Species-specific house productivity of appendicularians

Abstract

Appendicularians, pelagic tunicates that are common in world oceans, periodically produce new mucus houses and discard old ones. Discarded houses form macroscopic aggregates that constitute sites of biological activity in the water column and also contribute to the transport of organic matter to deeper water. In this study, we measured the house renewal rates of 10 appendicularian species cultivated in 30 μm-mesh-screened seawater at 17 to 29°C. In addition, the carbon content of the tunicates (CB), their newly secreted houses (CNH) and their discarded houses (CDH) were concurrently examined for some of the oikopleurid species. House renewal rates varied from 2 houses d-1 for Oikopleura cophocerca at 20°C to 40 houses d-1 for Fritillaria formica digitata at 23°C. The CNH of O. longicauda, O. fusiformis, O. rufescens and Megalocercus huxleyi were 0.16, 0.48, 1.6 and 8.8 μg, corresponding to 5.3, 9.2, 14.1 and 10.3 % of CB, respectively; the CDH of the first 3 species were 0.68, 1.2 and 3.9 μg, corresponding to 17.9, 30.0 and 32.8% of CB, respectively CDH was not measured in M huxleyi). House renewal rates decreased with increasing CNH/CB ratios for all species. Our calculations indicate that at 23°C, populations of O. longicauda, O. fusiformis, O. rufescens and M. huxleyi produced new houses corresponding to 112, 217, 75 and 89% of their biomass d-1, and that the first 3 species could discard houses corresponding to 380, 708 and 174% of their biomass d-1, respectively. The individual lifetime production of CNH and CDH for all 3 species was estimated to be 1.1 to 3.6 and 2.7 to 12 times greater than the CB of mature individuals. This high house-productivity, combined with large population sizes, indicates that appendicularians are major producers of macroscopic aggregates in marine ecosystems.