Aseasonality in the abundance and life history of an ecologically dominant freshwater crab in the Rift Valley, Kenya

Abstract

1. Freshwater crabs appear to show at least two alternative life history patterns, which differ in the timing of seasonal reproduction. Reproduction occurs during low flow among temperate lotic species, but during high water levels among wetland species. Crab biomass is often very high and both strategies would lead to spatial and temporal pulses in density and biomass. The life history and reproductive strategy adopted by tropical lotic species is poorly known, however, despite their importance in community and ecosystem dynamics. 2. In this study, we determined annual patterns of life history, density and biomass of a lotic freshwater crab in a small headwater stream in the East African highlands where it maintains high biomass. This crab is an as yet undescribed species of Potamonautes, here referred to as the Chinga crab. 3. Crabs were sampled non-destructively for 15 months using baited traps and benthic sampling with a Surber sampler. At the end of the study, an intensive hand search was carried out. Each method was biased towards different size classes of crabs and the efficiency of both long-term methods varied according to water levels in the stream. The intensive search was more effective than benthic sampling, but failed to record the large individuals caught by baited traps. 4. Population density and biomass remained constantly high throughout the study period. Reproduction, as evidenced by the presence of ovigerous females and small free-living juveniles, also showed no seasonality. As a consequence, the population size structure (size-frequency distribution) of crabs remained constant throughout the year. 5. The Chinga crab illustrates a third life history pattern, with no clear breeding season, and this may be common among tropical species. This is probably a consequence of the non-seasonal nature of its habitat: temperature varied little throughout the year and rainfall fluctuations were relatively small. This strategy allows the species to maintain high biomass without seasonal pulses and, perhaps, to dominate community and ecosystem processes.

Aquatic Invertebrates as Unlikely Vectors of Buruli Ulcer Disease

“…Most tropical species have multivoltine (multiple generations) and asynchronous (overlapping) life cycles throughout the year (39). For instance, all life stages of tropical naucorids have been reported through both wet and dry seasons over 2 years (38), and the same has been documented for other aquatic invertebrates in Kenya (36) and Lake Tanganyika (37). Therefore, although season might have had a small effect on the abundance variation of biting hemipterans and other invertebrates, this infl uence was unlikely to have limited our potential for detecting differences between BU+ and BU-sites.…”

“…Season may also play a role in invertebrate abundance patterns; however, in many tropical and subtropical regions, most invertebrate taxa show minimal seasonally based abundance patterns (36)(37)(38). Most tropical species have multivoltine (multiple generations) and asynchronous (overlapping) life cycles throughout the year (39).…”

Dieta del cangrejo dulceacuicola Bottiella niceforei (Schmitt & Pretzmann, 1968) (Decapoda: Trichodactylidae) y su relacion con el procesamiento de materia organica en una corriente intermitente del noroeste de Venezuela

“…Está bien documentado que los insectos acuáticos tienen un papel fundamental en el procesamiento de la MOPG en ríos de orden bajo, de Europa y Norteamérica (Allan & Castillo, 2007). Sin embargo, para las zonas tropicales de Asia, África y América se ha reportado una baja densidad de insectos fragmentadores en ríos tropicales de tierras bajas (Winterbourn et al, 1981;Bunn, 1986;Yule, 1996;Rosemond et al, 1998;Dudgeon & Wu, 1999;Tumwesigye et al, 2000;Mathuariau & Chauvet, 2002;Rincón et al, 2005;Greathouse & Pringle, 2006;Dobson et al, 2007;Wantzen & Wagner, 2006). Aunque esto no parece cumplirse en Australia (Cheshire et al, 2005), ni en ríos de altas elevaciones en Malaysia (Yule et al, 2009).…”

“…En una corriente intermitente del noroeste de Venezuela, se ha observado que la mayor parte de los peces son omnívoros, encontrando un consumo elevado de material alóctono vegetal (74,4-87,3%) (O. Pomares, datos sin publicar). Dobson et al (2007) reportan una alta biomasa de cangrejos dulceacuícolas (58-94%) en ecosistemas tropicales de África, indicando que un importante componente de su dieta lo constituye el detrito de origen vegetal.…”

Modifying Surber sampling technique increases capture of freshwater crabs in African upland streams

“…Different methods are often selective, being variously more effective at catching adults or juveniles, or even being biased in favour of different sexes (Rabeni et al 1997, Costa and Negreiros-Fransozo 2002, Taggart et al 2004, Gladman et al 2010. Baited traps in particular are biased toward large individuals (Turnbull-Kemp 1960, Disney 1971, Hill and O'Keeffe 1992, Somers and Nel 1998, Smith et al 2004, Dobson et al 2007a. Furthermore, use of baits as attractants means that the area sampled is unknown.…”