TANE 24, 1978 INTERTIDAL LIFE OF FAREWELL SPIT by S.H.Anderson, C.A. Hammerton and F.M. Town Department of Zoology, University of Auckland, Private Bag, Auckland SUMMARY The distribution and abundance of intertidal organisms has been studied at various points along the length of Farewell Spit. The species present and their abundance and ranges are discussed in relation to habitat and environment. INTRODUCTION During the Auckland University Field Club scientific trip to Farewell Spit in May 1977, a survey of the intertidal marine life of the Spit was carried out. A large tidal area is sheltered within the recurved arm of the spit, backed by stabilized and moving sand dunes. Since currents arc deflected by the tip of the spit, little wave action is experienced and a relatively undisturbed habitat exists at ebb tide. The low tide mark extends up to 3km out from high water level, with an accumulation of sediment towards the seaward edge allowing the eel grass (Zostera) to become established and constitute consolidated "Zostera flats". Trends observed in the distribution of inter-tidal life along transects extending from high to low water mark are examined. The location of transects is shown in fig. 1 of Wright (1978). Nomenclature follows Morton and Miller (1968) except that Paphies now replaces Amphidesma. Previous work of this nature has been carried out in this area by research students and staff of the Auckland University Zoology Department and is recorded in "The Marine Environment of Golden Bay" report (University of Auckland 1975). METHODS At three points along the spit transects were made, running from high tide level towards low tide mark as far as the receding tides would permit. Samples were taken at 30m intervals for the first 200m, and thereafter at 100m intervals with 0m considered as the interface between bare sand and marram grass at the base of the sand dunes. The initial smaller intervals were used in order to include variation within the narrower high and middle beach areas. The results of all three transects have been condensed into one account, since similar trends were observed in each transect. Samples consisted of diggings, approximately 0.5m 3 from which the sand was sieved or sorted, and the organisms that were encountered identified where possible and recorded according to an approximate abundance scale ranging from rare (1-2 individuals), through occasional (2-10 37
individuals), common (10-20 individuals), to abundant (more than 20 individuals). RESULTS The transects may be divided into three main zones; those of the upper, middle and lower beach. The upper beach zone lies entirely above high water mark and has a slope of approximately 4. At 0m the sand was wetted only occasionally by the tide and scattered with dead eel grass, shattered shells of the cockle (Chione stutchburyi), crab shells and other debris. The burrowing sandhopper (Talorchest The middle beach extends down to the level of extreme neap tide, sloping at approximately 1. It was composed of coarser materials, a drift line of cockle shells, unidentified crab fragments, fresh eel grass and the seaweed Colpomenia sinuosa washing up with the tide. The latter drift plants provided food and protection for the sandhoppers abundant in this area. At 30m the sand was wet and rippled, supporting only rare sandhoppers and a small amount of surface debris. The lower beach slopes almost imperceptibly seawards, and constituted the greatest extent of sand and Zostera flats. Patches of standing water were encountered in this area, forming shallow films inshore and deepening further seawards to merge with occasional channels. Eel grass also occurred in this area, at first sparse inshore but forming dense, sediment-trapping masses towards low water mark. Thus a mosaic of sand, eel grass and standing water occurred throughout the intertidal region. At 60m sparse young eel grass grew and no fauna was observed. By 90-100m the sand was either covered with a thin sheet of surface water or supported sparse eel grass growth. Worm casts were apparent on the surface, marking the tailshafts of polychaete worm tubes. The lugworm Abarenicola was first encountered in occasional abundance, along with several other polychaetes and nemertines: An orange amphipod (unidentified) and tiny individuals of the pipi (Paphies australe) were also found. At 200m the water pools were 2-4cm in depth. Polychaetes were dominant. Whelks (Cominella glandiformis) crawled over the surface sand in small numbers and occasional cockles were found living in the sand. Dead cockle valves and the horn shells of Zeacumantus lutulentus were closely packed in the sand and scattered on the surface. The unidentified orange amphipod of the upper beach was again encountered in the water tables here in local abundance, as was a small green crustacean. At 300m polychaete worms were common and the surface sand was crowded with their faecal coils. Here pools of standing water formed interconnecting channels through the wide areas of bare sand. A patchy covering of Colpomenia and eel grass occurred. Gastropods, Zediloma atrovirens and Cominella spp., could be observed in local abundance feeding on the surface 38
waters along with various small crustaceans. Dead cockle valves and tiny valves of the pipi were also encountered. Live cockles and pipis occurred, though the size of both species at this extreme of their shore range was rarely more than 5mm. At 500m tiny swimming amphipods and shrimps were common in the water pools. Polychaete worms were common, including Abarenicola, The unidentified orange amphipod was also observed here. At 600m sparse eel grass, large pools of 6cm depth and a scattering of cockle valves formed the surface environment. Cockles and pipis were occasional and slightly larger in size. The whelks and horn shells could be observed at the surface and two shrimp species were found in the pools. Polychaetes were occasional and the sea-cucumber (Trochodota dendyi) was first encountered, putting out worm-like faecal castings from the material ingested as it burrowed. At 700m we found sparse eel grass and bare sand. Cockles and the thin-shelled bivalve Macomona liliana were occasionally present, as were the polychaete and