Aspects of the population dynamics of Neosarmatium meinerti at Mgazana, a warm temperate mangrove swamp in the East Cape, South Africa, investigated using an indirect method

Abstract

Twelve 1 m² quadrats were randomly staked out in the N. meinerti zone throughout the mangrove swamp at Mgazana. Every month during 1995 and 1996, the numbers and positions of the holes falling within the quadrats were measured and recorded. A wide size range of N. meinerti was then caught within the mouths of their burrows and measured to yield a significant crab size-hole size relationship. Data was then analysed using cohort analysis and regression. Occupied burrow size ranged from 13 to 75 mm in diameter. Hole and hence crab frequency distribution was found to be polymodal throughout the year. Neosarmatium meinerti appears to live for about 5 years. There was a complete lack of holes or crabs in the 5–10 mm hole size class or Year 0 class. These ‘missing’ classes were found when resin casts of adult burrows revealed small burrows containing the missing size class looping off the main burrow shaft. Developing crabs can thus obtain both shelter and food here (N. meinerti is known to store leaves in burrows before consumption) before they are strong enough to break through to the surface and form their own fully independent burrows. These Year 1 cohorts emerge from April to August. The monthly position of holes did not vary considerably, but hoods produced over the holes did change position suggesting that burrows are relatively permanent. There was also evidence of re-occupation of existing holes by other crabs which represents a considerable saving of energy compared to de novo excavation. Density estimates ranged from 5.5 to 22 m⁻², with an overall mean of 11.4 m⁻² and showed little seasonal trend. Crab holes were significantly denser directly under trees compared to open ground.