Influence of surface texture and microhabitat heterogeneity in structuring nodule faunal communities

Manganese nodules provide an important habitat for sessile benthic organisms, many of them novel foraminifera, in the abyssal Pacific. The surface texture and microhabitat heterogeneity of nodules may play a significant role in structuring these communities. We analyzed the distribution of foraminiferal species on 20 nodules collected using either a USNEL box corer or the Nautile submersible sampling tool at a 5000-m-deep site in the Tropical North Pacific. The nodules had an upper region characterized by two microhabitats, namely raised surfaces and intervening depressed surfaces. The more or less vertical sides of the nodules were regarded as a third microhabitat. The upper region had a predominantly smooth texture while the sides had a rough texture. We tested whether the percent cover of each of the 34 species examined was distributed evenly over the three microhabitats and the two surface textures (smooth vs rough). More than half (20 out of 34) of species covered a greater area on smooth surfaces than on rough surfaces, while one-third of the species (12 out of 34) occurred more on raised microhabitats than on depressed surfaces and nodule sides. These two results are closely interrelated, since 78% of the raised surfaces were characterized by a smooth texture. The nodule-encrusting species were feeding either on suspended particles or on particles deposited on the nodule surfaces. Because most species had a mat-like or domed morphology, we suspect that nodule surface feeding was more prevalent. Current flow, which transports propagules and particulate food, may be the main environmental factor explaining the observed faunal distributions, as it is in the case of other hard-substrate assemblages.