Evidence for sperm limitation in the blue crab, Callinectes sapidus
Reproductive success of female blue crabs may be limited by the amount of sperm received during the female's single, lifetime mating. Sperm must be stored in seminal receptacles until eggs are produced and fertilized months to years after mating. Further, intense fishing pressure impacts male abundance, male size and population sex ratio, which affect ejaculate quantity. We measured temporal variation in seminal receptacle contents in relation to brood production for two stocks differing in both fishing pressure on males and latitudinal effects on reproductive season: Chesapeake Bay, Maryland and Virginia, experienced intensive fishing and relatively short reproductive season; and the Indian River Lagoon, Florida, experienced lower exploitation and longer reproductive season. Nearly all (>98%) females were mated, and mating prevalence did not vary among sites during 1996. Seminal receptacle weight declined markedly for 2 mo following mating as seminal fluid disappeared to leave only spermatophores for long-term storage, which suggests that seminal fluid serves as a short-term sperm plug. Seminal receptacle weight in upper Chesapeake Bay declined by 31% from 1992–1999, indicating that females received smaller ejaculates. In 1996, seminal receptacle contents were highest (3.75 g wet wt, 2.3 × 103 μg DNA, 1.2 × 109 sperm) in Florida, but were significantly lower by: 25% for weight and 50% for sperm number at the upper Chesapeake Bay site; and 30% for weight and 65% for sperm number at lower Chesapeake Bay sites. Generally, females receive 2–3 × 103 spermatophores and 108–109 sperm cells for a full ejaculate, whereas females produce ca. 3 × 106 eggs per brood. Chesapeake Bay females appear to live about 3.5 yr, producing 1–3 broods (up to 9 × 106 eggs) per year and up to 6–7 broods (2.1 × 107 eggs) per lifetime. In contrast, Florida crabs produced up to 6–7 broods (2.1 × 107 eggs) per year, and up to 18 broods (5.4 × 107 eggs) per lifetime. In Florida, last broods produced by lab-held females were often infertile, indicating that females became sperm limited at the end of their lifetime. Experiments showed that male mating history affected female reproductive success, with females mated late in a sequence having only one third the brood hatching success of females mated early in the sequence. Sperm : egg ratios were estimated at 100:1 to 400:1 for the first brood but only about 20:1 or 30:1 for maximum lifetime broods over 2 seasons, suggesting that about 67 × 106 sperm are used per brood of 3 × 106 eggs. A model of brood production and sperm depletion in blue crabs indicates that sperm limitation may be common in Florida as females age, and in Chesapeake Bay as a result of fishery-induced reductions in initial quantities of sperm transferred at mating.
No Supplementary Data.
Document Type: Research Article
Publication date: 2003-03-01
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