meroplankton fish eggs larvae

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MEROPLANKTON:

ICHTHYOPLANKTON

Fish Eggs and Fish Larvae

- important part of meroplankton

Reasons why ichthyoplankyon surveys are done:

i. Surveys are often directed towards a single target species(or a group of closely related species) in order to use their

distribution and abundance of pelagic eggs to obtain an estimate of the biomass of the adult spawning population;

ii. Larvae of the target species are studied in order to estimate the success of the year brood resulting from its

spawn and hopefully to understand the factors underlying fluctuations of survival (recruitment);

iii. Surveys are used to evaluate fish resources in general.

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- some fish attach their eggs to substrates

- most marine fish release free-floating planktonic eggs that

are fertilized externally and float individually near the sea

surface(ex. Sardines, anchovy, tuna & many other commercially

harvested species)

Planktonic eggs typically:

•spherical (some ovoid to oblong)

•transparent

•small (usually 1-2 mm in diameter; range 0.5-5.5 mm)

•contain varying amounts of clear yolk which is the food for developing embryos and newly hatched larvae

•contain one or more spherical oil globules which aside from aiding flotation, may also provide nourishment (eggs without oil globules are equally buoyant); newly fertilized eggs usually float with the oil globule uppermost.

Appearance in plankton:

• dependent on spawning cycles of adults• linked to environmental change

Rate of egg development:• species-specific

• closely tied to ambient seawater temperature (hatching delayed in colder waters)

• hatching generally occurs within a few days to a few

weeks after the eggs are spawned

Egg Number & Survival• fecundity high: 250,000; 500,000 or over 1 million

(no. of eggs spawned per fish per spawning season)

• survival is low – food for holoplankton & adult fish

Egg Number & Size

• inverse correlation between egg size & number

Large eggs but small number

• because of size & energy restrictions• large eggs with more yolk hatched young larger

• larger young higher survival rates� too large to be eaten

� more active� better able to evade predators

Small eggs in large numbers

•with little or no nutritive material for the developing embryo•hatch at small size vulnerable to predators begin

feeding immediately

•high mortality but compensated for by large numbers

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Yolk sac

•first few days after hatching retain yolk in sac under body•rely on yolk sac as food until mouth & gut develop

•yolk exhausted begin to feed totally dependent on

suitable food in the plankton

Plankton as Food•feed on plankton for several months

•until large enough (nekton) actively seek feeding areas

independent of current drift

Fish larvae as Plankton – vulnerable to pelagic predators –both large zooplankton & nekton

Mortality very high�Cod – mortality = 99.999%

Identification of Fish Eggs

Characters used for identification:

•presence or absence of oil globules

•homogeneous or segmented egg yolk•size of perivitelline space

•egg membrane with smooth or sculptured surface•size of the egg

•shape of the egg

•in late stages of development�presence or absence of pigmentation on yolk sac or

oil globule�degree of pigmentation in the eyes

�pigmentation pattern of the embryo

�presence or absence of yellow or red pigment when� examining living eggs

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In late stages of development

�presence or absence of

pigmentation on yolk sac or oil globule

�degree of pigmentationin the eyes

�pigmentation pattern of

the embryo

�presence or absence of

yellow or red pigmentwhen examining living

eggs

FERTILIZATION TO HATCHING

Released egg(protected by fairly tough chorion or egg case. Within the chorion is the cytoplasm & yolk covered by a vitelline membrane. Often 1

or more oil globules are present)

Fertilization(spermatozoa penetrates egg through microphyle resulting in fusion

of egg & sperm nuclei)

Development = Embryogenesis(Vitelline membrane separates from chorion creating a perivitelline

space & the microphyle is plugged – preventing spermatozoa from entering; chorion hardens to protect the egg. Chorion remains

permeable to water & small molecules; most species telolecithal –yolk concentrated at vegetative pole & cytoplasm at animal pole)

The zygote period.

A: The zygote within its uplifted chorion, a few minutes after fertilization.

B. The dechorionated zygote with the animal pole to the top, about 10 min after fertilization. Yolk-free cytoplasm has begun to segregate to the animal

pole. Scale bar: 250 µm.

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Cleavage & Morphogenesis(cells divide, form layers and then organs)

Blastulation(Holoblastic cleavage = entire egg divides to form smaller cells or

micromeres at animal pole and macromeres at vegetative pole; Meroblastic cleavage = cleavage at animal pole leads to blastoderm

or cap of cells; blastoderm overgrows the yolk (epiboly) eventually enclosing it to form

Gastrula(a hollow sphere of cells containing yolk with a small opening in the

perivitelline space = the blastopore)

Fig. 4. Embryos during the cleavage period. Face views, except for B, which shows the embryo twisted about the animal-vegetal axis, roughly 45 degrees

from the face view. A: 2-cell stage (0.75 h). B: 4-cell stage (1 h). C. 8-cell stage (1.25 h). D: 16-cell stage (1.5 h). E: 32-cell stage (1.75 h). F. 64-cell stage (2 h). Scale bar: 250 µm.







