We are where we eat! The food selectivity of Copepoda Harpacticoida species on mangrove benthic diatoms and cyanobacteria influences their spatial distribution at a microscale level

Abstract

Harpacticoida participate in important benthic processes but their specific interactions with other primary producers are still unknown. This study investigated Copepoda Harpacticoida's selectivity of diatoms and cyanobacteria and whether their microscale distribution is influenced by diatoms and cyanobacteria distribution. We conducted a laboratory controlled experiment, where diatoms and cyanobacteria and four Harpacticoid species (Cletocamptus cf. deitersi, Mesochra sp. Robertsonia mourei and Metis holothuriae) were isolated and cultivated from water and sediment samples collected in the midlittoral zone of the Santa Cruz Canal in Pernambuco, Northeast Brazil. The grazing experiment lasted 24 h. Concurrently, sediment samples were collected with 25 contiguous syringes (1.3cm²) at one centimeter depth to verify the microspatial relation in situ. We characterized the site through the levels of chlorophyll-a, phaeopigments, silt and clay percentage, organic matter percentage and salinity. We found that the species of Copepoda Harpacticoida from a mangrove area fed selectively on different diatoms and cyanobacteria species. Furthermore, our results also indicated that there was a very low overlap in the use of diatoms and cyanobacteria by these Harpacticoida species and that the size of diatoms and cyanobacteria seemed to partially influence their selectivity. For the set of nine diatoms for which there was strong or very strong evidence that the presence of harpacticoid animals decreased their densities due to herbivore-prey interactions, seven were grazed upon by a single harpacticoid species. Our field observational study demonstrates that the aggregated spatial pattern of diatom and cyanobacteria species explains, to a large extent, the spatial distribution of Copepoda Harpacticoida species in the estuarine mud flat. For all tested species, at least one of the diatoms and cyanobacteria selected as food in the laboratory was a significant predictor of harpacticoid species spatial distribution, showing the importance of species-specific relationships.