Class THALIACEA

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Class THALIACEA

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P. Kott, Queensland Museum, Brisbane, Queensland, Australia

Introduction

Thaliacea are the most speciose of the two planktonic tunicate classes. The geographic ranges of most species are vast, being defined by the courses of ocean currents, rather than geographic areas. No indigenous species are known in this group. New species are rare, and very few have been added to the list of known species in the last 50 years. Twenty-six species are known from Australian waters.

The class Thaliacea contains the salps and doliolids, both of which have solitary and colonial generations, and the pyrosomids which are only colonial. Each of these groups is given ordinal rank in the class as Cyclomyaria (or Doliolida), Desmomyaria (or Salpida) and Pyrosomatida. Each order contains only one family—the Doliolidae, the Salpidae and the Pyrosomatidae, respectively. Only the Salpidae are reasonably diverse, with two subfamilies, Cyclosalpinae and Salpinae.

Thaliacea are characterised by their translucent test, branchial and atrial siphons at opposite ends of the body and the atrial cavity posterior to the large pharynx. Budding takes place from an endostylar stolon. Species are separated by the numbers and arrangement of the muscle bands that encircle their bodies.

Thaliacea are most prolific. In certain seasons some species occur in vast swarms that exclude most other zooplankters from surface waters. Such swarms of Thalia democratica (Forskål, 1775) are characteristic of the Australian eastern coastal waters in autumn and spring. Thaliaceans gain the energy for their prolific sexual reproduction and vegetative replication from their voracious filter feeding activity as they pass through the water—jet-propelled by their ciliary feeding stream.

Most known taxa of the Thaliacea were described from the collections made on one or other of the great European scientific voyages of the 19th century, namely, the French Astrolabe (Quoy & Gaimard 1833, 1834), the British Challenger (Herdman 1888), the great German Plankton Expedition and the Deep Sea Expedition of the Valdivia (Traustedt 1893; Seeliger 1895; Apstein 1906a, 1906b; Neumann 1906, 1913), and the Dutch Siboga (Ihle 1910). Later, the American Fisheries Bureau Albatross (Metcalf 1919) also made collections in the western Pacific. The taxonomy and biology of Thaliacea have been reviewed in Bone (1998).

In Australian waters, a small plankton collection made by the German expedition to south-western Australia yielded only one thaliacean (Doliolium denticulatum) from Shark Bay (Lohmann 1909), and Russell & Coleman (1935) reported on collections made around Low Isles (Great Barrier Reef). Species occurring in eastern Australian waters from the Tropic of Capricorn to South Australia were determined and reported by Thompson (1945) from the collections made by the CSIRO research vessel FRV Warreen in just over two years (1938–1941). Since then, no systematic planktological work has been pursued in Australian waters; and neither have the western, southern or tropical Australian waters been surveyed for Thaliacea, although Heron (1972–1988), produced seminal work on the biology of Thalia democratica.

The whereabouts of the majority of the type specimens of thaliacean nominal species occurring in Australian waters are not known. Material reported by Quoy & Gaimard (1825, 1833, 1834) from the Voyage de l'Astrolabe was discovered in the Laboratoire de Biologie des Invertebrés marins et Malacologie in the Muséum National d'Histoire Naturelle, Paris. However, although there are vials labelled 'Biphores' and 'Salpes', the external labels have either been lost or never existed (C. Monniot, pers. comm.). Not one of the type specimens of the eleven species from Australian waters, ascribed to these authors, has been located. Type specimens of the four from the Challenger Expedition (1888) have not been located in the BMNH; nor are the types of the species described by Ritter (two species), Brooks (two species) and Metcalf (two species) to be found in the U.S. National Museum. Relevant type specimens from Tilesius, Otto, Desor, Dall, Vogt, Todara, Sigl, Borgert, Uljanin (each being the author of a single species) have not been located, and may never have been assigned. Similarly, the type specimens for species described by Bosc (two species), Sars (two species) and Traustedt (three species) and three of the four described by Apstein have not been located.

