Eggs and larval dynamics of finfish and shellfishes - Unit 3 of Developmental Biology of Finfish and Shellfish - FRM 507

1. Eggs and larval dynamics
Prepared by - Sourav Saha
FSM-2022-20-08
M.F.Sc. 1st Year, FRM

2. Reproductive cycle in fish
• Majority of teleost fishes are seasonal breeders, while a few
breed continuously.
• In order that survival of young be optimized, the timing of
spawning by the mature, adult fish must be closely linked to
the cycles of availability of prey consumed by the newly
hatched young.
• In many temperate and cold water species, spawning is an
annual event. These fishes have discrete spawning season, so
that the eggs hatch, and young are ready to consume
exogenous food, at a time when prey is abundant.

3. • In broad terms annual cycle can be divided into three
major periods, or phases:
1. A post-spawning period when the gonads are small
and appear to be in a resting phase;
2. A pre-spawning period in which the gonads begin
production of gametes (gametogenesis) and there is
production and incorporation of yolk into the oocytes
(vitellogenesis); this is accompanied by a gradual
increase in gonad size;
3. A spawning period, involving final maturation and
ripening of the gametes: this phase culminates in the
spawning act, with the release of gametes and
fertilization of the eggs.

4. Photoperiod, temperature and seasonal rainfall, among other factors, are
important in regulating reproductive cycles in teleost fishes. They show
considerable but precisely-timed annual fluctuations in temperate regions,
whereas in the tropics, dry seasons alternate with wet ones and lead to
seasonal differences in water quality and food availability.
In many Salmonids, that spawn in autumn, gradually increasing photoperiods
followed by decreasing ones or even short photoperiods play a dominant role in
the regulation of reproductive cycles.
In Cyprinid and Perciform fishes, temperature may also be a significant
regulatory factor in the reproductive cycling.
Thus, even among teleost fishes, mechanisms for reproductive timing vary
considerably.

5. Embryonic Development in Finfishes
1. Fertilization
 Fertilization is usually external in water, and sperms and ova are discharged close to each
other in water. Sperms become very active soon after they are released in water, but
survive only for a few minutes during which fertilization takes place.
 Fertilization is internal in a few teleostean species, in which the urinogenital papilla or
the anal fin is enlarged and modified for transferring the sperms.
Key stages in embryonic development:
1. Fertilization
2. Cleavage
3. Blastula formation
4. Gastrulation
5. Hatching and postembryonic
development

6. 2. Cleavage
 Cleavage is a period after fertilization, when a 1-cell embryo starts developing into a
multicellular organism.
 It consists of a series of mitotic divisions, which divide the large volume of a fertilized egg
into numerous smaller, nucleated cells—blastomeres.
 It is meroblastic (incomplete) being confined to the layer of cytoplasm.
 In early stages, the cleavage is vertical only and all the cells lie in one plane over the yolk.
 Later, horizontal cleavages take place and more than one row of blastomeres are formed.

7. 3. Blastula
 A blastula is an early embryonic stage characterized by a hollow, fluid-filled sphere or ball of
cells.
 As the zygote undergoes cleavage, it transforms into a multicellular structure with a central
fluid-filled cavity known as the blastocoel.
 The key characteristic of the blastula is its hollow, spherical structure. It consists of an outer
layer of cells called the blastoderm, which surrounds the central fluid-filled cavity.
 The blastula serves as a stage of development during which the embryo is organized into
distinct cell layers.
 The formation of the blastula is a critical step in embryonic development because it marks
the beginning of differentiation and specialization of cells.

8. 3. Gastrulation
 During gastrulation, the single-layered blastula transforms into a three-layered structure,
known as the gastrula. This process is essential for the formation of various tissues and
organs in the developing fish embryo.
 It results in the formation of three primary germ layers: ectoderm, mesoderm, and
endoderm. These germ layers give rise to different tissues and organs during the later stages
of fish development.
• Ectoderm: The outermost layer that gives rise to the skin, nervous system, and sensory
organs.
• Mesoderm: The middle layer that contributes to muscles, bones, circulatory system, and
reproductive organs.
• Endoderm: The innermost layer that develops into the digestive system and associated
organs.

