In the East African coast, the effects of anthropogenic activities combined with the lack of technical solutions have led to serious contamination problems in the marine ecosystems. However, not all contaminants can be considered pollutants. Determining when contamination results in pollution requires not only chemical but also biological measurements. Because the benthos integrates conditions over time, macrobenthic organisms are considered good bioindicators to assess local environmental quality. Crabs, and especially fiddler crabs, constitute the most important macrofaunal taxa in terms of abundance, species richness and biomass in mangrove ecosystems. In the present study, the reproductive potential of the mangrove crab Uca annulipes was compared between a population inhabiting a peri-urban mangrove, subjected to untreated domestic sewage discharges, and populations inhabiting two different pristine mangroves. For this an ACI (After Control-Impact) unbalanced design was followed. Fecundity, egg loss and potential fertility were evaluated and compared by sampling a representative fraction of ovigerous females found in each of the mangrove habitats for two East African countries with different climatic, geographic and social conditions. While for Mozambique sampling took place during the two seasons (February to March, 2006 - wet season ; and August to September, 2006 - dry season), for Kenya only during the end of the long rain season was sampled (July to August, 2005). For all mangrove populations, the morphometric relations showed no abnormalities and fecundity was similar to that already earlier described for fiddler crabs with an expected increase with female size. Volume increase of the eggs during embryonic development was not different and within the common range (50 - 150 %). For both impacted locations fecundity was higher compared to the pristine locations, but in Mozambique, however, egg loss was high during embryonic development, resulting in a similar potential fertility as in the pristine mangroves. This higher egg number may be related to a higher organic content found year-round at the peri-urban mangroves. In Mozambique the populations showed reproductive seasonality. While during the dry season no ovigerous crabs were found at the pristine mangroves, there was a higher reproduction during the wet season, probably due to a higher input of nutrients, leading to a year-round breeding in the contaminated mangrove. Although we cannot state that discharges done at peri-urban mangroves are affecting negatively its ecosystem - mangroves appear naturally resilient to a certain degree of contamination, it is influencing the reproductive dynamics of Uca annulipes populations, increasing fecundity and extending its reproductive season. |
