Dietary assimilation and food retention time in the herbivorous terrestrial crab Gecarcoidea natalis

The ability of Gecarcoidea natalis to assimilate salts, nitrogen, energy, and fiber components from fallen leaves of Ficus macrophylla and Erythrina variegata was examined. Dry matter and energy assimilation coefficients were very similar (41% for dry matter and 46% for energy). Assimilation of N was 39% ± 12.15% and 49.2% ± 16.03% on diets of Ficus and Erythrina, respectively, and respective N retentions were approximately 1.5 and 4.2 mmol · kg⁻¹ · d⁻¹. Neutral detergent-soluble materials were the main components of dry matter assimilation in both diets (assimilation coefcients of 42.7% ± 12.6% and 58.3% ± 9.3%, respectively). Hemicellulose in the leaves of Ficus was well assimilated (49.2% ± 10.2%) and was a major component of assimilated dry matter but was poorly assimilated from Erythrina leaves (14% ± 14.2%). A substantial ability to digest cellulose in both leaf types was evident (assimilation coefficients of 42.6% ± 9.7% and 37.7% ± 15.6%, respectively). Lignin assimilation was very low on leaves of Erythrina (8.7% ± 12.2%), and the high assimilation of lignin from leaves of Ficus (39.1% ± 11.9%) may have represented tannins. Assimilation of salts from the leaves was low for K (14% and 17% for Ficus and Erythrina, respectively), higher for Na (33.6% and 47.4%) and Ca (40.1% and 34.1%), and maximal for Mg (52.6% and 65.9%). The retention time measured on a diet of Erythrina was quite short (mean retention time of 11.6 ± 5.42 h), indicating a fairly rapid passage of digesta.