Abstract
Pyrosomas are colonial tunicates capable of forming dense aggregations. Their trophic function in the ocean, as well as their ecology and physiology in general, are extremely poorly known. During the ANTARES-4 survey (January and February 1999) their feeding dynamics were investigated in the south Indian Ocean. Results show that their in situ clearance rates may be among the highest recorded in any pelagic grazer, with up to 35 l h -1 per colony (length: 17.9 ± 4.3 (SD) cm). Gut pigment destruction rates, estimated for the first time in this tunicate group, are higher than those previously measured in salps and appendiculari- ans, ranging from 54 to virtually 100% (mean: 79.7 ± 19.8%) of total pigment ingested. Although individual colony ingestion rates were high (39.6 ± 17.3 (SD) µg pigment d -1 ), the total impact on the phytoplankton biomass and production in the Agulhas Front was rela- tively low, 0.01 to 4.91% and 0.02 to 5.74% respec- tively, as a result of the low abundance of colonies. Colonies showed higher retention efficiency for parti- cles larger than 10 µm. Low levels of lipids and per- centages of triacylglycerols and free fatty acids were found in zooids. Markers for diatoms (C16 PUFA), dinoflagellates (22:6n-3, 18:5n-3) and prymnesiophytes (18:1n-9, 18:4n-3) were observed. Stepwise discrimi- nant analysis of published data on phytoplankton fatty acid showed a strong similarity between the composi- tion of the 2 neutral lipid classes found in P. atlanticum and that of both dinoflagellates and prymnesiophytes. The sterol composition confirmed this result with the contribution of 24-methylcholest-5,22-dien-3β-ol, cho- lesterol, 24-methylenecholesterol and 24-ethylcholes- terol. Colonies exhibited large numbers of ciliate pro- tozoans (possibly Strombidium sp.). It is not clear whether this constituted a mere opportunistic intru- sion, or rather a more stable association between the 2
Highlights
IntroductionPyrosomas are a small group of holoplanktonic tunicates, currently including only 8 described species and 3 genera (van Soest 1981)
Low levels of lipids and percentages of triacylglycerols and free fatty acids were found in zooids
Stepwise discriminant analysis of published data on phytoplankton fatty acid showed a strong similarity between the composition of the 2 neutral lipid classes found in P. atlanticum and that of both dinoflagellates and prymnesiophytes
Summary
Pyrosomas are a small group of holoplanktonic tunicates, currently including only 8 described species and 3 genera (van Soest 1981). They are restricted to the warmer open ocean waters, between approximately 50° N and 50° S in all oceans (van Soest 1981). Harbison (1998) lists 62 fish and 3 turtle species that worldwide use pyrosomes as a significant food source. To this list, 2 albatross and 1 sea lion species from New Zealand and Australian waters have been added more recently (James & Stahl 2000, Childerhouse et al 2001, Hedd & Gales 2001). Three species of ciliate protozoans have been reported as parasitic hosts inside pyrosomas (Harbison 1998)
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