9. 3. Hatching and Postembryonic development
 A small embryo formed having more or less cylindrical, bilaterally symmetrical body.
 Body of embryo becomes distinct from yolk sac.
 The embryo grows further in size, while yolk sac shrinks.
 Finally hatching takes place and embryo become free swimming larvae.
 The ectoderm forms the epidermis and its derivatives like enamel of the teeth, lens of eye,
and internal ear. Brain, Spinal cord and retina also formed fro, ectoderm.
 The mesoderm divide into epimere, mesomere and hypomere. Epimere divide to form
vertebral column, muscles etc. Mesomere give rise to kidneys, gonads and their ducts.
Hypomere enclose the coelomic cavity.

10. Embryonic Development in Shellfishes
Embryonic development in shellfish, which include various species of mollusks (such as clams, oysters,
and mussels) and crustaceans (like shrimp, crabs, and lobsters), is a complex and highly variable
process. The specific stages and details of embryonic development can differ among different species of
shellfish. Here is a general overview of the stages involved:
1. Fertilization: The first step in embryonic development is the fertilization of eggs by sperm. In some
shellfish, fertilization is external, with eggs and sperm released into the water where fertilization
occurs. In others, it may be internal, with the male transferring sperm to the female.
2. Cleavage: Cleavage patterns can be classified into two main types: holoblastic and meroblastic
cleavage.
i. Holoblastic Cleavage: This type of cleavage occurs when the entire zygote or egg undergoes cleavage.
Bivalve mollusks (e.g., clams and oysters)

11. A. Radial Holoblastic Cleavage: In radial cleavage, the cleavage planes are
perpendicular or parallel to the animal-vegetal axis. This type of cleavage is commonly observed
in echinoderms.
B. Spiral Holoblastic Cleavage: In spiral cleavage, the cleavage planes are diagonal to
the animal-vegetal axis, leading to a distinctive spiral arrangement of cells. Spiral cleavage is more
characteristic of annelids and some mollusks.
ii. Meroblastic Cleavage: In meroblastic cleavage, only a portion of the zygote undergoes
cleavage, typically due to the presence of yolk. This type of cleavage is common in organisms with
large amounts of yolk. Crustaceans (e.g., crabs and shrimp) and cephalopod mollusks (e.g., squids
and octopuses) show meroblastic cleavage.

12. 3. Blastula Formation: The cleavage-stage embryo develops into a blastula,
which is a hollow, fluid-filled sphere of cells. The blastula is the early
embryonic stage that follows cleavage.
4. Gastrulation: Gastrulation is the stage where the blastula undergoes a
reorganization of cells to form three primary germ layers: ectoderm,
mesoderm, and endoderm. These germ layers give rise to different tissues
and organs in the developing shellfish embryo.
5. Organogenesis: Following gastrulation, the embryo continues to develop,
and specific organs and structures start to form. The timing and details of
organogenesis can vary significantly among different species. For instance, in
bivalve mollusks like clams and oysters, the formation of the shell and gills is a
critical part of organogenesis.

13. Spawning stock and recruitment
It is the rate at which individuals recruit to the population that determines how rapidly
the numbers within a disturbed population will return to the equilibrium level.
Density-
dependent
factors
The absolute
numbers of fish
recruiting to the
population
influenced by
Population density
Spawning stock
Density dependent
factors related to
amount of food available
to individuals
Individual fecundity
Even when the individual fecundity is reduced, the population fecundity
may be higher for a large spawning stock.

14.  The population fecundity gives an indication of the potential numbers of
recruits immediately after spawning (N0).
 The eggs and larvae are vulnerable to predation, and mortality rate (M)
may be relatively high, so that there is a relatively rapid decline in
numbers with time.
 The change in numbers with time (D in days) may be described by -
where ND is the number of eggs or
larvae remaining after time D days
ND = N0 e-MD
When initial number (N0) are large, the newly hatched larvae may deplete
their food base and, consequently, individual growth rate may be low. Slow
growing larvae need more time to reach the size of metamorphosis
(recruitment), therefore, vulnerable to predation for a longer period of time.

