Life history and population structure of the exploited mangrove crab Ucides cordatus cordatus (Decapoda: Brachyura) in the Caete Estuary, North Brazil.

Abstract

The semiterrestrial crab U. cordatus is one of the most heavily exploited resources of Bra-zilian mangrove forests. The growing interest in protecting and managing U. cordatus populations increases the need for research on socioeconomical and biological aspects. In the present study, reproduction, growth, population structure and size of commercially landed crabs were investigated in the mangrove fringed estuary of the river Caeté, situated at the North Brazilian coast about 200 km east-north-east of Belém. The Caeté peninsula is primarily covered by dense Rhizophora mangle dominated forest stands which are only inundated during spring tides. Since 15 years access to the forest and exploitation of U. cordatus is facilitated by a tarred road crossing the peninsula. Today, the species constitutes the main income source for 50% of the rural households, which was demonstrated by parallel running socioeconomical investigations. The results of the here presented study will contribute to the evaluation of the need for protecting and managing the U. cordatus population of the Caeté estuary and are of direct significance for local environmental insti-tutions who consider to implant temporal and spatial capture restrictions.
Reproduction of U. cordatus was seasonal and followed a strict lunar rhythm. Each study year (1997-1999) four to five andança events (times of mate searching) took place within four days after new moons between December and April with a peak occurring in either January or February. Shortly after andança activities females extruded eggs and incubated them until they spawned within four days around the following new moon. Zoea larval re-lease was precisely timed. Females spawned in the flooded mangrove forest around slack spring high tides. Peak spawning activity always occurred one day before new moon. Larvae were rapidly washed out of the forest into estuarine channels where initial zoea den-sities of up to 230000 larvae per m³ were recorded. Ebbing water masses exported the zoeae from tidal channels to coastal waters. Later stage zoeae were absent in estuarine water courses which indicates that larvae remained offshore throughout their three to four weeks of development until the postlarval stage (megalopa). Estuarine re-invasion of U. cordatus megalopae was recorded for the first time. Up to 128 individuals per m³ were found within distinct recruitment phases for a maximum of eight days before until four days after new moons.
U. cordatus grows according to the indeterminate pattern. Moulting continues after sexual maturity and occurred over the whole size range of the specimens (0.13 - 8.25 cm carapace width) studied in the laboratory and in a 100 m² field enclosure. Relative growth increment and moulting frequency decreased distinctly with size. First instar crabs had a carapace width of 0.12 to 0.17 cm and attained a size between 0.87 and 1.15 cm after six months. Within this time span crabs had moulted at least eight to ten times and mean biweekly growth increment was 23%. In males and females between 2.0 and 4.0 cm carapace widthmean size increment per moult was 13% and 14%, respectively, and decreased to 5% (males) and 2% (females) in crabs with a carapace width