Latest arrival from Australia: the flatback mangrove goby

Latest arrival from Australia: the flatback mangrove goby

Around the world, gobies are a major component of the benthic fish fauna of coastal tropical and temperate seas and estuaries – except around New Zealand. Here, triplefins (family Tripterygiidae) have evolved 30 species which have radiated to fill almost every ecological niche more usually occupied by gobies elsewhere. It appears that we only have one endemic goby species, the black goby Gobiopsis atrata – widely distributed from Northland to Rakiura Stewart Island – and there are six alien goby species. Five were recorded relatively recently over the period 1968–2012, and Assistant Curator Andrew Stewart and Research Fellow Clive Roberts discuss the latest one and how and where it was found.

The latest discovery, just published in the New Zealand Journal of Marine and Freshwater Research, is the east Australian flatback mangrove goby, Mugilogobius platynotus.

Fig. 1: New Zealand specimen of an Australian flatback mangrove goby (Mugilogobius platynotus). 41.2 mm SL, male. Scale bar 10 mm. NMNZ P.061608. Photo by Carl Struthers

This alien species (Fig. 1) was found as part of a primary school education outreach by the private owners of a wetland at the Ngunguru Old Mill Road tidal wetland drains. These photos are taken about low tide, showing fine mud, bricks, and mānawa mangrove with pneumatophore roots above the water.

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Clive, living locally, was asked to talk onsite to school children about fishes of the nearby wetlands, and the importance of these threatened habitats. Using unbaited fish traps (Fig. 6 and 7), the usual suspects of small eels, freshwater bullies, mosquitofish, and whitebait were caught, but there was also a surprise with the addition of three unusual, small (39–43 mm SL) gobies.

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Documenting the values of over 24 characters were counted or measured under a binocular microscope at our fish lab. These confirmed this goby to be different from all other species previously reported from New Zealand. Checking goby literature in Te Papa library, and discussions with goby taxonomic specialists in Australia, we conclusively nailed the species down to the temperate eastern Australian endemic Mugilogobius platynotus.

How did they get here?

Most of the gobies have become established here after hitch-hiking their way across the Tasman Sea from Australia. One probably arrived in the Bay of Islands via the fouled hulls of sailing vessels pre–1844! Based upon the distance between their native habitat and Ngunguru, and the time required for currents to carry eggs and larvae across the Tasman Sea, the only feasible method of transport was in sea chests and/or ballast water of cargo vessels. Gobies in general are among the most prolific of fishes spreading worldwide via shipping.

In addition, although small, Mugilogobius gobies are voracious predators. Mugilogobius cavifrons, a close relative to M. platynotus and native to the area of Taiwan to Timor, has become established throughout Oahu Island, Hawaii. It is now more abundant than the native species, readily eating native goby larvae and juveniles, as well as other native food sources.

Recent underwater photographs appearing on iNaturalist show an unidentifiable goby similar to Mugilogobius platynotus. These suggest that the flatback mangrove goby (or a closely related species) has spread much further than the Ngunguru area.

This highlights the need for surveying northern tidal channels of mangroves, and collection of specimens for accurate identification. In order to establish the extent and potential ecological impact of invasive species in New Zealand.

It is highly probable that there will be further incursions of the flatback goby, and perhaps its sister species, Mugilogobius stigmaticus, also native to east Australia living in the same mangrove habitat.

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