1. Biodiversity of Fishes Sex under Water
Rainer Froese
GEOMAR

2. Importance of Sexual Reproduction
Recombination of alleles during sexual reproduction creates new, unique genomes which then are subject to natural selection
From the perspective of individual fitness, surviving offspring are likely to have improved genomes
High genomic variability increases the chances of persistance at the population level in changing environments

3. Importance of Sexual Reproduction
In unpredictable fluctuating physical environments such as most of the aquatic realm, having many offspring with different capabilities is better than having many offspring with the same capabilities
(bet-heding)
At the population level, evolution is faster if natural selection can act on a wide range of unique genomes

4. The Mechanics of Sex under Water
Eggs have to be fertilized (or activated) by the right sperms
Eggs are few and large (< 10 cm) or numerous and small (< 1 mm), internal, attached or drifting
Sperms are very small, very numerous, mobile, active outside of male body
Survival of gametes in water is short (few minutes), travel of sperms is short (few cm)
Courtship and mating procedures first help in the selection of matching mates, and then aim to increase fertilization rate

5. Modes of Reproduction in Fishes

6. Powell, M. L. et al. Integr. Comp. Biol. 2005 45:158-165; doi:10
Powell, M. L. et al. Integr. Comp. Biol : ; doi: /icb/
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7. Vertebrate Sex through the Ages
Hagfish: keeping a secret for 600 million years
Eggs are large (~ 4 cm) with horny shell, produced in batches of 20-30; iteroparous
Males have no penetrating organ
Cloacal gland may envelope sperms and eggs in slimy mass
Eggs found in females were not (yet?) fertilized

8. Lampreys (450 Million Years)
Genital papilla
in males;
semelparous

9. Lungfishes (400 Million Years)
External fertilization of large eggs
Males guard eggs in burrow (Protopterus, Lepidosiren)
Eggs are deposited among plants (Neoceratodus)

10. Coelacanths (400 Million years)
Another secret: internal fertilization without special male organ (despite lobes)
Young hatch from large eggs within the female; older embryos feed on unfertilized eggs;
Gestation takes 3 years (Froese & Palomares 2000)

11. Chimaeras (400 Million years)
Finally: the advent of male intromittent organs (claspers) and separate urogenital openings
Females lay few, large (~10 cm), horny eggs

12. Sharks and Rays (200 Million Years)
Internal fertilization with claspers, but separate urogenital opening lost
Few large young
From egg-laying (oviparity) to hatching after birth, to hatching within the mother (viviparity), feeding on other embryos, to a placenta-like arrangement

13. Video start

14. Ray-finned Fishes (150 Million Years)
Separation of anus and urogenital opening
Wide variation in strategies for fertilization and development of eggs and larvae
New strategies include:
Millions of small eggs
In-mouth fertilization and brooding (cichlids, Apogon)
Put eggs in other animal (bivalve, Rhodeus armarus, ovipositor)
Outside of the water (Leuresthes tenuis)
Monogamy, polyandry, harems
Sex change (protogyny, protandry, simultaneous)
Cloning by self-fertilization (Kryptolebias marmoratus)
Cloning by gynogenesis (Poecilia formosa)

15. Synchronizing Release of Eggs and Sperm
Parallel swimming, courtship, dancing
Trial-spawning
Fake-eggs near male genital opening in female mouth-brooders (e.g. Astatotilapia burtoni)
Genital tassel attracts mouthbrooding female to male’s genital opening in Tilapia macrochir
Male bends around female and turns her on her back to “squeeze” the eggs into bubble nest (Betta splendens)

16. Indigo Hamlets

17. Mandarin fish
Video:

18. Sockeye salmon

19. Betta imbellis

20. Exercises
In FishBase, select a species of your choice and discuss its reproductive strategy with respect to phylogeny, size, and environment (check for online photos or video)