15. There are mainly two theories about relation between spawning stock and
recruitment:
The Beverton-Holt model, which says that the recruitment increases with the
size of the spawning stock to a certain level, then it flattens out. Further
increase of the spawning stock does not lead to higher recruitment.
Then we have the Ricker model, recruitment increases with the size of the
spawning stock to a maximum, then recruitment decreases as the spawning
stock increases.
Stock-recruitment relationship
There may be fewer recruits to the
population when the spawning
stock is large than when it is small.

16. How Recruitment Fits in Fish Life Cycles
The fish life cycle can be broken up into several notable physical stages,
in each of which mortality can be described as density-dependent or
density-independent.
Mortality for egg and larval stages is often considered density-
independent because these tiny fish have less control over the habitats
they occupy compared to larger fish.
Once larval fish have developed enough to direct themselves, they
often settle into structural habitat (like aquatic vegetation, corals,
shallow areas, etc.) or aggregate into schools. This settlement phase is
when most scientists think mortality begins to be density-dependent,
and the recruitment period starts.
Eventually the fish will grow large enough that density-dependent
mortality ceases and the fish are considered
as recruited or recruit size.

17. all larval fish must still survive the density-dependent period to become a
recruit. For this reason, recruitment is sometimes referred to as a "bottleneck"
because it limits the number of fish surviving to larger sizes where they can be
caught by fishers and eventually spawn to replenish the population.

18. Environmental cues
Environmental cues are subtle signals in the physical environment that influence
behavior, emotions and decisions of an organism.
Different fish species have been found to use different environmental cues to
time their reproductive migration, such as photoperiod, temperature, discharge,
hour in a day, lunar cycle, atmospheric pressure, or precipitation.
gonadal maturation
and reproductive
migrations
linked to
Photoperiod
Temperature
variation

19. Temperature: Water temperature plays a critical role in fish reproduction. Many fish
species have specific temperature ranges at which they spawn. Warmer temperatures
often trigger the onset of spawning, while cooler temperatures can delay or inhibit it.
Proper temperature is also essential for the development and survival of fish eggs and
larvae.
Photoperiod: The length of daylight hours, or photoperiod, can signal the appropriate
time for fish to reproduce. Changes in day length with the seasons can trigger
spawning behavior in many species. Some fish, like salmon, rely heavily on
photoperiod cues to determine when and where to spawn.
Food Availability: The availability of prey organisms is crucial for the survival of fish
larvae. Fish often time their spawning to coincide with periods of increased food
availability, such as the hatching of planktonic organisms. Adequate food resources are
necessary for the growth and survival of larval fish.

20. Water Flow and Currents: Water flow and currents can help disperse fish eggs and
larvae, increasing their chances of survival. Some species rely on specific water flow
patterns to transport their offspring to suitable nursery areas.
Oxygen Levels: Adequate oxygen levels are essential for fish egg development and
larval survival. Poor water quality, with low oxygen levels, can be detrimental to fish
reproduction and early life stages.
Salinity: For fish that inhabit estuaries and coastal areas, changes in salinity can be an
important cue. Some species require specific salinity levels for successful reproduction
and larval development. For example, many euryhaline species need lower salinities
for egg fertilization and larval survival.

21. Predator Avoidance: Avoiding predators is essential for the survival of fish eggs and
larvae. Many fish species time their spawning to minimize exposure to predators, often
choosing nighttime or low-light conditions.
Social Interactions: In some species, social interactions among individuals can
influence the timing of reproduction. Hierarchies within fish populations may
determine which individuals get to spawn first.
Chemical Signals: Chemical cues, such as pheromones released by adult fish, can play a
role in attracting potential mates and synchronizing spawning efforts.

22. In the tropics there are minor seasonal changes in photoperiod and water
temperature, so these factors unlikely to act as a major cues in tropics. In some
species, there are no distinct spawning season. In other species, spawning
activities vary on a seasonal basis, with factors such as variations in rainfall,
current weather conditions and tidal strength acting as the environmental cue.
Weather changes are also an essential component of environmental cues that
can trigger migrations and reproductive onsets in fishes.
Sudden changes in water discharge due to hydropeaking may have profound
effects on reproductive success.
Hour in a day was an important variable affecting the size of reproductive
aggregations.
Predation is often the reason why reproductive migrations and onsets occur
during the night and twilight hours.
Multiple cues may interact in complex ways to cause additional variation in
spawning intensity.

