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Geographic trends in mangrove crab abundance in East Africa

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Abstract

The aim of this work was to determine the abundance of crabs inmangrove communities along a latitudinal gradient along the eastern coastof Africa from 4°S to 32°S. Surveys were made atMombasa (Kenya), Zanzibar (Tanzania), Maputo (Mozambique) and in theTranskei (South Africa). Crabs were estimated at three designated levelsin the mangroves by visual census using a common protocol, and numberswere converted to biomass.Even after standardising the selection of sites and methods of censusthere was still extensive variability in the data, emphasising the complexheterogeneity of mangrove ecosystems. Lunar phase (full versus new moonsprings) did not have a consistent effect on results, but shore height hadseveral effects. Total crab biomass was similar in the two lower shore strataexamined, but about twice as high at the top-Avicennia level. Theratio of grapsid biomass:ocypodid biomass also changed with height: fromnear unity in the lower mangrove, to 0.14 in the middle strata, but to 15at the top.There was no consistent latitudinal trend in total crab numbers, but totalcrab biomass increased from north to south. In addition there was aconsistent and marked change in the grapsid biomass:ocypodid biomassratio: this swung from 0.65 at Mombasa to 6.8 in the Transkei. This hasimplications for the transfer of primary production through the food chain. Grapsids are important macrophagous feeders on the leaves and other partsof mangroves, whereas ocypodids are microphagous deposit feeders.

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Authors and Affiliations

  1. Port Erin Marine Laboratory, University of Liverpool, Isle of Man, IM9 6JA, British Isles

    R.G. Hartnoll

  2. Dipartimento di Biologia Animale e Genetica `Leo Pardi', Università degli Studi di Firenze, Via Romana 17, I-50125, Firenze, Italy

    S. Cannici & S. Fratini

  3. Department of Zoology, University of Transkei, P. Bag X1, Umtata, East Cape, South Africa

    W.D. Emmerson & F. Porri

  4. Department of Biological Sciences, University Eduardo Mondlane, C.P. 257, Maputo, Mozambique

    A. Macia

  5. Institute of Marine Science, P.O. Box 668, Zanzibar, Tanzania

    Y. Mgaya & J.P. Shunula

  6. Kenya Marine and Fisheries Research Institute, P.O. Box 81651, Mombasa, Kenya

    R.K. Ruwa

  7. Port Erin Marine Laboratory, University of Liverpool, Isle of Man, IM9 6JA, British Isles

    M.W. Skov

  8. Museo di Storia Naturale, Sezione di Zoologia `La Specola', Università degli Studi di Firenze, Via Romana 17, I-50125, Firenze, Italy

    M. Vannini

Authors
  1. R.G. Hartnoll
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  2. S. Cannici
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  3. W.D. Emmerson
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  4. S. Fratini
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  5. A. Macia
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  6. Y. Mgaya
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  7. F. Porri
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  8. R.K. Ruwa
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  9. J.P. Shunula
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  10. M.W. Skov
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  11. M. Vannini
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Hartnoll, R., Cannici, S., Emmerson, W. et al. Geographic trends in mangrove crab abundance in East Africa. Wetlands Ecology and Management 10, 203–213 (2002). https://doi.org/10.1023/A:1020123713133

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