Carbonate production by two New Zealand serpulids : Skeletal allometry, mineralogy, growth and calcification of Galeolaria hystrix and Spirobranchus cariniferus (Polychaeta: Serpulidae), southern New Zealand

Abstract

This study investigates the skeletal allometry, mineralogy, growth and calcification of two New Zealand serpulids, Galeolaria hystrix and Spirobranchus cariniferus. Tube allometry (length, diameter, wall thickness, carbonate weight) was studied for G. hystrix from Otago Harbour, Doubtful Sound and Big Glory Bay, Stewart Island and S. cariniferus from Otago Harbour and Doubtful Sound. The tubes length-to-weight and diameter-to-weight relationships are shown to have highest correlation coefficients (at least 0.92), allowing calculation of the carbonate weight of serpulid tubes simply by analysing photographs without the need of collecting and killing the animals. Both G. hystrix and S. cariniferus tubes in New Zealand are made of high-Mg calcite (9.5 & 10.8 wt% MgCO3 respectively) with little (max. of 2 wt% for G. hystrix and 12 wt% for S. cariniferus) or no aragonite. Differences in tube mineralogy among environments suggest an environmental control superimposing the more important genetic control of serpulid mineral precipitation. Opercula of both serpulids show different mineralogy from the tubes, being completely high-Mg calcite (~15 wt% MgCO3) for G. hystrix and almost completely aragonite for S. cariniferus. Perhaps the serpulids secrete these more durable opercula to contend with the high-energy environment in the intertidal (S. cariniferus) and to withstand attacks from predators (G. hystrix). Tube mineralogy was not found to fluctuate seasonally along the tubes’ length and therefore cannot be used to calculate tube growth or age of serpulids. Mean annual tube growth at Harington Point, Otago Harbour in 2011 is 4.1 cm (range: 2.0-6.7 cm, n = 28) for G. hystrix and 1.7 cm (range: 0.4-3.4 cm, n = 24) for S. cariniferus. Tube growth is slower in winter compared to summer and slows with serpulid age. G. hystrix tubes reach lengths of ~6 cm one year post-settlement, while tubes of S. cariniferus are only about ~3 cm long after one year. In Otago Harbour both serpulids are believed to live as long as about 10-12 years, during which G. hystrix produces a tube of ~21 cm and S. cariniferus a tube of ~11 cm in length. Mean annual calcification rates are 1.5 g/year for G. hystrix and 0.3 g/year for S. cariniferus individuals in Otago Harbour. G. hystrix aggregations in the subtidal in Big Glory Bay contain 4500-8500 living worms/m2, deposit up to 6.75-12.75 kg CaCO3/m2/year, take 9-50 years to form and could involve up to 31 generations of worms. S. cariniferus aggregations in the intertidal at Banks Peninsula, Canterbury contain 30,000-40,000 living worms/m2, deposit up to 9.0-12.0 kg CaCO3/m2/year, take at least 26 years to form and could involve ~15 generations of worms. Serpulid aggregations in New Zealand are important temperate reefs, the counterpart of tropical coral reefs. Protection of these habitat-forming biodiversity hotspots is strongly recommended.