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I'm just realizing that several shark and snake species also give birth to live young, but I don't know whether they have placentas or similar structures. I'm s

I'm just realizing that several shark and snake species also give birth to live young, but I don't know whether they have placentas or similar structures. I'm sure they couldn't be homologous to mammal placentas, right? (Certainly not if this article is right about the timeline.) To what extent do viviparous animals converge when it comes to the mother/child interface? So some sharks have placentas, as mentioned in this article about tiger sharks secondarily losing theirs: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4906603/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4906603/ I wonder if I'll ever run out of huge, obvious questions I haven't even begun to explore.

jelliclesfarm 6y ago
Sharks are viviparous, oviparous and also ovoviviparous. [..] There are over 500 species of shark living in waters around the world and the majority give birth to live young. The remainder are oviparous, meaning they lay eggs. [..] [..]Whale sharks (Rhincodon typus) are the largest species of shark. Although these animals produce eggs, they don't lay them. Instead, the young hatch while still in the female's body and are born as miniature adults. This is known as ovoviviparity.[..] In some species, the female will produce unfertilised eggs, which are eaten by embryos. This is known as oophagy ('egg eating') and occurs in species including the shortfin mako (Isurus oxyrinchus) and bigeye thresher sharks (Alopias superciliosus). Embryos of other sharks survive by feeding on their smaller siblings. This is called intrauterine cannibalism or sometimes as embryophagy ('embryo eating'). This is known to occur in sand tiger sharks (Carcharias taurus). Placental viviparity occurs in some species of shark - once the yolk sac has been depleted, it attaches to the uterine wall, acting as a pseudoplacenta.[..] https://www.nhm.ac.uk/discover/do-sharks-lay-eggs.html https://www.nhm.ac.uk/discover/do-sharks-lay-eggs.html
- dr_orpheus 6y ago
  Thank you for this link on how sharks give birth. I never thought the subject would be so fascinating. Also led me to this interesting article on the Basking shark which has an absolutely gigantic mouth for filter feeding plankton. https://www.nhm.ac.uk/discover/basking-shark-cetorhinus-maximus.html https://www.nhm.ac.uk/discover/basking-shark-cetorhinus-maxi...
  - jelliclesfarm 6y ago
    I remember that! Also whales do that too iirc.
  - jelliclesfarm 6y ago
    Addendum to previous comment: I am reminded of Leviathans of Jupiter by Ben Bova(we lost him to covid pneumonia and stroke on 29 nov 2020) The Leviathans..city sized sentient whale like creatures that replicate by splitting themselves ..a process called ‘budding’. A very enjoyable read. This thread made me look it up to read again. If you like good old fashioned sci-fi, I recommend it. It’s part of Ben Bova’s Grand Tour series. https://aliens.fandom.com/wiki/Leviathan_(Grand_Tour) https://aliens.fandom.com/wiki/Leviathan_(Grand_Tour)
shellfishgene 6y ago
Seahorses are also interesting in this regard, not only do they also have internal pregnancy and a placenta-like structure, it's also the males that get pregnant.
- dmos62 6y ago
  Sorry for asking a simple question, but how do we distinguish sexes? If male seahorses can be pregnant, it's obviously not by which sex gives birth.
  - goalieca 6y ago
    I am not a biologist but what I remember from biology is that female seahorses have the eggs. Sexual reproduction happens when two haploid cells combine and the sex is determined based on this system.
  - chongli 6y ago
    It’s entirely determined by the gametes. Males produce sperm, females produce eggs.
    - jessriedel 6y ago
      And the thing that distinguishes eggs from sperm is what? The relative size? (Are there no species where they are roughly the same?) Or that the mitochondria come from the egg? Edit: Looks like it's merely relative size, and when neither gamete is bigger biologists throw up their hands. Sort of a bummer that there isn't a more robust definition. https://en.wikipedia.org/wiki/Isogamy https://en.wikipedia.org/wiki/Isogamy
      - chongli 6y ago
        The relative size but also the structure and the process of fertilization. The sperm enters the egg, not the other way around. Mitochondria come from the egg, yes. Also note that sperm vary in shape. Mammalian sperm have the familiar tadpole shape with a flagellum for swimming. Other species, such as nematodes, have ameboid sperm which move along surfaces rather than swimming freely.

