Thaliacea

Franco, P., Dahms, H. - U., Lo, W. - T. & Hwang, J. - S., 2017, Pelagic tunicates in the China Seas, Journal of Natural History 51 (15 - 16), pp. 917-936 : 923-926

Class Thaliacea

Cristian and Madin (2004) stated that thaliaceans tend to aggregate in areas of high abundances of nano- and ultra-plankton. However, dominant thaliacean species have a tendency to concentrate, resulting in an uneven distribution of individual numbers of various species at different sampling stations ( Xu and Zhang 2006). According to the worldwide literature, distribution patterns of thaliacean assemblages are associated not only with temperature, salinity and food availability, but also with their own physiological and ecological traits, such as swimming ability, growth rate, reproductive behaviour ( Gibson and Paffenhöfer 2002) and feeding behaviour ( Huskin et al. 2003). Salps show vertical migration ( Tsuda and Nemoto 1992), which may allow them to approach the surface in order to feed and reproduce in optimal conditions and then migrate to deeper waters. Salps favour waters of higher salinities and higher temperatures – 24~31° C – which provide suitable conditions to enhance their asexual reproduction ( Liao et al. 2013). It is the family which shows the highest diversity in the China Seas. In fact, apart from the northern Yellow Sea, where only one species, namely Salpa fusiformis , was found ( Franco et al. 2014), the other areas surveyed showed a higher diversity of salp species. In fact, in the East China Sea, a total of 11 species were recorded ( Table 3). In the South China Sea, a total of 13 species were observed. Liao et al. (2013) recorded 13 species in the waters around Taiwan, whereas Zhang et al. (2003) identified a total of eight species in the Taiwan Strait and Nanwan Bay. In our own faunal inventory, we found a total of 10 salp species in Taiwanese waters during summer, which correlated with elevated surface temperatures (about 25–28°C). Thalia orientalis was most abundant in Taiwan waters. The stations with higher species diversity were the ones in the South of Taiwan, corresponding to a coastal area belonging to the South China Sea. We hypothesised that the intrusion of the South China Sea surface current could have an influence on species diversity.

Doliolids (belonging to the class Thaliacea) can live at different water depths although they do not show a clear diel vertical migration ( Gibson and Paffenhöfer 2000). Doliolids also show an alternation between sexual and asexual reproduction. Each life stage tends to stay at a different water depth ( Tew and Lo 2005). In good environmental conditions they can migrate to the surface and spawn in high-density patches ( Deibel and Paffenhöffer 2009). According to worldwide published records, the development of their populations also depends on food availability and hydrodynamic stress ( Gibson and Paffenhöfer 2002).

The results of our study showed a total of three species in the China Seas: Doliolum denticulatum , Dolioletta gegenbauri and Doliolina muelleri . Doliolum denticulatum showed the highest densities. Doliolina muelleri was present only at the two southern stations ( Table 2). In the East China Sea, the three species listed above were also identified. At the northern stations we could not find Doliolum nationalis , which has been recorded from the East China Sea before. In the South China Sea, the literature mentions only two species. In the northern Yellow Sea, only one species was identified. Liao et al. (2013) showed a total of four species of doliolids in their research in the waters around Taiwan. These authors state that doliolids could be scarce in the waters surrounding Taiwan, probably due to their soft bodies and weak swimming abilities, not being adapted to higher current velocities and turbulences. Another possibility is that they occur in deeper waters which were rarely sampled in Taiwan and elsewhere.

Our study confirms the hypothesis of Franco et al. (2014) and Liao et al. (2013) who identified D. denticulatum as an indicator species for the China Coastal Current. This current originates in the East China Sea and flows towards the south along the China coastline ( Liu et al. 2003). It is characterised by low-temperature waters. It is reported to be responsible for the transport of copepod species to Taiwan and Hong Kong from the East China Sea and North Yellow Sea ( Hwang and Wong 2005). Doliolum denticulatum seems to prefer waters of lower temperatures and salinities and higher Chl- a concentrations ( Liao et al. 2013). In our survey, we found this species at every station of western

Zooplankton Location

Survey Species found in East South North of Taiwan Strait Taiwanese waters China China Yellow Taiwan and Nanwan (summary after Sea Sea (Li Sea (after Bay in authors ’

(Xu et al. (Franco Liao Taiwan (after observations, Zhang

Class: Larvacea Family: et al. 2010, et al. et al. Zhang et al. et al. 2003; Liao

Appendiculariidae 2006 ) 2011) 2014) 2013) 2003) et al. 2013)

