Reproduction in monogonont rotifers

This is a marine rotifer from the genus Synchaeta (according to a key in Wallace & Snell 2010). Rotifers are a phylum of small, mostly fresh-water invertebrates. This adult individual is 0.7 mm long. When I found it in the plankton sample, it was carrying two egg capsules attached to its foot (a little stalk at posterior end of the organism-see photo). While I watched it, one of the egg cases ruptured, releasing a fully formed and seemingly completely functional miniature (~ half a millimeter long) rotifer. The empty case remained attached to the foot of the adult, while the newly hatched rotifer (shown below) began swimming immediately.   

Interestingly, rotifers can reproduce both sexually and asexually. One class, Bdelloidea, appears to lack males altogether (Wallace & Snell 1991). Sexes are separate in the Class Monogononta, to which the marine genus Synchaeta belongs. For the majority of the year, females produce offspring asexually by generating diploid embryos that develop without fertilization into females. Under certain conditions though, females produce haploid eggs. As in certain social insects (e.g. bees), if these eggs remain unfertilized, they develop into haploid males. If fertilized, diploid eggs can remain in a diapause (resting state) for up to 20 years (Fradkin 2007), and eventually develop into females who feed and grow before becoming sexually mature (Ricci & Melone 1998). These resting eggs allow the population to outlive adverse conditions (e.g. desiccation, freezing etc.). Haploid males are sexually mature at birth, do not feed and live a short life (usually about half as long as females) with the apparent sole purpose of fertilizing females via hypodermic insemination (Ricci & Melone 1998). Another fascinating fact is that most rotifers have constant cell numbers as adults (~ 1000) (Fradkin 2007). In other words, no cell divisions take place after embryogenesis is completed, and growth is accomplished solely by enlarging existing cells. In contrast, humans start out as one cell, but end up with trillions in the adult body, the vast majority of which are born post-embryonically. Additionally, in some asexually reproducing rotifer genera, generational clones have progressively shorter life spans (i.e. daughters live a shorter life than their mothers, grand-daughters live even shorter, and so on), which eventually leads to extinction of the line (see King 1969 for a review).

A dearth of information about development, distribution, and ecology in NE Pacific rotifers leave members of this phylum prime candidates for future research.

Fradkin, S.C. (2007). Rotifera. Light's manual; intertidal invertebrates of the central California coast. 4th ed. pp 280-282. J.T. Carlton (ed.). University of California Press.

King, C.E. (1969). Experimental studies of aging in rotifers. Experimental Gerontology 4: 69-79.

Wallace, R.L., Snell T.W. (2010). Rotifera. In: Ecology and classification of North American freshwater invertebrates. 1st ed. pp 173-235. J.H. Throp and A.P. Covich, eds. Academic Press.

Ricci, C., Melone, G. (1998). Dwarf males in monogonont rotifers. Aquatic Ecology 32: 361-365.