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6. Reproduction and Development of Fishes
- Johns Hopkins University Press
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74 Chapter 6 Reproduction and Development of Fishes How do fishes reproduce? Most fishes reproduce sexually, males fertilizing the eggs of females. Most fish individuals are one gender throughout life, either male or female (minnows, catfishes, salmons, black basses, perchlike fishes, tunas). Although it is often hard (for us) to tell the genders apart, in many species the difference is obvious, especially during the breeding season. Regardless , reproduction in fishes takes many twists and turns along paths not followed by amphibians, reptiles, birds, or mammals. Some fishes reverse sex, from male to female or from female to male. Individuals of other species are both male and female, including some that can self-fertilize. Sex reversal occurs in at least 34 fish families. It is most common in marine fishes, especially on coral reefs. The prevailing pattern is for an individual to mature first as a female and then later switch to male (seabasses, wrasses, parrotfishes, gobies). In most species with this pattern, males fight over females, and large, dominant males fertilize the eggs of many females. Small males are by comparison unsuccessful reproducers, but every female mates. Hence there is little cost to being small and female, and little benefit to being small and male, which helps explain why the switch occurs. Cleaner wrasses (Labroides, Labridae) form harems of one large male and a peck order of up to 10 females. The largest female mates most with the male, on down the list. If the male dies, the largest female changes to male within two weeks. In species that mature first as male, little fighting occurs over females (moray eels, loaches, snooks, porgies, threadfins, some damselfishes). Because larger females produce more eggs, the bigger the animal, the greater 75 Reproduction and Development of Fishes the advantage in being female. Small males still mate successfully and thus incur minimal cost; they wait until they are much larger to switch to female. Male-first species include the popular anemone or clownfishes (Amphiprion, Pomacentridae). Anemone fishes live in groups of two large and several small individuals in an anemone. Only the two largest fish are sexually mature, the largest one being female and the next largest being male. If the female dies, the male changes sex to female and the next largest fish in the group quickly matures as a male. If Finding Nemo had been true to life, Nemo’s dad, Marlin, should have become Nemo’s mother shortly after his original mother was eaten by a barracuda. Explain that to your little brother. Hermaphroditism is the condition of being a functioning male and female at the same time. In hamlets (Hypoplectrus, Serranidae), fishes form long-lasting pairs. During spawning, individuals switch back and forth, one fish fertilizing the eggs of the other, then vice versa. One species of topminnow (Kryptolebias marmoratus, Rivulidae) is the only fish capable of fertilizing its own eggs. Self-fertilization produces clones of genetically identical offspring. A few other unusual sexual patterns occur in livebearers and deep-sea anglerfishes. In livebearers such as some mollies (Poecilia, Poeciliidae), females mate with males but the male’s sperm only stimulate egg development without contributing any genes. All offspring are copies of the mother. Deep-sea anglerfishes (Ceratioidei) do not change sex but females may be 10 to 60 times larger than males. The males are small and parasitic, permanently fused to the side of the female, connected to her bloodstream. The female provides food and oxygen via her blood, and the male provides sperm when the female spawns. Finding a mate in the deep sea is challenging given the vast space involved, so this arrangement assures males and females are close (very close) when the time is right. Do all fishes lay eggs? Most fishes (in fact, 98% of bony fishes) lay eggs. In about 2% of bony fishes and half of sharklike fishes, fertilization is internal in the female’s reproductive tract and young are born rather than hatched. Males deposit sperm in females via a penis-like structure that is a modified fin (pelvic fins in sharks, anal fin in guppies and goodeids). Embryos use nutrients from the yolk that was originally part of the unfertilized egg (many sharks, coelacanths, scorpaenid rockfishes). In others, including goodeids, bythitid brotulas, and embiotocid surfperches, the mother provides nutrition for the young in addition to the yolk. In some sharks (e.g., lamnid White Shark), this nutrition involves a form of cannibalism. After using up their own yolk reserves, developing young...