http://www.straightdope.com/mailbag/mseahorsesex.htm
In this article, it is stated that genders are discernable depending on which one lays the egg. Are there characteristics that all eggs share that all sperm do not, to make this easy to do, or are there species where the types of cells are similar enough or odd enough that there is a debate?
As far as seahorses, pipefishes and their allies go, none of them have sperm even remotely close to the size of the eggs. I’m pretty sure you could say the same of all fish, I think the smallest of fish eggs are still about 0.5 mm, far larger than any sperm.
To get sperm that are similar in size to the egg, you’d have to get away from the vertebrates, I’d think. Heck, even echinoderms have sperm vastly different in size from their eggs.
Wow, there must be something in the ether. I was going to start a discussion very much like this one.
I understand that in fungi and some plants (mosses, I believe), the life cycle is a bit different and that spores are released, these grow into one form of the organism, which will develop different kinds of node (I think they’re just called + and - or something) - either different ones on the same organism, or all the same on each, which bind together and initiate formation of the fruiting body. Although there is some kind of differentiation there, I’m not sure if it’s anything we could rightly call male or female.
Are you thinking of sporophytes and gametophytes?
Gametophytes are similar to us in that they produce sperm and egg, even in plants, and that the sperm and egg are haploid compared to the diploid gametophyte. Sporophytes produce spores, some of which are differentiated in size (heterosporous) and others are not differentiated in size (homosporous.) The spores can also have the same ploidy number as the sporophyte if they are produced by mitosis, and will have a different ploidy number if they are produced by meiosis. Unlike gametes, spores do not need to combine with each other to produce a new individual, they can germinate on their own asexually.
I try to think of it this way: imagine a human sperm (haploid) living for years or months on its own as an individual, and eventually giving rise to its own gamete to produce the next generation. That’s what a sporophyte is like - and it happens in a huge number of plants and fungi, it’s extremely common anywhere you find life on Earth.
That’s a bit like the lifecycle of the (OK, somewhat implausible) titular species in the film Alien, I guess - egg>facehugger>embryo/parasite>adult.
But my understanding is that fungi do combine with each other to produce the next generation of spores - and that the union involves two different types, but they’re not called ‘male’ and ‘female’
Also… I was thinking about the (plant) genus Primula - as an example of heterostyly - plants are hermaphrodites, but must be of opposite flower morphology in order to fertilise one another - it’s almost like another system of gender superimposed over the top of the existing one.
Slightly more complicated than that - fungal hyphae (the main body part of many fungi when they’re not reproducing) are often haploid already.
So, in many cases of fungal reproduction:
Haploid hyphae join to form a dikaryon - technically not really sex since the cytoplasm of the cells joins but not the nuclei.
This is where mating strains come in - not really male or female, but biochemical barriers to inbreeding or self-fertilization. There can be more than two mating types in one type of fungus.
Inside the dikaryon, fusion of the nuclei occur, creating a diploid structure.
Meiosis from the diploid structure creates haploid gametes.
Gametes fuse, creating a diploid zygote (and the sporophyte.)
Zygote undergoes meiosis again, creating haploid spore that will germinate into new haploid hyphae.
Depending on the organism, the zygote may be short-lived, or microscopic, or both, making it diffcult for people to think of as a “fungus” or as a major part of the life cycle. Also, fungi encompasses so many diverse groups that it may be good to envision this post as containing lots of weasel words like “many,” “some,” “often,” because you can doubtless find exceptions to the above. 
Here are some images of fungal life cycles:
http://www.fungionline.org.uk/7sexual/1intro.html
And a quote from a helpful cite: (my own formal mycology education is about 11 years in the past)
Thanks for that - I thought it might be more complex than my vague description of it.
What amazes me is: who even thought to check?
I mean, you’ve got a pair of seahorses, one has a penis-like appendage, the other one gets pregnant when that appendage is inserted…what could make even the most curious scientist look at that arrangement and say, “Let me examine these little critters more closely, to determine what sex the pregnant one is?”
Basically the distinction is based on the differences I mentioned: eggs are larger (usually much larger) and non-mobile, while sperm are small and mobile. In most multicellular organisms sperm and eggs are pretty distinct, and there’s not much room for confusion.
However, within the green algae there is a progression of types that may show the steps by which differentiated gametes arose.
The unicellular Chlamydomonas is an example of the most primitive condition. All gametes are morphologically identical. This condition is known as isogamy. However, there are several different mating strains; a gamete can unite with a different mating strain but not its own, which ensures cross-fertilization. The gametes are also identical to adult cells, just smaller. A haploid cell produces and releases gametes, which have flagella and can swim. Once a gamete meets another of a different strain, they fuse to produce a diploid sporelike zygote. The zygote later releases haploid daughter cells, which grow to become haploid adult cells and repeat the process. *Chlamydomonas * can also reproduce asexually.
The colonial alga Gonium is also isogamous. Panodorina is a colonial form that is slightly more complex. There are two types of gametes, but they differ only in size. Both have flagella and can swim. The condition of having two different kinds of gametes is known as heterogamy. This particular form, in which gametes differ in size but not in morphology is known as anisogamy.
The next step up is found in Eudorina, in which the large female gametes do not swim about but are retained within the colony, where they are fertilized by the small free-swimming male gametes.
Finally we get to the much larger colonial form Pleodorina. In some species of this group the female gametes not only are retained in the colony, they also lose their flagella and can thus be considered true egg cells. This condition is known as oogamy.
As I mentioned, some other unicellular organisms and multicellular fungi also have undifferentiated gametes. As far as I know, all multicellular animals have differentiated eggs and sperm.
In doing initial studies on species, taxonomists routinely do dissections in order to determine if the individual they are describing is a male or female. This can be determined by the gonad type - the texture and form of testicular tissue characteristically differs from ovarian tissue, especially under a microscope. This is particularly important in species without external genitalia, but even for other species biologists have learned not to take anything for granted. (For example, female hyenas and spider monkeys have very large clitorises that appear to be penises.) No doubt some ichthyologist dissected a pregnant seahorse and noticed that it had testicular tissue rather than ovarian.