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Distribution of invasive Magallana (Crassostrea) gigas and native Ostrea edulis oysters in natural habitats of the Northern Adriatic Sea

Oyster species are distributed worldwide in tropical and temperate waters and are predominantly coastal, occupying various types of hard substrate in the intertidal and/or shallow subtidal zones of enclosed, wave sheltered estuaries and bays. Lim Bay in the Croatian northern Adriatic provides such suitable habitat and is hence also one of the main shellfish aquaculture areas of the region. The native flat oyster, Ostrea edulis, is the only oyster species traditionally cultured in Croatia. Nevertheless, in natural habitats of Lim Bay and along the coastline of Istria peninsula, feral populations of the non-native Magallana (Crassostrea) gigas oysters have been recorded. At present it is unclear whether the introduction of M. gigas resulted from short-lived intentional aquaculture attempts in the Lim Bay in 1970s or via dispersion from neighbouring Italian coasts where M. gigas is cultivated commercially. Given the high invasion success of M. gigas the aim of this study is to investigate whether the native and non-native oysters are in competition for space in their natural habitats. The distribution and abundance of oysters was determined by in situ visual counts at 3 depth zones (surface, 3 m, 6 m) of 6 locations (2 within the Lim Bay, 2 on Istria coastline north and 2 on coastline south to the estuary). O. edulis mostly occurred as solitary specimens and density slightly increased with depth. M. gigas were found exclusively at the surface layer where it formed thick, dense clusters resulting in much higher densities compared to O. edulis in all locations (10.4±4.9 to 141.7±31.2 ind/m2 vs. 0.5±1.8 to 1.0±2.4 ind/m2) except to the north of estuary where the distribution and abundance of two species resembled (0.5±1.8 to 3.6±5.0 ind/m2 vs. 1.6±2.8 to 2.1±3.1 ind/m2). Interestingly, the coexistence of oyster species at surface was such that M. gigas were located always in the medio-littoral zone while all recorded O. edulis were below the low–tide line. There seems to be no overlap in depth distribution of the two oyster species, while their spatial distribution with higest densities of M. gigas within the bay and locations to the south might indicate the local introduction origin. However, to corroborate this, further studies on a larger spatial scale, as well as population genetic studies, are needed. This study is the first quantitative assessment of M. gigas in the Adriatic and provides a base for monitoring changes in its distribution.

Crassostrea gigas, Ostrea edulis, Adriatic Sea, distribution

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