The community structure of the deep-sea nematode community associated with polymetallic nodules in the Central Indian Ocean Basin

Abstract

Technological advances have led to an increase in the number of investigations in the deep sea over the last two decades. However, despite a large number of scientific cruises, few published data on the nematode biodiversity and community structure are available from the abyssal nodule fields of the Central Indian Ocean Basin (CIOB). It is well known that CIOB harbors one of the largest high-grade nodule reservoirs and has therefore gained the most attention from industries and governments. Therefore, to adequately assess the potential environmental impacts of deep-sea polymetallic nodule mining in CIOB, more quantitative samples are needed to better understand trends in nematode diversity, abundance, and community structure in the nodule fields. Recently, on the basis of detailed surveys and analysis, a Test Mine Site (TMS) and Reference Mining Site (TRS) have been identified within the First Generation Mine Site (FGM) for further detailed studies. According to ISA guidelines, the TMS and TRS need to be similar in faunal abundance, composition and diversity, nodule abundances, metal grade and environmental data. As per the deep-sea environment conservation point of view, particularly to plan mining operations, the study has been done to check whether TMS and TRS have a similar type of faunal assemblage so that impacts and the ensuing loss of biota are minimized. To address this, we compared the composition and distribution of nematode communities at genus/species level from TMS and TRS. The average nematode density ranged from 71.68 ± 1.7 inds/10 cm2 in the TRS and 91.23 ± 1.3 inds/10 cm2 in the TMS. A total of 63 nematode genera belonging to 24 families were identified from TMS. The TRS samples supported 52 genera belonging to 20 families. A homogeneous community was observed at family/genus level from the TMS and TRS, particularly dominated by cosmopolitan nematode genera like Acantholaimus, Halalaimus, Thalasomonhystera and Leptolaimus. The TRS and TMS samples did exhibit a similar evenness (J'), and diversity (H’) suggesting a homogeneous nematode community throughout the area. Taxon diversity and richness estimators revealed that the current sampling design was able to characterize the majority of the nematode genera present. It can be suggested that the entire area of sampling is inhabited by a single, uniformly distributed, nematode assemblage at the genus level.This study also addresses whether or not nematode assemblages at genus/ species level inhabiting the bare sediment (soft sediment without nodules) in abyssal nodule fields are unique and differ from those found in the crevices of polymetallic nodules. As expected, there were fewer numbers of total genera recorded from the crevices with the differences being found in the dominant genera and families. Thalassomonhystera, Acantholaimus and Desmoscolex were noted as characteristic of the bare sediment while Leptolaimus and Camacolaimus showed the highest likelihood in nodule crevices. Hard nodule crevice substratum does favor some species level morphotypes of the genus Leptolaimus and Camacolaimus since these morphotypes were found exclusively in nodule crevices. However, owing to the still limited sampling effort in the deep sea, it is premature to suggest that these species are endemic to nodules. Implications of the present findings for environmental management and future research needs are provided.