Fig. 8. Face views of embryos during the blastula period. A: 256-cell stage (2.5 h). B: high stage (3.3 h). C. transition between the high and oblong stages (3.5 h).

D. transition between the oblong and sphere stages (3.8 h). E: dome stage (4.3 h). F. 30%-epiboly stage (4.7 h). Scale bar: 250 µm.

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Fig. 11. Development during the gastrula period. Left

side views, except where noted, with anterior up and

dorsal to the left. A: 50%-epiboly stage (5.25 h). B.

Germ ring stage (5.7 h). C. Animal pole view of the

germ ring stage; the arrow indicates the germ ring; the

embryonic shield will probably developed from the

flattened region of the ring at the lower right. D: Shield

stage (6 h). The embryonic shield, marking the dorsal

side is visible as a thickening of the germ ring to the

left. E: Animal pole view of the shield stage; the arrow

indicates the embryonic shield. F: 70%-epiboly stage

(7.7 h). The dorsal side of the blastoderm, to the left, is

thicker than the ventral side, to the right. The anterior

axial hypoblast, or prechordal plate, (arrow) extends

nearly to the animal pole. G: 70%-epiboly stage, ventral view, but tipped slightly forwards anteriorly to reveal the

now well delineated axial hypoblast (arrow) of the

prechordal plate. H: 75%-epiboly stage (8 h). The arrow

indicates the thin evacuation zone on the ventral side. I:

80%-epiboly stage (8.4 h), dorsal view. The arrows

indicate the boundaries between axial mesoderm in the

midline, and the paraxial mesoderm flanking to either

side. J: 90%-epiboly stage (9 h). The tail bud (arrow)

becomes visible in some embryos at this stage. K:

90%-epiboly stage, ventral view. The anterior

prechordal plate (compare with G) enlarges as the

polster. L: Bud stage (10 h). The arrow shows the

polster, and the arrowhead shows the tail bud. A

distinctive region just ventral to the tail bud (i.e. just to

the left in this view) shows where the yolk disappears as epiboly ends. Scale bar: 250

Fig. 14. Fate map of the deep cell layer (DEL) at gastrula onset, at the 50%-epiboly stage before formation of the germ ring and hypoblast. The

blastoderm now has the shape of an inverted hemispherical cup overlying the yolk cell

Organ Formation / Organogenesis(The optic cups (the future eyes) and the heart are the first organs to

be identified and in some cases the eyes become pigmented & functional; the embryonic fish often wriggles & rotates within the

chorion)

Neurula stage(embyonic axis laid down in relation to dorsal lip of blastophere =

future head, spinal cord and body musculature soon visible; the tail region moves away from the neurula & coils round inside the

perivitelline space)

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Fig. 15. Development during the segmentation period. Fig. 15. Development during the segmentation period.

Softening of the Chorion(chorion softened by enzymes secreted by glands on the head)

Hatching(the embryo breaks away from the chorion)

Larva

Postlarva

Fry(young fish is capable of active swimming

so that its strictly planktonic life is over)

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LARVA

• early larva – yolk sac prominent; about half of body length

• short incubation – eyes not pigmented, mouth not functional & anus not open

• marginal primordial fin – no fin rays yet – whole length of

body, from crown of head in dorsal to caudal to ventral side

• some species – pigmentation pattern + absence/presence

of oil globules & their position = identification• living specimens – coloured pigmentation other than black

melanophores – on body only; on primordial fin; yolk sac

& oil globule

• eyes become fully pigmented, mouth & anus open (position

of anus useful character for identification)

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• during development gradually use up yolk sac & oil

globule • yolk sac completely gone all organs necessary for

searching & devouring food are fully functional;

• availability of right food organisms is critical

complete utilization of yolk = end of larval period

POSTLARVA

Anatomic & morphometric features of postlarva

POSTLARVA

• most of essential organs are functional

• able to catch food• gradually assumes adult characteristics

• earlies postlarval stages = characteristic pigmentation

pattern for species has appeared• pigmentation pattern persists until adult meristic

characteristics are developed – pigmentation becomes

diffuse & silvering occurs

Melanophore pigmentation of postlarva

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FRYactive swimming

= nekton

• remain pelagic &

form shoals

• become benthic

(on seafloor)

• in inshore waters

• associate with jellyfish

Amali 8

Meroplankton: Fish Eggs & Larvae

March 11-12 (Thursday to Friday)

• Report to Makmal BioD at 9:30 P.M. on Thursday.• Work in groups of 3 students.

• Each group needs a digital camera, a log book, and acompound microscope.

• Calibrate your microscope first for measurement of

egg & larvae dimensions.• You are not allowed to leave the lab until 8:00 A.M. of

Friday, so bring your food & drinks when you come on Thursday night.

• Behave in the lab by not making too much noise.

• Bring references on fish embryogenesis.

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Examples of Fish Egg & Larvae Report

meroplankton fish eggs larvae

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