

Final maturation of spermatids into functional spermatozoa

Germinal Epithelium

Membrane-bound tissue containing multiple spermatocysts

Spermatogonia

Early germ cells that develop into primary spermatocytes

Primary Spermatocytes

Germ cells that develop from spermatogonia during spermatogenesis

Secondary Spermatocytes

Cells produced from primary spermatocytes

Spermatids

Cells produced from secondary spermatocytes that mature into spermatozoa

Spermatozoa

Mature sperm cells released into the testicular lumen

Spermatogenesis Sequence

Spermatogonia → primary spermatocytes → secondary spermatocytes → spermatids → spermatozoa

Testes Stage 0

Immature cells present; no spermatozoa

Testes Stage 1

Immature cells predominate with sparse spermatozoa

Testes Stage 2

Spermatocytes, spermatids, and spermatozoa occur in similar proportions

Testes Stage 3

Mature spermatozoa predominate

Testes Stage 4

Spent testes with loose connective tissue and remnant sperm

Hormone

Internal chemical signal transported through the bloodstream to target organs

Pheromone

Chemical signal released externally that affects other individuals

Estrogen

Rises in winter/early spring and stimulates vitellogenesis

Gonadotropin

Spring/early summer surge that triggers ovulation

Progestogen

Spikes around ovulation and supports final reproductive maturation

Female Spawning Hormone Sequence

Estrogen → gonadotropin → progestogen

Testosterone

Rises in winter/spring and initiates spermatogenesis

11-Ketotestosterone

Peaks in spring and promotes secondary sex traits and spermatogenesis

Male Progestogen

Spikes in spring/early summer and triggers final sperm maturation

Male Spawning Hormone Sequence

Testosterone → 11-ketotestosterone → progestogen

Spawning Manipulation

Aquaculture can alter temperature, photoperiod, or use hormone injections

Sex Determination Manipulation

Hormonal or environmental treatments can alter sex during the labile period