2. OVIPARITY: EGG LAYERS AND
EXTERNAL FERTILIZATION
• By far the vast majority of fishes are oviparous,
that is they produce eggs that are fertilized
after they have been laid
• About 96 percent of all living fishes are egg-
layers. Fishes exhibit a great variety of egg
types and adaptations. Morphologically and
physiologically they are tremendously
diverse
3. OVIPARITY IN MARINE
FISHES
• Very generally speaking eggs come in two kinds:
• 1. Pelagic eggs: eggs that float
• 2. Demersal eggs: eggs that sink
• By far the majority of marine fishes start out life as pelagic
eggs.
This includes:
• 1. Most all fishes that live over the continental slope
• 2. Nearly all those that range over surface waters of the
open ocean
• 3. All pelagic deep-sea fishes
4. • The eggs of these kinds of fishes are made buoyant by low-
density fluids acquired from the follicle cells of the ovary
or they develop an oil droplet independent of ovarian
tissue.
• The kinds of fishes that develop floating eggs must be able to
produce large numbers of small eggs. A fair-sized hake
(Merlucciusproductus) lays about 1 million eggs; fecundity
in cod (Gadusmorhua)ranges from 2–9 million eggs;
5.
6. • High numbers of eggs are necessary for successful
recruitment because thousands of eggs and larvae
are dispersed into areas far beyond the optimal
conditions for survival, and thousands die long
before hatching or metamorphosis to juvenile
stages
7. Demersaleggs
• Some marine fishes lay demersale eggs, that is, eggs
that are heavier than water and thus sink to the
bottom after being laid, or they are laid directly on
the bottom, or placed in nests, or fastened to
rocks, shells, seaweed, sponges, and a whole host
of other objects
8. OVIPARITY IN FRESHWATER
FISHES
• While most marine species lay pelagic eggs, demersal or
non-floating eggs are the rule in freshwater—they sink to
the bottom. There are several reasons for this:
• 1.It is physiologically more difficult to produce an egg with
a specific gravity less than freshwater.
• 2. Freshwater does not provide the rich food resource in its
upper layers as does the marine environment.
• 3. Fast moving rivers and streams would remove nearly all
eggs and larvae from a local population preventing
recruitment
9. OVOVIVIPARITY AND VIVIPARITY: INTERNAL
FERTILIZATION
• Ovoviviparous and viviparous fishes are similar in that both
are live-bearing forms that require internal fertilization.
However, they differ fundamentally with regard to the
source of nutrition for the developing young.
• In ovoviviparous forms the eggs are retained and fertilized
within the body, but the young receive no nutrients from the
mother—they must rely solely on what is provided in the
yolk. In viviparous forms, the young are nourished by some
kind of placental connection with the mother.
10. Ovoviviparity and viviparity are relatively rare among fishes—they
include only about 4 percent of all living fishes, but they are among the
most interesting when it comes to reproduction.
. Representatives are found among the following taxa: Chondrichthyes
(sharks and their allies), live-bearers(i.e., guppies and their allies,
family Poeciliidae and the coelacanths(genus Latimeria).
. Internal fertilization involves the use of some kind of organ, a
structure used to pass sperm to the female. Most live-bearing fishes
have males with such an organ. They are usually modified analog
pelvic fins.
11.
12. The guppy (genus Poecilia) is the best known case of
ovoviviparity in fishes—the eggs are fertilized within the egg
follicles of the ovary where they develop for some time.
• Some marine forms and many more freshwater forms retain their
eggs after they are laid, that is, they practice parental care.
Parental care takes on a host of different modes, from simple to
highly complex
PARENTAL CARE
13. Forms of parental care:
A.Male parental care: sea catfishes (Ariidae), sticklebacks (Gasterosteidae),
pipe fishes (Syngnathidae), and greenlings (Hexagrammidae)
B. Female parental care:
1.Oviparity with post-spawning care: the cichlidaegenus Oreochromis
2. Ovoviviparity without post-spawning care: rockfishes, genus Sebastes
3. Viviparity without post-spawning care: Elasmobranches, livebearers (e.g.,
Genus Poecilia), surfperches (Embiotocidae)
C.Biparental care: bullheads (Ictaluridae), several cichlid genera
(e.g.,Cichlasomaand Symphysodon)
D. Juvenile helpers: some African cichlids (e.g., genus Lamprologus)
14. Reproductive strategies
several males to each female (Salmoniformes, lampreys)
several females to each male (Gobiidae)
single-pair matings (guppies)
24. Reproductive strategies
bearers
- internal bearers (viviparity)
facultative - killifishes
obligate - Lake Baikal sculpins,
marine rockfishes (Scorpaenidae)
livebearers - Poeciliids, many sharks
gradient of nutrient supply from mother
superfetation
placental viviparity - sharks
25.
26. Alternative reproductive strategies
Hermaphrodites
synchronous (or simultaneous) hermaphrodites
Myctophiformes: (laternfishes) - several families
Atheriniformes: Aplocheilidae, Poeciliidae
Perciformes: Serranidae (sea basses, hamlets),
Labridae (wrasses), and others
"Egg-trading" in black hamlets Hypoplectrus nigricans (serranid)
27. Alternative reproductive strategies
Hermaphroditism
consecutive (sequential) hermaphrodites
first male (protandrous) – less common
Stomiiformes (lightfish, dragonfish)
Scorpaeniformes: Platycephalidae
Perciformes: Serranidae, Labridae, and others
blue-headed wrasse
29. Alternative reproductive strategies
parthenogenesis:
females produce diploid eggs, no sperm used
premeiotic endomitosis - mitotic division without cytokinesis
gynogenesis:
females produce diploid eggs, use sperm to stimulate development
male genome not used
congeneric species are used for sperm
hybridogenesis: one genome from female in egg,
male genome discarded - then uses sperm to restore ploidy
- no crossing over
example: Poeciliopsis monacha-lucida
30. Natural polyploids
triploids - Cyprinidontiformes: Poeciliid triploids
tetraploids (autotetraploids vs. allotetraploids)
hexaploids and octaploids (rare in carp)
