Th e General Natural History of the African Lungfi shes

Abstract

Th e four African lungfi shes: Protopterus aethiopicus, P. amphibius, P. annectens and P. dolloi, typically inhabit fringing weedy areas of lakes and rivers, where dissolved oxygen levels are low, daytime temperatures are high, and seasonal drying is common; conditions they are adapted to because of their broad generalist diet, wide tolerances to variations in temperature and salinity, parental care of eggs and young, aerial respiration, and the ability to aestivate. Collectively they have a broad geographical distribution but sympatric populations are probably rare and are not well documented. Th e adaptations that enable protopterids to inhabit harsh environments may also be important in the recolonization of areas subject to local extinction, and in their competition for resources with actinopterygian fi shes. In this chapter we discuss these adaptations against what is known about the natural history of African lungfi shes, an approach limited by the lack of comparative data for the four species. Growth in sub-adult African lungfi shes appears to be rapid and has been estimated at 0.3% body mass per day for a 1000 g P. aethiopicus in Lake Baringo, Kenya. Males may become mature at a larger size than females, possibly because they must construct and defend nest sites, and provide care to eggs and young. Female P. aethiopicus in lake populations only move to inshore spawning areas when ready to spawn and return to open waters soon aft er spawning, which probably accounts for skewed sex ratios in favor of mature females reported in open waters. Spawning is likely seasonal in most African lungfi sh populations but appears to be less so in lake populations of P. aethiopicus. Age atmaturity for female P. aethiopicus is approximately 3 years. Th e instantaneous mortality rate for the same species in an exploited population (Lake Baringo) is estimated at 0.4 year-1. Th ere are no comparative data for other protopterid species or populations. Ultrasonic tracking of this lake population of P. aethiopicus recorded daily movements of up to 5.2 km and indicated that this species is active throughout the diel cycle. Some individuals had large (5.8-19.8 km2) home ranges which they occupied for 8 to 18 weeks. Prey availability appears to be the most important factor infl uencing movement in open waters but intra-specifi c aggression and the presence of predators may also infl uence the use of space. Individuals are capable of long linear movements in very turbid water suggesting a well developed navigational ability. While laboratory studies have shown that protopterids are obligate air breathers, a recent fi eld study suggests that in well oxygenated water wild P. aethiopicus can meet their oxygen demands solely through aquatic respiration, thereby reducing both their metabolic costs and exposure to aerial predators. Although this result needs to be verifi ed, it illustrates the importance of a thorough knowledge of lungfi sh natural history for understanding lungfi sh biology. Keywords: ecology, behavior, distribution, growth, reproductionTh e four African lungfi sh species, the West African lungfi sh Protopterus annectens (Owen 1839), the marbled lungfi sh P. aethiopicus (Heckel 1851), the slender lungfi sh P. dolloi (Boulenger 1900), and the gilled lungfi sh P. amphibius (Peters 1844) are the only extant members of the family Protopteridae. Th e protopterid lungfi shes are placed with the South American lungfi sh Lepidosiren paradoxa (Fitzinger 1837) (Lepidosirenidae) in the suborder Lepidosirenoidei and with the more distantly related Australian lungfi sh Neoceratodus fosteri (Krefft 1870) (Ceratodontidae) in the order Ceratodontiformes, which also includes a number of fossil forms, some dating back to the late Permian or early Triassic (Nelson 2006). Although the Ceratodontiformes are generally accepted as the closest living relatives of the Tetrapods (Rosen et al. 1981; Forey 1986; Brinkman et al. 2004), other extinct non-dipnoan sarcopterygian groups are thought to be more closely related to the tetrapods (Schultze 1986; Clack 2002). Th e earliest lungfi shes may have been largely marine (Miles 1977; Schultze and Chorn 1998), but the ceratodont lineage appears to have been a freshwater one since the Permian (Campbell and Barwick 1986; Cavin et al. 2007) and all extant ceratodonts are primary freshwater fi shes. Within the Ceratodontiformes the split between the Neoceratodus and Protopterus/ Lepidosiren lineages predates the breakup of Gondwana in the Cretaceous (Cavin et al. 2007), with the later split between the South American and African lungfi shes possibly coinciding with the fi nal breakup of Gondwana in the Late Cretaceous (Lundberg 1993; Tokito et al. 2005). A continuous fossil record of protopterid lungfi shes in Africa exists throughout the Cenozoic (Stewart 2001), although only recently have molecular techniques been used to resolve the relationships among the extant species (Tokito et al. 2005) (Figure 1).