Bdelloid rotifers (class Bdelloidea), with about 350 or so known species, are the largest group of animals that reproduce exclusively by parthenogenesis. Every individual bdelloid is a female that produces unfertilized eggs from which more females hatch and so on. There is no published record of a male bdelloid having ever been observed. Genetic studies done with a limited number of species also indicate that they have evolved without sexual reproduction (Welch & Meselson, 2000).
However, the absence of evidence is not the evidence of absence of male bdelloids. There could be some species that may have males, but for various reasons they may not have been detected so far.
About 3 weeks ago I received an interesting request from Bill Birky at the University of Arizona. He was asking everyone working with bdelloids a rough estimate of the total number of bdelloids that they have "examined without ever seeing an individual with sperm". It appears that Bill will try to come up with some sort of estimate of the likelihood that a male bdelloid may be found.
Today I finally had time to respond to Bill's request. However, I had never searched for sperm in any of them. So instead of the number of individuals without sperm, I sent him the number of individuals that were definitely female. I counted as a female those whose vitellaria or ovaries I had seen or those whom I had seen carrying or laying eggs or live embryos (some are viviparous). Fortunately, I have kept a written record of almost every bdelloid specimen I ever examined during the days I was working with them. These notes were almost always supplemented with drawings and often with photographs. So it wasn't too difficult to come up with a rough estimate of the number of definitely female bdelloids I have seen, which was 250. I am curious to know what Bill will eventually come up with.
Bdelloid rotifer Macrotrachela ehrenbergi. An experienced worker would recognize the bag-like organ (arrow) along the lower body wall as a vitellarium (the other vitellarium would be on the opposite side). This animal was, therefore, a female. The length of this specimen was about 250 μm (its foot is cut off in this picture).
I have one criticism of Bill's underlying premise, though. Let us assume there are indeed male bdelloids. Now we have 2 possibilities:
1. The external morphology of the males is roughly similar to those of the females. Therefore, when someone isolates a particular specimen using only external morphology discernible at low magnification to assure that it is indeed a bdelloid, the probability of it being a male is roughly identical to the ratio of males to females in the population from which that specimen came. In other words, the specimen selection process is not biased towards females, because males look the same. I have seen 250 females, but no males, therefore, the overall ratio of males to females must be less than 1 in 250.
2. The external morphology of male bdelloids is much different than those of the females and even an experienced worker may have difficulty recognizing them as bdelloids without a careful examination at high magnifications. Therefore, when someone isolates a particular specimen the probability of it being a male is much less than the ratio of males to females in the population from which that specimen came. Consequently, the overall "examined bdelloids" sample is highly biased towards females and the total number of females seen is meaningless for comparison purposes.
The egg of Macrotrachela ehrenbergi, about 80 μm long. What is the function of the spines on it?
I have a feeling that if there are male bdelloids they are very different morphologically from the females. They may be much smaller and even parasitic on or inside the females. Furthermore, they may appear during only certain times of the year and may have very short lifespans. Moreover, they may be present only in some species, most likely in those that live in permanent waters. These factors would all make it difficult and unlikely to spot them if they existed.