Environmental correlates of the distribution and size of the land snail Caracolus marginella (Gmelin 1791) and the number of micro-invertebrates associated with its feces in a secondary forest at Mayaguez, Puerto Rico

Abstract

Phoretic micro-invertebrates use animals, such as snails, as hosts or for dispersal mechanisms through host feeding and feces. Here we studied the number of micro-invertebrates associated with the feces of the land snail Caracolus marginella (Gmelin 1791) during two seasons (wet and dry) in a secondary forest at Mayagüez, Puerto Rico. Per season, a total of 70 snails were collected, their sizes determined, and the substrates and elevation where the snails occurred were noted. Snails were housed individually for 24 to 48 hours. Feces (10 mm of fecal content diluted in 1 mL of water) were collected from each individual (140 in total). The number of micro-invertebrates was expected to be greater in snail feces during the wet season, due to the increased amount of water in the surroundings for the development of organisms that rely on water to exit cryptobiosis. Relationships analyzed were: (1) snail dimensions (length, width, volume); (2) snail abundances versus (a) season, (b) environment (forest, anthropogenic), (c) location within forest (interior, peripheral), and (d) surface (tree, wall, soil); and (3) the environment with the number of micro-invertebrates found in snail feces. Season had an effect on the environment (p = 0.0001), surface (p = 0.0002) and elevation above ground (p = 0.0001) where snails were found. Nematodes (Rhabditidae) found in the samples were of the parasitic type, while rotifers (Habrotrochidae) are associated with moss, which could suggest phoresy. Contrary to the hypothesis, these micro-invertebrates were more abundant in the dry season than in the wet season.

Los micro-invertebrados foréticos utilizan animales, como caracoles, para llevar a cabo mecanismos de dispersión a través de la alimentación del hospedador y las heces. Aquí estudiamos el número de micro-invertebrados asociados con las heces del caracol terrestre Caracolus marginella (Gmelin 1791) durante dos temporadas (húmeda y seca) en un bosque secundario de Mayagüez, Puerto Rico. Por temporada, se recolectaron un total de 70 caracoles, se determinaron sus tamaños, y se anotaron los sustratos y la elevación sobre el suelo donde se encontraban. Los caracoles se alojaron individualmente en envases por 24 a 48 horas. Luego, se recolectaron heces de cada individuo (10 mm del contenido fecal diluido en 1 ml de agua), para un total de 140 muestras. Se esperaba que la cantidad de micro-invertebrados en las heces recolectadas de los caracoles fuera mayor en la temporada húmeda, debido a la disponibilidad de agua en los alrededores para el desarrollo de organismos que dependen de esta para salir de la criptobiosis. Se analizaron las siguientes relaciones: (1) dimensiones de los caracoles (largo, ancho y volumen); (2) la cantidad de caracoles con: (a) la temporada, (b) ambiente (bosque, antropogénico), (c) localidad en el bosque (interior, periferia), (d) superficie (árbol, pared, suelo); y (3) el ambiente con el número de micro-invertebrados encontrados en las heces de los caracoles. La temporada tuvo un efecto en el ambiente (p = 0.0001), la superficie (p = 0.0002) y las elevaciones (p = 0.0001) en las que se encontraron los caracoles. Los nematodos (Rhabditidae) encontrados en las muestras son del tipo parasítico, mientras que los rotíferos (Habrotrochidae) parecen estar asociados a los musgos, los que sugiere foresis. Contrario a la hipótesis, estos micro-invertebrados fueron más abundantes en la temporada seca que en la húmeda.