Pondering sex has a long history... that’s not quite right either. Let me start in a different way.
Some Gastropod species change sex during their lifetime. One species with perhaps the most interesting name is the slipper shell, Crepidula fornicata — did Linnaeus use this name (the fornicata part) because he thought they had a lot of sex? These snails stack on top of each other so perhaps. Linnaeus described this species in 1758 in his famous work Systema Naturale where he outlined the zoological naming system still used today.
FYI: Solo settlers, those not finding a current stack of snails, may develop into females, which takes longer, and begin developing a new stack of snails.
One focus of researchers has been to parse out the selective advantage of changing sex. The thinking goes something like this: Producing sperm is cheaper (doesn’t take as much energy) than producing eggs, so small/young snails can start enhancing their fitness more quickly by developing sperm while still saving some energy to grow their body. Once they are larger, snails put more energy into producing eggs and less into body growth. If most snails are male, since it requires less energy to be male, then a new (newly large) female, has enhanced fitness.
Side note: The term fitness, used here, is not Arnold Schwazenagger fitness but evolutionary fitness, defined as the number of offspring you leave to the next generation.
This idea of changing sex to take advantage of size/energy relationship and reproductive opportunities can be diagrammed effectively.
The idea of fitness advantage was outlined by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection. Since, it has remained a topic of research with some very ‘heavy hitters’ of science weighing in (in the readings section below I list a few, Ghiselin, Hamilton, but there are many more).
The idea, as I’ve outline above with Slipper Shells, seems pretty straight forward: When small be male, when large be female; thus these snails take advantage of the best fitness situation for their age/size.
In a 2004 paper, researchers wondered about the same set of advantages in snails that are simultaneous hermaphrodites. That is, rather than changing from male to female as they age, snails are, instead, both at the same time. Among the air-breathing snails this is a very common lifestyle. They asked, do snails prefer the role of one sex over another? and under what situations?. These researchers setup a series of experiments that effectively answered these questions:
First, they observed matings among groups of snails. Overall observation — between mating pairs the one acting as female tended to be larger.
Second, they paired up snails — large with large, small with small, and large with small.
Third, groups of three snails were placed together. large/large/small, large/small/small.
Overall observations from pairings and trios of snails — Small snails actively sought the male role and would reject playing the female role, large snails would accept the female role.
The authors take home message: The preferred sex role is male, but when a larger snail encounters a smaller snail, the large snail accepts the female role.
Being a simultaneous hermaphrodite would seems to be advantageous to being a sequential hermaphrodite under some situations. In a population that is sparse or spread out sequential hermaphroditism would mean you could mate with any adult you encounter. Maybe not an advantage for the stuck-in-place Slipper Shells.
Interested in how hermaphroditism evolves in the first place? See Ghiselin’s work below.
Last; a quick note on sex/love darts:
In discussing snail sex on any level it’s difficult to not bring In sex darts (also called love darts). Among terrestrial snails (sequential hermaphrodites) some groups have darts that are ‘fired’ (not really a harpoon — they cannot reach very far — they must be touching the target) into the perspective mate. Two snails will ‘dance’ around each other both trying to impale the other with its dart. The dart does not carry the gametes, instead in carries hormones that help make fertilization by sperm more likely. It turns out that most darts do not find a target, and mating may take place regardless.
How did this, that is sex darts, come about: Is there a book on the evolution of sex darts — time for some searching.
Readings:
Systema Naturale - from the Biodiversity Heritage Library
https://www.biodiversitylibrary.org/item/10277#page/3/mode/1up
Fisher, R.A. 1930. The Genetical Theory of Natural Selection, Clarendon Press, Oxford
Ghiselin, M.T. 1969. The evolution of hermaphroditism among animals. The Quarterly Review of Biology 44: 189–208.
Hamilton W.D. 1967. Extraordinary sex ratios. A sex-ratio theory for sex linkage and inbreeding has new implications in cytogenetics and entomology. Science 156: 477–488.
Ohbayashi-Hodoki K., F. Ishihama, M. Shimada. 2004. Body size–dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta, Behavioral Ecology 15: 976–981