nemertine worms. The unidentified orange amphipod was again encountered. By 800m the eel grass cover had become thicker. Two species of crab appeared in abundance, a spider crab and the mud crab (Hemiplax hirtipes). Occasional whelks crawled on the surface. Polychaete worms were common and the holothurian Trochodota was again present. Cockles had reached a size of 3.5cm and supported individual acorn barnacles (Elminius modestus) on the outer shell. At 900m eel grass distribution was clumped into dense areas between patches of bare sand. Swarms of whelks feeding on cockles were observed. Cockles and horn-shells were present, both supporting growths of acorn barnacles. Polychaete worms were common, together with Trochodota dendyi. The mud crab was also present in occasional abundance. By 1000m the water was guided into large channels and the eel grass distribution was distinctly clumped. Cockles and pipis were occasional, as were the polychaete worms. At 1100m dense eel grass mats trapped the sidement. The molluscs Zediloma, Macomona and cockles, all occasional, formed the dominant fauna. At 1200m were dense areas of eel grass and root-laced sand. Cockles were common with the anemone Edwardsia tricolor on many of the shells. Macomona was occasional, many with acorn barnacles on the outer shell. Whelks were found in feeding groups, and the hornshells and Zediloma were occasional. Polychaete worms were common and numerous small shrimps could be found in the water pools. By 1300m dense eel grass areas formed the dominating flora. Cockles and whelks were both common, with acorn barnacles and the snake-skin chiton (Amaurochiton) living on the shells of both these species. 39
SPECIES LIST Polychaete worms: Holothurians: Nemertines: Molluscs: Crustaceans: Coelenterates: Abarenicola affinis Aglaophanius macroura Axiothella quadrimaculata Glycera sp. Hemipodus sp. Lepidasthenia sp. Magelona papillicornis Orbinia papulosa Perenereis nuntia Platynereis australis Scolecolepsis sp. Travisia olens Trochodota dendyi numerous - unidentified Amaurochiton glaucus Chione stutchburyi Cominella adspersa C. glandiformis Macomona liliana Paphies australe Zeacumantus lutulentus Zediloma atrovirens Amphipod - unidentified Elminius modestus Hemiplax hirtipes Shrimps - unidentified Edwardsia tricolor DISCUSSION The extent of the Farewell Spit tidal flats offers a range of intertidal environments. Throughout the relatively dry sand and drift materials of the upper and middle beach only the semi-terrestrial sandhopper has found a niche and occurs in significant numbers. The lower beach, however, supports a diversity of fauna. The sand flats, where Zostera growth is sparse and scattered, shelter beds of the cockle Chione stutchburyi. These are small nearer the tide mark and become progressively larger and more numerous seawards. Another deposit-feeding bivalve Macomona liliana also occurs here but occupying a different niche its vacuum cleaning operation of feeding is impeded by cockles in the upper layer and also by cockle faeces on the surface. The most common gastropod is the carnivorous Cominella found clustering around cockles upon which it preys. Associated with the molluscs are the chiton Amaurochiton, the barnacle Elminius, and the anemone Edwardsia all of which are dependent on a hard shell substrate for attachment and thus become more evident with increased bivalve-gastropod density. Although some silt adheres to the sand and the water table is at surface level, promoting anaerobic conditions just below the surface, the sediments are easily 40
penetrated and rich in polychaete fauna. Found in the sand here are the proboscis burrower Aglaophanius macroura and the thinly stretched Orbinia papillosa. In the coarser shell sand Platynereis australis occurs, its tube stabilized by attachment to shell fragments and sand grains in the sediment. The errant scale-worm Lepidasthemia is found roving the surface and in the fine sand, the bamboo-worm Axiothella quadrimaculata projects its tubes above the ground. Long, pink, Magelona papillicornis may be dug up here, or Travisia olens found ingesting the sand to glean from it any organic matter. The lugworm Abarenicola is also very common, carrying out its activity with tidal rhythmicity. Gradual enlargement of the standing water pools towards the sea allows an increasing variety of Crustacea to exist, mainly the sandlicking amphiphod filterers and several species of shrimps. As the Zostera cover thickens, new species appear. The burrowing worm fauna is again rich carnivorous Glycera sp., Platynereis australis with its parchment tube anchored to the rhizomes, Axiothella quadrimaculata in its fine sand tube and the free-living deposit feeder Orbinia papillosa all occur here. The mud crab Hemiplax hirtipes is also closely associated with the Zostera beds, benefitting as a mud feeder from the accompanying sediment accumulation. A general trend in the composition of the sand profile was also observed, a result both of biological activity and the instability of the dunes from which sand was blown and deposited by the prevailing wind. Near high tide mark the profile was of uniform aerobic semi-dry sand. In the middle beach, this was scattered with dead and shattered shells. At the commencement of the lower beach, a fawn band of decomposing roots and leaves was observed at the base of the profile. Progressing further seawards, the band rose consistently towards the surface, followed by a band of black anaerobic sand. By the end of the sand flat, the aerobic sand layer was a thin veneer covering recently smothered Zostera and dead roots and a thick band of anaerobic sand. Within the Zostera flat area, the profile was of black anaerobic sand and silt. ACKNOWLEDGEMENTS The authors would like to thank Mr A.E. Wright for assistance in preparation of this paper and Dr B.A. Foster for critically reading the manuscript. REFERENCES Morton, J.E. & Miller, M.C. 1968: "The New Zealand Seashore". Collins, London. University of Auckland 1975: "Marine Environment of Golden Bay". Unpublished report prepared for Shell, B.P. and Todd Oil Services Ltd. Wright, A.E. 1978: Auckland University Field Club scientific trip to Farewell Spit, May 1977. Tane 24: (this issue). 41