TANE 22, 1976 CRABS OF GREAT MERCURY ISLAND (AHUAHU) by Gillian F. Puch Department of Sociology, University of Auckland, Private Bag, Auckland SUMMARY Twenty crab species (eighteen brachyuran and two anomuran) are recorded from Great Mercury Island. A detailed study of the Huruhi Harbour shows the habitat range of eleven of these species. INTRODUCTION AND METHODS The work recorded here was undertaken during the Auckland University Field Club's Scientific Trip held at Huruhi Harbour, northern Great Mercury Island, 10-17 May 1975. As it was not feasible to cover all areas of the island, work was confined to the north-western areas only (Fig. 1). First, an annotated species list was compiled after sampling three types of habitat: boulder beaches, mudflats, and sandy beaches (Fig. 1), this list being supplemented by results from offshore dredgings (Grace and Grace 1976). A study of the Huruhi Harbour was then conducted in order to gain an understanding of the specific habitat range of some of the species (Figs. 2 and 3). ANNOTATED SPECIES LIST TRIBE BRACHYURA Family Leucosidae Ebalia laevis (nut crab) One specimen found, subtidally, in muddy sand near the entrance to Huruhi Harbour. Family Dromidae Petalomera wilsoni (hairy sponge crab) One specimen in shell gravel south of Motururuku Island. Family Portunidae Ovalipes punctatus (common swimming crab) One specimen found at the southern end of White Beach (width of carapace 4cm). Also one large carapace (width 6cm) found on the beach immediately north of Stingray Point. Liocarcinus corrugatus (corrugated swimming crab) Five specimens found in offshore dredgings. Family Majidae Notomithrax peroni (large spider crab) One carapace (width 2.5cm) found on beach immediately north of Stingray Point. 71
Notomithrax ursus (masking crab) Three carapaces (widths 2, 2, and 1cm) found on beach immediately north of Stingray Point. Paramithrax minor (lesser seaweed crab) Present in offshore dredgings, mostly in shelly sediments. Family Hymenosomidae Halicarcinus varius (offshore spider crab) One specimen found at LWM on White Beach. Common in offshore dredgings. Halicarcinus whitei (mudflat spider crab) Frequently found in the mid-shore zone of the Huruhi Harbour mudflats. 72
Family Xanthidae Ozius truncatus (black-finger crab) One large specimen found at LWM on boulder beach south of Tamewhera. Heterozius rotundifrons (pebble or big-handed crab) Found at LWM south of Tamewhera and along the boulder beach coastline at the western entrance of the Huruhi Harbour. Frequent. Family Grapsidae Leptograpsus variegatus (large purple shore crab) Found at Poteku, north and south Tamewhera, Motururuku Island, and the more exposed coastline immediately south of the Old Beacon. The adults are always found towards LWM where they can hide in the large crevices of rock platforms. The greyish-black juveniles, however, are found under boulders in the lower to mid-shore zone. Frequent. Cyclograpsus lavauxi (smooth high-tidal crab) Found mainly in the mid-shore to HWM zone at Poteku, north Tamewhera, Motururuku Island, and the coastline extending from south-east of the Old Beacon to Te Raieawhare Bay. Common. Cyclograpsus insularum (orange boulder crab) Found in the mid-shore zone at Poteku, north Tamewhera, Motururuku Island, and the coastline south-east of the Old Beacon. Common. Plagusia chabrus (red rock crab) Formerly Plagusia capensis. Found amongst algae at sub-tidal level on the exposed boulder beach of Poteku and the coastline south-east of the Old Beacon. Occasional. Hemigrapsus crenulatus (hairy-handed crab) Found at HWM in the more muddy areas of the Huruhi Harbour. Occasional. Helice crassa (tunnelling mud crab) Found in burrows at HWM on the large sandy mudflats of the upper Huruhi Harbour. Both adults and juveniles are abundant. Family Ocypodidae Hemiplax hirtipes (stalk-eyed mud crab) Found at LWM in burrows among patches of Zostera capricorni (eel-grass) on the sandy mudflats of the upper Huruhi Harbour, and also found extending into the shallow sublittoral zone. One crab found was carrying a mass of orange-coloured eggs. Occasional. TRIBE ANOMURA Family Paguridae Pagurus novaezelandiae (common hermit crab) Found the whole length of a one-metre-wide muddy strip adjacent to the landward side of a stone wall built across Te Raieawhare Bay. This stone wall is just above LWM and is covered at high tide. The crabs are abundant and occupy many types of gastropod shells. 73
