Reproduction in Fishes

I. Lifetime spawning frequency Reproduction in Fishes Semelparity — spawn once Iteroparity — spawn multiple times I. Lifetime spawning frequency II. Spawning cycles semelparous III. Modes of spawning Why? iteroparous IV. Sex change and mating systems II. Spawning cycles III. Modes of reproduction • semelparous — annual (or multiples thereof) • oviparity — egg laying • iteroparous — annual, lunar, daily; & cycles within cycles • ovoviviparity — livebearing, no maternal nourishment daily • viviparity — livebearing with maternal nourishment monthly yearly Oviparity Tradeoff: number vs. size of eggs/offspring Number of offspring (eggs or young) and degree of parental care Species Relative Egg or Offspring Size Mola mola very small cod very small haddock very small sturgeon medium salmon medium sculpin medium stickleback medium medaka medium sand shark very large Mean # of Eggs or Offspring Degree of Parental Care 28,000,000 none, free floating pelagic eggs 9,000,000 2,000,000 200,000 eggs buried, no other care 5,000 140 60 high 30 1-2 extreme: uterus w/ cannibalism • external fertilization — gametes shed into water - most common mode - ancestral condition - simple - small, but numerous eggs • internal fertilization — eggs fertilized internally, but later released into water - uncommon - many elasmobranchs - often very large eggs in cases 1 Internal Fertilization in Oviparous species Ovoviviparity • internal fertilization — eggs fertilized internally & hatch internally, but no direct maternal nourishment advantage - young are generally larger and more advanced disadvantages - low dispersal of offspring - few offspring produced - if mother dies while pregnant, offspring die, too examples: - most sharks and rays - coelacanth - rockfishes copulation: white tip reef sharks Viviparity • internal fertilization & direct maternal nourishment - ovarian fluids - placenta (some sharks) advantage - young are very large and advanced disadvantages - same as for ovoviviparity - energetic cost to mother 2 Internal Fertilization: males have an intromittent organ 1. modifications of pelvic or anal fins: - claspers: elasmobranchs, chimeras - gonopodia: guppies, mollies 2. genital papilla: non-bony tube - rockfishes, surfperches, cottids viviparity - lemon shark birth Viviparity viviparity - shiner surfperch birth examples: - requiem sharks (family Carcharhinidae) & hammerheads - guppies & other freshwater livebearers - marine surfperches Embiotocid embryos with trophotaenia Return to Oviparity: Modes of Oviparity 1) broadcast spawning 2) demersal, non-guarding 3) demersal, guarding 4) brooders 3 1) Broadcast Spawning - most marine fishes • eggs & sperm shed into water column & drift away • larval & juvenile/adult habitat completely different (bipartite life cycle) 1) Broadcast Spawning • advantages - zero energy expenditure after gametes released - best chance of long distance dispersal • disadvantages - very low survival of offspring - caused by: * lack of parental protection * small larval size * inability to find or return to appropriate adult habitat 1) Broadcast Spawning: Timing & Location • maximize transport of eggs & larvae away from the reef - spawn during outgoing tide - downcurrent side of reef - upward spawning rush to place gametes high in water column • two hypotheses: - minimize exposure to predators - maximize dispersal (“spread the risk”) 2) Demersal Non-Guarding • hiders and non-hiders • eggs usually placed or glued on bottom Brook trout eggs • common in freshwater fishes (minnows), but not in marine fishes advantages - zero energy expenditure after gametes released - eggs may be better protected - greater control over where offspring end up disadvantages - still quite low survival of offspring - high risk of loosing entire batch of eggs - lower dispersal - nest sites can become limited 4 Grunion spawning Grunion spawning Skate egg cases Swell shark egg case 3) Demersal Guarding • common among reef fishes (e.g., gobies, damselfish, triggerfish) • males usually guard 3) Demersal Guarding advantages - increased survival due to parental protection - increased survival due to parental care (cleaning and oxygenation) disadvantages - high cost to parent(s): energy and time - increased risk of mortality in parent(s) - nest sites may become limited 5 4) Brooding • young brooded in: – mouths (males of cardinalfishes, jawfishes, catfishes; female cichlids) – on body (some male pipefishes) – pouches (male seahorses & some pipefishes) • offspring usually (but not always — e.g., cardinalfishes) well developed when released Haplochromis spawning Haplochromis mouthbrooding female seahorse oviposition & male giving “birth” 6 4) Brooding advantages • increased survival due to parental protection • increased survival due to parental care (cleaning and oxygenation) • no need to find or maintain a nest IV. Sex change - among vertebrates, found only in fishes disadvantages • high cost to parent(s): energy and time • if parent dies, so do offspring • low dispersal Sexual Strategies • gonochorism — separate sexes, no sex change - advantage: allows sexual specialization - disadvantage: may not maximize fitness Why are fish able to change sex? • hermaphroditism - simultaneous • simple gonads and reproductive tracts - sequential • sex chromosomes are similar or identical - protogynous = female then male - protandrous = male then female Why do fish change sex? Why do fish change sex? • to maximize fitness by maximizing lifetime reproductive output Size advantage model - predicts sex change if your current reproductive value (RV) is less than what it would be if you were the opposite sex RV = (expected fecundity at a given size) × (probability of surviving to that size) 7 RV changes with body size and mating system: Why would RV differ between the sexes? 1) female — increases linearly with size 1) relative cost of gametes • sperm are cheap, eggs expensive 2) male — depends on mating system • therefore, females usually limited by ability to produce eggs or hold them or embryos in the body • competition for mates or sperm competition (69 human offspring; Russian woman, 1700’s) Gonochorism males usually limited by access to females (850+ children: Emperor of Morocco 1600-1700’s) 2) mating system... • monogamy - one partner • polygamy - multiple partners male female # eggs produced or fertilized • - polygyny - many females, one male - polyandry - many males, one female Size (body mass) RV changes with body size and mating system: immature 1) female — increases linearly with size Protogyny -- California sheephead 2) male — depends on mating system • competition for spawning sites or limited access to females Protogyny female # eggs produced or fertilized male female male Size (body mass) RV changes with body size and mating system: Protandry -- anemonefish 1) female — increases linearly with size 2) male — depends on mating system • no defensible resources • limited # of sperm needed • no direct competition between males • can occur if matings are random or there is stable monogamy # eggs produced or fertilized Protandry female male Size (body mass) 8 Trimma okinawae Paragobiodon echinocephalus Reversible Sex Change or Simultaneous Hermaphroditism Reversible sex change is very rare (a handful of gobies) # eggs produced or fertilized female male Why? • energetically expensive? • lost mating opportunities? Lythrypnus zebra Size (body mass) Gobiodon histrio Simultaneous hermaphroditism is common in two groups of fishes: • deep sea fishes Simultaneous hermaphroditism seems like best strategy • small serranids • Why is it so rare? Cheating Small serranids (Hypoplectrus, Serranus) barracudina barracudina • egg trading allows cheaters to be detected and punished • cheaters abandoned Deep sea fishes • population densities very low • must be able to act as either sex when you encounter a conspecific • abandoned cheaters lose mating opportunities while searching for new mate Serranus tigrinis • selection so strong that cheating species went extinct? Hypoplectrus indigo lancetfish Serranus tabacarius 9