Oikopleura dioica × × × × × ×

Oikopleura (Coecaria) longicauda × × × × × ×

Oikopleura (Vexillaria) rufescens × × × ×

Oikopleura intermedia × × × ×

Oikopleura fusiformis × × ×

Oikopleura albicans ×

Oikopleura cophocerca ×

Oikopleura parva ×

Stegosoma magnum ×

Althoffia tumida ×

Family: Fritillariidae

Fritillaria aberrans × ×

Fritillaria borealis × ×

Fritillaria abjornseni ×

Fritillaria formica ×

Fritillaria megachile ×

Fritillaria pellucida ×

Tectillaria fertilis ×

Class: Thaliacea Family: Doliolidae

Doliolum denticulatum × × × × × ×

Dolioletta gegenbauri × × × × ×

Doliolina muelleri × ×

Doliolum nationalis × × × ×

Family: Salpidae

Thalia democratica × × × × ×

Thaliaorientalis × × × ×

Thalia rhomboides × × ×

Thalia cicar × ×

Iasis cylindrica × × × × ×

Brooksia rostrata × × × ×

Soestia zonaria × × ×

Salpa fusiformis × × ×

Salpa maxima ×

Salpa younti × ×

Traustedtia multitentaculata × × ×

Pegea confoederata × × ×

Ihlea punctata × ×

Ritteriella amboinensis × × × ×

Ritteriella picteti × ×

Cyclosalpa pinnata × × × ×

Cyclosalpa affinis × × ×

Cyclosalpa polae ×

Cyclosalpa quadriluminis f. quadriluminis ×

Cyclosalpa sewelli × × ×

Cyclosalpa floridana × × × ×

X = Occurrence of the organism in the specific area.

Taiwan, particularly during the warm season. Its abundances, however, were extremely low. Considering that our sampling was made only in surface waters, we hypothesised that doliolid concentrations could be higher in deeper and colder waters. We also speculated that this species would be more abundant in autumn, in winter, or in deeper waters.

Analysing and comparing the studies from all China Seas with distributional records from Taiwan, we observed that there are species which were recorded in the China Seas that could not be identified from Taiwan waters as yet: Oikopleura albicans , Oikopleura cophocerca , Stegosoma magnum, Altho ffi a tumida and Oikopleura parva , belonging to the family Appendiculariidae ; Fritillaria abjornseni , Fritillaria borealis , Fritillaria formica , Fritillaria megachile , Fritillaria pellucida and Tectillaria fertilis belonging to the family Fritillariidae ; and Salpa maxima , Cyclosalpa sewelli , Cyclosalpa quadriluminis f. quadriluminis and Cyclosapla polae belonging to the class Salpidae . The reason for this might be that there have been only a few investigations that covered only limited areas in Taiwan. Deep-water or seasonally restricted occurrences are other possibilities.

There are possible limitations of our study that underestimate overall pelagic tunicate diversity. The collection of organisms was carried out at restricted regions covering only the summer season and surface waters. As mentioned, there are tunicate species which are restricted to certain depths or which migrate or reproduce in deeper waters. Through sampling in deeper waters, we could expect to have found a higher species diversity for salps and doliolids. Salps perform vertical migration and doliolids reach deep waters in order to reproduce. Perhaps this is the reason that we could not identify any of the following salp species: Thalia rhomboides , Thalia cicar , Salpa younti , Traustedtia multitentaculata , Ritteriella amboinensis , Cyclosalpa a ffi nis and Cyclosalpa sewelli . In fact, the listed species were already monitored in the waters of Taiwan by Liao et al. in 2013 and Zhang et al. (2003). As for the doliolid species, thanks to sampling in deep waters, we found Doliolum nationalis , also identified in the past through the same studies listed above.

Furthermore, the mesh size of the plankton net (333 μm) used in six of a total of seven stations was probably not enough to collect small-sized taxa such as small representatives of Oikopleura and neonates or juveniles.

We expect our study to be a starting point for future research. It would be interesting to study abundance and distribution shifts caused by climate change in addition to coastal thermal discharge effects in different seasons. Such information would be useful for a better understanding of expected changes in the community structure of pelagic tunicates over longer periods of time, taking as an example the study conducted by Lavaniegos and Ohman (2003) which focused on long-term changes of pelagic tunicates following the California Current. We might even be able to predict their patches, similar to the approach of Deibel and Paffenhöfer who analysed the Gulf Stream (2009). We would also gain a better understanding of diversity, abundance and distribution of zooplankton generally within pelagic systems in the China Seas.