Skov Martin W, Hartnoll Richard G
Port Erin Marine Laboratory, University of Liverpool, Isle of Man, IM9 6JA, UK.
Oecologia. 2002 Mar;131(1):1-7. doi: 10.1007/s00442-001-0847-7. Epub 2002 Mar 1.
Sesarmid crabs dominate Indo West-Pacific mangroves, and consume large amounts of mangrove litter. This is surprising, since mangrove leaves have high tannin contents and C/N ratios that far exceed 17, normally taken as the maximum for sustainable animal nutrition. This paradox has led to the hitherto untested hypothesis that crabs let leaves age in burrows before consumption, thereby reducing tannin content and C/N ratio. We excavated burrows of Neosarmatium meinerti within high-shore Avicennia marina mangroves, and investigated whether burrow leaves had C, N or C/N values significantly different from those of senescent leaves. Leaves were found in <45% of burrows, mostly only as small fragments, and N concentrations and C/N ratios of burrow leaves never varied significantly from senescent leaves. The leaf-ageing hypothesis was therefore not supported. In the field N. meinerti and Sesarma guttatum fed on sediment in 76% and 66-69% of observations, respectively, and on leaves in <10% of observations. Sediments from two A. marina mangroves had a mean C/N ratio of 19.6. Our results, and the literature, show that mangrove leaves are unlikely to fulfil the N requirements of crabs, whether or not leaf ageing takes place. Sediment detritus could be a richer source of N, as shown by lower C/N ratios and regular ingestion by crabs. By fragmenting leaves crabs may be elevating the nutritional quality of the substrate detritus.
方蟹科螃蟹在印度-西太平洋的红树林中占主导地位，并且会消耗大量红树林凋落物。这令人惊讶，因为红树林树叶的单宁含量和碳氮比都很高，远远超过17，而17通常被视为可持续动物营养的最大值。这一矛盾现象导致了一个迄今为止未经检验的假说，即螃蟹在食用树叶之前会让树叶在洞穴中老化，从而降低单宁含量和碳氮比。我们在高潮带的白骨壤红树林中挖掘了迈氏新方蟹的洞穴，并调查洞穴中的树叶与衰老树叶相比，其碳、氮或碳氮值是否存在显著差异。在不到45%的洞穴中发现了树叶，而且大多只是小碎片，洞穴树叶的氮浓度和碳氮比与衰老树叶相比从未有过显著变化。因此，树叶老化假说未得到支持。在野外观察中，分别有76%和66 - 69%的情况下，迈氏新方蟹和斑点方蟹以沉积物为食，而以树叶为食的情况不到10%。来自两个白骨壤红树林的沉积物平均碳氮比为19.6。我们的研究结果以及相关文献表明，无论树叶是否老化，红树林树叶都不太可能满足螃蟹对氮的需求。沉积物碎屑可能是更丰富的氮源，较低的碳氮比以及螃蟹对其的定期摄食就表明了这一点。通过破碎树叶，螃蟹可能提高了底物碎屑的营养质量。
