Abstract
The abundance and biomass of benthic foraminifera are high in intertidal rocky‐shore habitats. However, the availability of food to support their high biomass has been poorly studied in these habitats compared to those at seafloor covered by sediments. Previous field and laboratory observations have suggested that there is diversity in the food preferences and modes of life among rocky‐shore benthic foraminifera. In this study, we used the stable nitrogen isotopic composition of amino acids to estimate the trophic position, trophic niche, and feeding strategy of individual foraminifera species. We also characterized the configuration and structure of the endobiotic microalgae in foraminifera using transmission electron microscopy, and we identified the origin of endobionts based on nucleotide sequences. Our results demonstrated a large variation in the trophic positions of different foraminifera from the same habitat, a reflection of endobiotic features and the different modes of life and food preferences of the foraminifera. Foraminifera did not rely solely on exogenous food sources. Some species effectively used organic matter derived from endobionts in the cell cytoplasm. The high biomass and species density of benthic foraminifera found in intertidal rocky‐shore habitats are thus probably maintained by the use of multiple nitrogen resources and by microhabitat segregation among species as a consequence.
Highlights
The study of environmental adaptation of organisms is essential to provide accuracy in our understanding of the distribution, ecology, population dynamics, species diversity, and evolution of the organisms
We conducted ultrastructural observations by transmission electron microscopy (TEM) to clarify the structures and configurations of endobionts, and we performed molecular characterizations via nucleotide sequencing to determine the origin of the endobionts
For the compound- specific stable nitrogen isotope analysis of amino acids, we used 10–200 individual foraminiferal specimens for amino acid extraction depending on their cell size, because the amounts of glutamic acid and phenylalanine were too small to detect within a single specimen
Summary
Several previous studies have investigated the adaptation and habitat preference of foraminifera in algal microhabitats based on field and laboratory feeding experiments in rocky shores (e.g., Kitazato, 1988; Langer, 1993) Those studies have identified four modes of life (based on attached substrates and way of locomotion) in the microhabitats of coralline algae and the associated trapped detrital sediments: phytal (living between the coralline algal stems), crawling (attached, mobile on the thalli of coralline algae), attached (immobile, fixed on both coralline algae and substrate), and free- living forms (living in and on the sediment, as well as on the coralline algae) (Kitazato, 1988, 1994). We conducted ultrastructural observations by transmission electron microscopy (TEM) to clarify the structures and configurations of endobionts, and we performed molecular characterizations via nucleotide sequencing to determine the origin of the endobionts From these results, we discuss the trophic hierarchy among species of foraminifera with respect to their modes of life, food preferences, and the nutritional relationships between algal endobitons or kleptoplasts
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