Critters of Ohinau Island

Te Papa vertebrate curator Dr Colin Miskelly recently spent 12 days on Ohinau Island (east of Whitianga, Coromandel Peninsula) as part of a Te Papa seabird research team. This blog reports on some of the more impressive invertebrate species that he found on the island.

Ohinau is a 43 ha forested island owned by Ngati Hei, and lying about 7 km south of other islands in the Mercury Island group. The island formerly held populations of Pacific rats (kiore), mice and rabbits, but has been free of introduced mammals since these were eradicated by Ngati Hei and the Department of Conservation (DOC) in 2005. The forest is low in stature and diversity, with canopy species including mahoe, mapou, karaka and pohutukawa. The island was very dry during our visit, and it is likely that some species (e.g. Rhytida snails) were staying in retreats deep underground.

Shell of Rhytida greenwoodi, Ohinau Island, January 2014. The dozen or so shells we found were a new record for the island. Image: Colin Miskelly, Te Papa

Shell of Rhytida greenwoodi, Ohinau Island, January 2014. The dozen or so shells we found were a new record for the island. Image: Colin Miskelly, Te Papa

Although rodents were eradicated from Ohinau nearly a decade ago, evidence of their former presence was readily apparent among the invertebrate fauna. In addition to the scarcity of Rhytida shells, we did not see or hear any tree weta or ground weta, and the only Mimopeus darkling beetles that we saw were on rock stacks accessible at low tide. These two stacks also had healthy populations of common geckos (which were scarce on the main island), suggesting that the Pacific rats were absent or not permanently resident on the stacks, even when present on the main island. (Mice were only detected on Ohinau Island in 2003, two years before they were eradicated, along with Pacific rats and rabbits.)

Darkling beetle (Mimopeus elongatus: Tenebrionidae) on a rock stack off Ohinau Island, February 2014. The presence or absence of Mimopeus beetle on islands off northern New Zealand is a sure sign of whether rodents are (or were) present. Image: Colin Miskelly, Te Papa

Darkling beetle (Mimopeus elongatus: Tenebrionidae) on a rock stack off Ohinau Island, February 2014. The presence or absence of Mimopeus beetles on islands off northern New Zealand is a sure sign of whether rodents are (or were) present. Image: Colin Miskelly, Te Papa

Among the larger insects on the main island, clapping cicadas (Amphisalta cingulata) and variable cicadas (Kikihia muta) were common, and we found a single New Zealand praying mantis (Orthodera novaezealandiae).

A variable cicada (Kikihia muta) climbs a tree trunk at night, in order to emerge as a flighted adult. Ohinau Island, January 2014. Image: Colin Miskelly, Te Papa

A variable cicada (Kikihia muta) nymph climbs a tree trunk at night, in order to emerge as a flighted adult. Ohinau Island, January 2014. Image: Colin Miskelly, Te Papa

A subadult New Zealand praying mantis (Orthodera novaezealandiae) on Ohinau Island, February 2014. Image: Colin Miskelly, Te Papa

A subadult New Zealand praying mantis (Orthodera novaezealandiae) on Ohinau Island, February 2014. Image: Colin Miskelly, Te Papa

The most conspicuous large invertebrates on Ohinau Island were purple rock crabs (Leptograpsus variegatus), which were abundant all around the rocky shoreline.

A purple rock crab (Leptograpsus variegatus) forages along the shore of Ohinau Island at night. Image: Colin Miskelly, Te Papa

A purple rock crab (Leptograpsus variegatus) forages along the shore of Ohinau Island at night. Image: Colin Miskelly, Te Papa

However, the rock stars among the invertebrates on Ohinau Island were the Mercury Island tusked weta (Motuweta isolata). This rare species was discovered on 13 ha Middle Island (Mercury Islands) in 1970, and named in 1997. Following eradications of Pacific rats on nearby islands, they were translocated to six other islands (Red Mercury, Double, Korapuki, Stanley, Ohinau and Cuvier Islands) between 2000 and  2008.

An adult male Mercury Island tusked wets (Motuweta isolata) on the forest floor at night, Ohinau Island, January 2014. Image: Colin Miskelly, Te Papa

An adult male Mercury Island tusked wets (Motuweta isolata) on the forest floor at night, Ohinau Island, January 2014. Image: Colin Miskelly, Te Papa

The entire range of Mercury Island tusked weta. This rare species survived only on 13 ha Middle Island, the small island left of centre in this image. Following eradications of Pacific rats (kiore) on nearby larger islands, they were successfully translocated to Cuvier Island (on the distant horizon), Korapuki Island (immediately below Cuvier Island, and to the lower right of Great Mercury Island), Stanley Island (the large island to the right of Middle Island), the western end of Double Island (which appears as two small islands to the right of Stanley Island), Red Mercury Island (the long, low island on the right), and Ohinau Island (foreground). Image: Colin Miskelly, Te Papa

One hundred tusked weta were released on Ohinau Island in November 2007. They are thriving there, and we found about 20 adults during an evening of searching. Only the adult males have the elephant-like tusks, which are used to fight other males for access to mates. The tusks are also used to stridulate (create sound), by rubbing ridged sections together.

A pair of Mercury Island tusked weta on the forest floor at night, Ohinau Island, January 2014. The large male (with tusks) is on the left; the long appendage at the rear of the female is her ovipositor, used to lay eggs in the soil. Image: Colin Miskelly, Te Papa

A pair of Mercury Island tusked weta (Motuweta isolata) on the forest floor at night, Ohinau Island, January 2014. The large male (with tusks) is on the left; the long appendage at the rear of the female is her ovipositor, used to lay eggs in the soil. Image: Colin Miskelly, Te Papa

Close-up of the tusks of a male Mercury Island tusked weta, showing the ridges that are rubbed together to create sound. Image: Colin Miskelly, Te Papa

Close-up of the tusks of a male Mercury Island tusked weta (Motuweta isolata), showing the ridges that are rubbed together to create sound. Image: Colin Miskelly, Te Papa

The Mercury Island tusked weta is the only animal species that has been translocated to Ohinau Island following eradication of Pacific rats, mice and rabbits.

With thanks to Ngati Hei for permission to visit Ohinau, and DOC for logistic support. Rob Chappell and Chris Green (DOC) provided information on mice and tusked weta, and Bruce Marshall (Te Papa) and Fred Brook advised on the distribution of Rhytida greenwoodi.

Related blogs

Life through a burrowscope lens (Part 4) – subterranean Ohinau Island

Seabirds of Ohinau Island

Landbirds of Ohinau Island

Lizards of Ohinau Island

Flesh-footed shearwater surveys at Ohinau Island, Coromandel

Critters of the Poor Knights Islands

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