Cuvier (six species) did not designate types, nor apparently did Forskål (six species), Blainville (four species), Chamisso (four species) and Pallas (one species). Of the six species described by Lesson, and by Péron and Lesueur in the early part of last century, only one (Pyrosoma giganteum Lesueur, 1815) has been located in the Muséum National d'Histoire Naturelle, Paris. The only other thaliacean type specimens located are Thalia cicar Van Soest, 1973 and Thalia rhinoceros Van Soest, 1975 in the Zoological Museum of Amsterdam, Salpa amboinensis Apstein, 1904 in the Muséum d'Histoire Naturelle, Geneva, and Pyrosoma triangulum Neumann, 1909 in the Museum für Natuurkunde, Berlin.

General References

Apstein, C. 1904. Salpes d'Amboine. Revue Suisse de Zoologie 12: 649-656 pl. xii

Apstein, C. 1906a. Salpen der deutschen Tiefsee-Expedition. Wissenschaftliche Ergebnisse der Deutschen Tiefsee-Expedition auf dem Dampfer "Valdivia" 1898–1899 12(3): 245-290

Apstein, C. 1906b. Die Salpen der Deutschen Südpolar-Expedition. Deutsche Südpolar Expedition 1901–1903 9(1): 155-203

Bone, Q. (ed.) 1998. The Biology of Pelagic Tunicates. New York : Oxford University Press 340 pp.

Forsskål, P. 1775. Salpa. pp. 112-117 in, Descriptions Animalium …; quae in itinere orientali observavit. Hauniae : Heineck & Faber. 164 pp. [publication date established from Melville, R.V. 1969. Salpa Forskål, 1775 (Tunicata): Validated under the Plenary Powers with designation of a type species for Thalia Blumenbach, 1798. Bulletin of Zoological Nomenclature 26(3): 136–138]

Herdman, W.A. 1888. Report upon the Tunicata collected during the voyage of H.M.S. 'Challenger', during the years 1873–1876. Report on the Scientific Results of the Voyage of H.M.S. Challenger 1873–1876, Zoology 27(76): 1-166 pls i-xi

Heron, A.C. 1972. Population ecology of a colonizing species: the pelagic tunicate Thalia democratica. I. Individual growth rate and generation time. Oecologia (Berlin) 10: 289-293

Heron, A.C. 1972. Population ecology of a colonizing species: the pelagic tunicate Thalia democratica. II. Population growth rate. Oecologia (Berlin) 10: 294-312

Heron, A.C. 1973. A new type of heart mechanism in the invertebrates. Journal of the Marine Biological Association of the United Kingdom 53: 425-428

Heron, A.C. 1973. A specialised predator-prey relationship between the copepod Sapphirina angusta and the pelagic tunicate Thalia democratica. Journal of the Marine Biological Association of the United Kingdom 53: 429-435

Heron, A.C. 1975. Advantages of heart reversal in pelagic tunicates. Journal of the Marine Biological Association of the United Kingdom 55: 959-963

Heron, A.C. 1976. A new type of excretory mechanism in the tunicates. Marine Biology, Berlin 36: 191-197

Heron, A.C., McWilliam, P.S. & Dal Pont, G. 1988. Length-weight relation in the salp Thalia democratica and potential of salps as a source of food. Marine Ecology Progress Series 42: 125-132

Heron, A.C. & Benham, E.E. 1983. Individual growth rates of salps in three populations. Journal of Plankton Research 6: 811-828

Heron, A.C. & Benham, E.E. 1985. Life history parameters as indicators of growth rate in three salp populations. Journal of Plankton Research 7: 365-379

Ihle, J.E.W. 1910. Die Thaliaceen der Siboga-Expedition. In, Siboga - Expeditie Monogr. 56d, 58 pp. Leiden.

Lesueur, C.A. 1815. Mémoire sur l'organisation des Pyrosomes et sur la place qu'ils semblent devoir occuper dans une classification naturel. Bulletin de la Société Philomathique de Paris 4: 70-74 pl. i