The lugworm Abarenicola affinis (Arenicolidae, Polychaeta) in tidal flats of Otago, southern New Zealand.

Abstract

Lugworms (Polychaeta: Arenicolidae) occur in coastal sediments worldwide and can dominate the macrofauna of intertidal sand and mud flats. They have been recognised as ecosystem engineers due to their bioturbating and bioirrigating activities, which can profoundly influence sediment properties and other biota. In view of the potentially significant role of lugworms, the present study examined aspects of the biology and ecology of the endemic lugworm Abarenicola affinis in coastal environments of southern New Zealand. Abarenicola affinis occur in tidal inlets along the Otago coast with a patchy distribution. Mean abundance ranged between 4 and 21 individuals per m² across four different tidal flats (Papanui, Hoopers, Purakaunui inlets, and Harwood in Otago Harbour), resulting in an overall mean abundance of 11 individuals per m². Two investigated lugworm populations in neighbouring inlets were stable across seasons, but exhibited differences in terms of their spatial distribution, biomass, body size, and burrow depth. Lugworm populations appeared to be limited by intertidal seagrass (Zostera muelleri), which had a significant negative influence on Abarenicola affinis abundance and biomass in one inlet. In laboratory experiments, seagrass root‐rhizome matrices imposed restrictions on the burrowing ability of Abarenicola affinis but did not prevent lugworms from burrowing and feeding similar to those in unvegetated sediment. Lugworms in seagrass treatments, particularly small individuals which stayed within the root‐rhizome matrix, processed less sediment than those in unvegetated treatments, suggesting that they may have exploited seagrass detritus as an additional food source. Sediment turnover by Abarenicola affinis was found to be stable over seasons, with lugworms being mostly active when burrows were submerged during high tide. Defaecation frequencies were shorter for small lugworms than for large ones, whereas the faecal amounts increased with increasing lugworm size. An annual sediment turnover estimate for an intertidal Abarenicola affinis population was calculated at 24.4 kg sediment dry weight per m², equivalent to a sediment depth of 2 cm. Habitat modification by lugworms had little influence on the macrofaunal assemblage composition in one tidal flat, and abiotic factors such as tidal level and proportion of sediment fines best explained assemblage patterns. Manipulative small‐scale exclusion of lugworms from otherwise densely populated areas did not result in significant changes in macrofaunal assemblages, but showed a subtle promotional effect of Abarenicola affinis on abundance of macrofauna, in particular dominant amphipods, at one of two sampling occasions. The effect was inferior to the high spatial variation in macrofaunal assemblages at the other sampling occasion. The study indicates that the impact of Abarenicola affinis on sediment and associated biota is spatially dependent and may be generally weak. As the distribution of this species is influenced by abiotic and biotic habitat variables, those factors will have, in turn, a profound influence on its engineering capacity. Abarenicola affinis does not reach the dominance and ecological importance as documented for lugworm species in other parts of the world (e.g. Arenicola marina in Europe), due to smaller and patchier populations, relatively smaller sediment turnover capacity, and less distinct influences on macrobenthic infauna. Future research is needed to gain more information on the species’ population dynamics, and to elucidate ways in which these lugworms interact with their abiotic and biotic environment in coastal ecosystems of New Zealand.