23. • In the Indian subcontinent, a vast majority of freshwater fishes
breed during the monsoon season when rainfall is heaviest.
• In Cyprinus carpio, start of maturation process is triggered by a
combination of increasing daylength and the commencement of the
spring - rise in water temperature. Cessation of spawning activity
due to inhibitory effects of high water temperature during summer.
• Salmonids that spawn in autumn are influenced by gradually
increasing photoperiods followed by decreasing ones or even short
photoperiods.

24. The hypothalamic-pituitary-gonadal axis indicating the major endocrine factors
involved in the control of the reproductive cycle

25. Natural food of commercially important finfish and shellfish
larvae from egg to adult
Yolk
Protein or Fat
Yolk granules smaller at the periphery towards
center, unite and forming a homogenous mass.
Eggs with amount of yolk
Microlecithal eggs- are small with little yolk. Eg:
Bivalves
Mesolecithal eggs- have relatively more yolk
than microlecithal eggs. Eg: Lampreys
Macrolecithal eggs- have a large yolk. Eg:
Cephalopods

26. Larva and juvenile in fishes
Yolk sac larva or Pro larva – which retains the yolk. The yolk sac
contains all the essential nutrients to feed the larvae and to help
it grow. Larva will feed from yolk sac until it grows bigger and
becomes able to feed by itself.
Pre-larva – which starts feeding on external food like phyto or
zooplankton, mouth will be well developed, gut will be long in
case of herbivorous and short in case of carnivorous.
Post-larva – Pre adult stage, resembles adult. Herbivorous or
carnivorous – mouth may be superior, inferior or terminal –
digestive system well developed.

27. Important live food organisms
Unicellular organisms
Yeast
Algae
Live animal prey
Copepod
Cladocerans
Rotifer
Artemia

28. Common rotifers

29. Common Cladocerans
Copepods
Ostracods

30. Indian Major Carps

31. Tilapia Indian Oil Sardine
Mullet

32. Anchovies
Pearl Spot
Milk fish

33. Groupers Elasmobranchs

34. Food and feeding habits of Shellfishes
Penaeid Shrimps
Penaeid shrimps are mostly omnivorous, feeding at the muddy bottom.
Their post-larvae and juveniles feed on detritus but sub-adult prawns prefer
polychaetes, bivalves, gastropods, benthic copepods, ostracods, amphipods and
foraminifers.
The adults of larger penaeids become predaceous and feed on cephalopods and
smaller species of prawns and fishes.
Non-penaeid Shrimps
Feeds on detritus

35. .
Marine Crabs
• Crabs feed mainly on smaller crustaceans, fishes, molluscs, polychaetes, detritus,
bits of plant and other organic materials.
Lobsters
• Lobsters generally prefer mussel and clam. Occasionally, they eat smaller
crustaceans, polychaetes, fishes while scavenging.
Cephalopods
• The cephalopods are generally carnivorous and their food consists of teleost fishes,
crustaceans and other cephalopods.
• Cannibalism is common among cephalopods. Feeding intensity decreases during
the spawning season

36. Reference
• Laprise, R., & Pepin, P. (1995). Factors influencing the spatio-temporal
occurrence of fish eggs and larvae in a northern, physically dynamic
coastal environment. Marine Ecology Progress Series, 122, 73-92.
• Tanaka, S. (1974). Significance of egg and larval surveys in the studies of
population dynamics of fish. In The Early Life History of Fish: The
Proceedings of an International Symposium Held at the Dunstaffnage
Marine Research Laboratory of the Scottish Marine Biological Association
at Oban, Scotland, from May 17–23, 1973 (pp. 151-157). Berlin,
Heidelberg: Springer Berlin Heidelberg.
• Khanna, S. S., & Singh, H. R. (2016). A textbook of fish biology and fisheries.
Naraendra Publishing House.
• Kunz-Ramsay, Y. (2013). Developmental biology of teleost fishes (Vol. 28).
Springer Science & Business Media.
• Scrimshaw, N. S. (1945). Embryonic development in poeciliid fishes. The
Biological Bulletin, 88(3), 233-246.