Manawa '< Parauri 1 station u 500 metres I ' I ' ' I o pebbles «small angular rocks ^ g y small platey rocks large angular rocks O large boulders ::: sand stream * rushes Fig. 2. Huruhi Harbour, showing location of the thirteen stations and nature of the shore substrate. 74
STATIONS 1 2 3 U 5 6 78 9 10 11 12 13 1 i i i i i i i i i I Fig. 3. The specific habitat range of the eleven crab species found between Stations 1 and 13. The crabs are not drawn to scale. 75
Family Porcellanidae Petrolisthes elongatus (blue-green half crab) Found mainly in the mid to lower-shore zone at Poteku, south Tamewhera, and the coastline extending from south of the Old Beacon to Te Raieawhare Bay. Abundant. HABITAT STUDY OF THE HURUHI HARBOUR Part of the coastline of the Huruhi Harbour was chosen for additional study because it contained habitats ranging from an exposed hard shore and boulder beach through to a protected soft shore and harbour. Thirteen stations, established along this coastline, were examined in detail from HWM to LWM. Numerous informal checks were also made between the stations, and the nature of the shore substrate was noted (Fig. 2). Altogether, eleven crab species (nine brachyuran and two anomuran) were found, and their habitat ranges recorded (Fig. 3). From this study the following pattern became apparent. The three crabs, Cyclograpsus insularum, Leptograpsus variegatus, and Plagusia chabrus, are found only in the more exposed parts of the hard shore and boulder beach coastline. Stations 1, 2, and 3 therefore form one extreme end of the habitat range for these three species. However the pebble crab, Heterozius rotundifrons, is found extending into the more sheltered areas of the boulder beach coastline, its habitat range ending at the entrance to the Huruhi Harbour. The high-tidal crab, Cyclograpsus lavauxi, and the blue-green half crab, Petrolisthes elongatus, are found mainly between Stations 1 and 9, although C. lavauxi is not nearly so abundant as P. elongatus. Between Stations 9 and 12, where the ground becomes more muddy and there are less rocks for crabs to crawl under, the juveniles of both these species predominate. The hairy-handed crab, Hemigrapsus crenulatus, is also found in the muddy area between Stations 9 and 12, but its habitat range is more restricted than those of C. lavauxi and P. elongatus. The common hermit crab, Pagurus novaezelandiae, is found only in one particular habitat, near the landward side of a stone wall built across Te Raieawhare Bay (Station 9), where the ground is very muddy. The two mud crabs, Helice crassa and Hemiplax hirtipes, are found only in the northern reaches of the Huruhi Harbour (Station 13), H. crassa having replaced H. crenulatus in the upper-shore zone. Halicarcinus whitei is also found on these mudflats. However, it should be noted that the Huruhi Harbour mudflats are not true mudflats; they can best be described as sandy mudflats with patches of eel-grass in the lower-shore zone. ACKNOWLEDGEMENTS I would like to thank Dr B.W. and Mrs G.C. Hayward for contributions to the species list and for field assistance. Also, I am especially grateful to Dr R.V. and Mrs A.B. Grace for help with identifications, contributions to the species list, and for critically reading the manuscript. REFERENCE Grace, R.V. & Grace, A.B. 1976: Benthic Communities west of Great Mercury Island, northeastern New Zealand. Tane 22: (this issue). 76