jelliclesfarm 6y ago

Sharks are viviparous, oviparous and also ovoviviparous. [..] There are over 500 species of shark living in waters around the world and the majority give birth to live young. The remainder are oviparous, meaning they lay eggs. [..] [..]Whale sharks (Rhincodon typus) are the largest species of shark. Although these animals produce eggs, they don't lay them. Instead, the young hatch while still in the female's body and are born as miniature adults. This is known as ovoviviparity.[..] In some species, the female will produce unfertilised eggs, which are eaten by embryos. This is known as oophagy ('egg eating') and occurs in species including the shortfin mako (Isurus oxyrinchus) and bigeye thresher sharks (Alopias superciliosus). Embryos of other sharks survive by feeding on their smaller siblings. This is called intrauterine cannibalism or sometimes as embryophagy ('embryo eating'). This is known to occur in sand tiger sharks (Carcharias taurus). Placental viviparity occurs in some species of shark - once the yolk sac has been depleted, it attaches to the uterine wall, acting as a pseudoplacenta.[..] https://www.nhm.ac.uk/discover/do-sharks-lay-eggs.html https://www.nhm.ac.uk/discover/do-sharks-lay-eggs.html

dr_orpheus 6y ago
Thank you for this link on how sharks give birth. I never thought the subject would be so fascinating. Also led me to this interesting article on the Basking shark which has an absolutely gigantic mouth for filter feeding plankton. https://www.nhm.ac.uk/discover/basking-shark-cetorhinus-maximus.html https://www.nhm.ac.uk/discover/basking-shark-cetorhinus-maxi...
- jelliclesfarm 6y ago
  I remember that! Also whales do that too iirc.
- jelliclesfarm 6y ago
  Addendum to previous comment: I am reminded of Leviathans of Jupiter by Ben Bova(we lost him to covid pneumonia and stroke on 29 nov 2020) The Leviathans..city sized sentient whale like creatures that replicate by splitting themselves ..a process called ‘budding’. A very enjoyable read. This thread made me look it up to read again. If you like good old fashioned sci-fi, I recommend it. It’s part of Ben Bova’s Grand Tour series. https://aliens.fandom.com/wiki/Leviathan_(Grand_Tour) https://aliens.fandom.com/wiki/Leviathan_(Grand_Tour)

shellfishgene 6y ago

Seahorses are also interesting in this regard, not only do they also have internal pregnancy and a placenta-like structure, it's also the males that get pregnant.

dmos62 6y ago
Sorry for asking a simple question, but how do we distinguish sexes? If male seahorses can be pregnant, it's obviously not by which sex gives birth.
- goalieca 6y ago
  I am not a biologist but what I remember from biology is that female seahorses have the eggs. Sexual reproduction happens when two haploid cells combine and the sex is determined based on this system.
- chongli 6y ago
  It’s entirely determined by the gametes. Males produce sperm, females produce eggs.
  - jessriedel 6y ago
    And the thing that distinguishes eggs from sperm is what? The relative size? (Are there no species where they are roughly the same?) Or that the mitochondria come from the egg? Edit: Looks like it's merely relative size, and when neither gamete is bigger biologists throw up their hands. Sort of a bummer that there isn't a more robust definition. https://en.wikipedia.org/wiki/Isogamy https://en.wikipedia.org/wiki/Isogamy
    - chongli 6y ago
      The relative size but also the structure and the process of fertilization. The sperm enters the egg, not the other way around. Mitochondria come from the egg, yes. Also note that sperm vary in shape. Mammalian sperm have the familiar tadpole shape with a flagellum for swimming. Other species, such as nematodes, have ameboid sperm which move along surfaces rather than swimming freely.