All earthworm species, to my current knowledge and including mostly for the purpose of discussion, the most popular bait and/or composting species, E fetida and E hortensis, contain both male and female reproductive organs. They do not fertilize themselves though, more like cross pollination of some plants. At least two individual worms need to come together and exchange sperm which is stored in each respective worm for the use in reproduction. If you raise worms you will undoubtedly see at one point or another, two worms all entertwined together so that they look much like a pretzel. They are mating and sometimes three or more will all pile up into a big worm orgy!
Not to get into a big debate on which worm species reproduces the most and fastest, but to offer up an observance of particular worm habit and how it may be more of a factor in population density than just saying E fetida reproduces the fastest compared to E hortensis, which may be a deciding factor when considering what species of worm to acquire when desiring to raise worms for whatever purpose.
For the sake of fact, it is a fact that E fetida produces more offspring per cocoon and at a faster frequency than E hortensis, thus lending to the notion that they must be the worm of choice for the perspective worm farmer or home vermicomposter. But, I believe we need to consider natural habits and what is the desired end goal of the respective individual.
E fetida, AKA red wiggler, manure worm. The natural habit of this particular worm in itself adds to the frequency and speed of its reproductive cycle. They will naturally gather together in and under a food source such as a manure pile out in a pasture, said more to be "surface feeders". They didn't gather together for the purpose of mating, rather their being together for the purpose of feeding makes it convenient that they should also easily find a mate with which to copulate as they cross paths in the food source.
On the other side of the coin we have E hortensis, AKA European Nightcrawler, a worm who's natural habit is to tunnel and burrow or come topside to crawl at night, finding food as it goes about and is in a much less favorable situation to happen across a mate.
When we desire to raise worms for whatever purpose, E fetida may seem like the natural choice versus E hortensis since we are providing an artificial environment that takes advantage of the natural habit but I believe when we also provide an artificial environment for other worm species which provides them with their requirements of food, moisture and air, by stocking in densities sufficient to provide a more commonplace "meeting" of two individuals, we may end up with more worm mass overall and greater composting efficiency since the size is greater of adult worms that are processing more foodstuffs by weight per individuals.
However, we can only have so many worms of either species in a given square ft of bedding right? Well, E fetida will naturally inhabit mostly the top few inches of bedding versus E hortensis which will not only inhabit the top layers of bedding where their food supply is being concentrated but will also just as easily tunnel and burrow throughout the bedding and so I believe we can expect more worm mass overall per sq ft of bedding with E hortensis. Both species will somewhat regulate their own population densities depending on available area and food supply and so I believe E hortensis is a superior worm all around for vermicomposting and bait raising applications, given that it also will excel in a wider range of temperature extremes than E fetida and is a much larger and robust bait worm.