Male Aquarius remigis (Hemiptera: Gerridae), produce giant sperm with an unusual morphology: unlike most animal sperm, in which the length of the tail far exceeds that of the head, the heads and tails of A. remigis sperm are approximately equal in length. The acrosome of A. remigis sperm contains a matrix rich in flavin adenine dinucleotide (FAD), a riboflavin-containing molecule. Previous studies have shown that giant sperm are costly to produce, leading to a trade-off between sperm size and number. Furthermore, insects are unable to synthesize riboflavin and must obtain it through their diet; given their FAD-rich sperm, A. remigis males likely require large amounts of dietary riboflavin for sperm production. Thus, given their large, costly, nutrient-rich gametes, I hypothesized that male A. remigis would respond to food restriction with a decrease in sperm number.
Here, I tested the hypothesis that sperm number, and hence male fertility, is limited by food availability in A. remigis with a series of four studies examining (1) sperm numbers and egg fertility in wild males and females in natural populations, (2) the effect of food restriction on sperm number and mating behavior in wild-caught males, (3) the effect of food restriction on sperm number and mating behavior in lab-reared males, and (4) long-term sperm storage in females.
I found that males and females in natural populations often contain few or no sperm, and the fitness of both sexes may, at times, be limited by sperm supplies. Food restriction had a drastic effect on sperm numbers in wild males, with fed males containing much more sperm than unfed males. Mating behavior was also subject to the effects of food restriction, with fed males mating more frequently than unfed males. No such effect of food restriction was seen on sperm numbers or mating behavior in males reared in the laboratory with abundant food. Finally, I found that viable sperm can be stored in the female reproductive tract for at least seven weeks, which is even longer than previously known.