Assessment of non-native marine invertebrates in fouling communities in Hong Kong

Abstract

Biological invasions by non-native species represent one of the major threats to the conservation of marine ecosystems. With a high volume of international marine traffic, Hong Kong is considered to be one of the most invaded harbors in the world. However, little is known about its status with respect to invasion by non-native marine species. This thesis aimed to assess the current spatial and temporal distribution and abundance of previously reported non-native species in local fouling communities and elucidate their interactions with key environmental and biological factors in an attempt to understand their invasion mechanisms in Hong Kong.

Field assessment conducted in fouling communities on pier and mariculture facilities revealed that four of the six previously reported non-native species are still present in Hong Kong, including the solitary ascidian, Ciona intestinalis; the slipper limpet, Crepidula onyx; the Caribbean bivalve, Mytilopsis sallei; and the isopod Sphaeroma walkeri. The bryozoan Bugula californica and the mussel Mytilus galloprovincialis likely failed to establish. During the course of this study, the recently established non-native bivalve Xenostrobus securis was also found. The spatial distribution and abundance of the non-native species were closely related to seawater quality and habitat disturbance. The abundance of S. walkeri increased with low seawater quality. The sedentary non-native species were abundant in disturbed environments in Victoria and Tolo Harbors. Typhoon shelters with low seawater quality (due to estuarine conditions, pollution and/or habitat alteration) were the most invaded.

The abundance of the non-native species was also affected by the subtropical seasonality of Hong Kong with distinctive warm-wet summer and cool-dry winter seasons. Seasonal predation experiments to investigate recruitment of solitary ascidians demonstrated that their composition and abundance in fouling communities change between seasons according to their original tropical or temperate distribution, and that predation pressure on the communities drastically decreases during winter. The temperate ascidian Ciona intestinalis that recruited and became dominant in winter finds a temporal niche opportunity under such temperate seawater conditions with low predation pressure.

Laboratory experiments revealed that the non-native bivalves Xenostrobus securis and Mytilopsis sallei tolerate and respond better than the native Brachidontes variabilis under the different temperature and salinity conditions that occur in Hong Kong. As such, these non-native species could adapt to a wide range of conditions and potentially expand outside their current habitats. As demonstrated in a further experiment, the predatory whelk Reishia clavigera, which commonly occurs in oceanic and less disturbed habitats, can limit the establishment of non-native species. However, these predators are less common in polluted and disturbed environments where the non-native species could establish under low predation pressure.

In summary, non-native species in Hong Kong find spatial and temporal niches to establish in fouling communities where environmental conditions reduce the ability of native species to resist invasion. As the tropical or temperate distribution of native and non-native species have important implications on their abundance and activity in subtropical regions, the role of seasonality in species responses and biological interactions must be accounted for understanding the mechanisms and potential management of biological invasion.