Abundance and distribution of larval Euphausiids at the western Antarctic Peninsula

Matilda Haraldsson¹, Volker Siegel², Denis Sologub³, Katya Uryupova⁴

1 Gothenburg University

2 Johann Heinrich von Thünen-Institut

3 Russian Federal Research Institute of Fisheries and Oceanography

4Moscow State University/N.K. Koltsov Institute of Developmental Biology Objectives

Knowledge about krill and Euphausiid larval ecology is an essential part to understand the variations found in adult Euphausiid populations. Still little is known about the general Euhapusiid larval ecology and almost nothing about what triggers the spawning of adults, the mortality at different larval stages and how their abundance and distribution is affected by the physical environment (Siegel 2000). For these kinds of questions long-term data is important as it gives the opportunity to investigate variations between years. Meanwhile, estimates of larval abundances give a direct measure of the spawning success at that specific year, which is also important information for stock assessment work. The main objective of this study is to get an overview of Euphausiid larval abundance and distribution at the western Peninsula in relation to their adult population, with detailed information of the larval development during this time of the year. Furthermore, genetic data on the population structure of Euphausiids in relation to their larval development is missing. A second part of this investigation aims to establish the knowledge of distribution of Antarctic krill larvae associated with particular water masses. For general understanding of the role of Euphausiids in evolution history of the Antarctic biota, a subset of samples will be used for molecular bar-coding analysis.

Finally, samples of amphipod Themisto gaudichaudii has been collected for bar-coding studies and subsequent reconstruction of evolutionary history of the Antarctic Hyperoidea with particular emphasis on relationships between the Southern and the Northern Hemisphere species. With these objectives, the krill monitoring program was an excellent opportunity for Euhapusiid larval studies, as the program offers a standardized sampling, coverage of large geographical area during subsequent years.

Work at sea Net sampling

The Euphausiid larval study was part of the krill monitoring program performed during this expedition (see section on RMT 8 sampling), and all sampling of krill larvae were done simultaneously to the adult krill sampling using the RMT 1 net (Rectangular Midwater Trawl). The RMT 1 is a smaller zooplankton net mounted on top of the RMT 8 (for description see section on RMT 8 sampling) with a mouth opening of 1 m² and 330 μm mesh size designed to catch Euphausiid larvae and other meso-zooplankton.

The station grid, timing and procedure of sampling are described by Siegel et al. (see section 5.4).

Treatment of samples

Immediately after net retrieval the RMT 1 net sample was split into two parts using a Folsom plankton splitter (van Guelpen et al. 1982) and preserved in a 4 % formalin-in-seawater solution. One half was preserved for analysis onboard, and one half stored away for further detailed analysis at the Russian Federal Research Institute of Fisheries and Oceanography in Moscow. At selected stations, one of the two halves was split further into two quarters. One of the subsequent quarters was either frozen down to -80°C for further chemical analysis, or fresh larvae and amphipods were picked out and preserved in ethanol for further molecular analysis.

After 1 - 2 days of preservation the subsequent analyses were performed. Due to the size of the RMT 1 samples they generally required further sub-sampling which was done using the Folsom plankton splitter. Larval Ephausiids were picked out from the subsample using a stereomicroscope and classified into developmental stages (nauplius, metanauplius, calyptopis 1 to 3, furcilia 1 to 6) according to Kirkwook (1982) and Baker et al. (1990). Damaged larvae which could not be identified into their exact stage were categorized into as detailed groups as possible, belonging either to unidentified calyptopis, unidentified furcilia, early furcilia (furcilia 1-3) or late furcilia (furcilia 4-6). Damaged larvae were most common for the older Thysanoessa macrura specimens, as this is smaller and more fragile species. Finally, all larval count data were standardized into density (m^-2) using filtered water volumes from the flow meter readings.

Fig. 5.4.8: Distribution of larval Euphausiids (given in m^-2) in the Antarctic Peninsula during January 2011. a) Euphausia superba, b) Thysanoessa macrura, c) E. crystallorophias and d) E. frigida. APF = Antarctic Polar Front, SACCF = South Antarctic Circum Polar Front and SBF = South Boundary Front.

Note the different scale in the figure.

Preliminary results

Larval samples were successfully analyzed from all 81 stations of the krill grid, and larvae from four of the possible five Euphausiid species were found; Euphausia superba, E. crystallorophias, E. frigida and Thysanoessa macrura, while larval E. triacanta was totally missing.

Larval E. superba were found at 57 of the stations, and had the highest densities out of the four species with an arithmetic mean of 2449.5 m^-2. Their geographical distribution were concentrated to the southernmost and offshore stations of the grid (Fig. 5.4.8a).

A high density patch was found around 66° S and 72° W with a maximum of 66,971 m^-2. The frequency distribution of developmental stages were highly dominated by calyptopis 1 (Fig. 5.4.9a). The mean developmental stage with time (Fig. 5.4.10a) shows that calyptopis 1 were dominating during the whole sampling period, while significant amount of calyptopis 2 and 3 were only found during the second part of the

cruise. This indicates that E. superba most likely started spawning in mid December (based on developmental time from Ikeda 1984 and Witek et al. 1980) and that they had been continuously spawning at least during the first part of the sampling period due to the steady stream of calyptopis 1.

Fig. 5.4.9: Frequency distribution of larval developmental stage for a) Euphausia superba, b) Thysanoessa macrura, c) E. crystallorophias and d) E. frigida. N = Nauplius, MN = Metanuaplius, C =

Calyptopis, F = Furcilia.

T. macrura larvae were the most widespread species found at every station of the grid.

Their geographical distribution was even over the whole sampling area with slightly lower densities close to shore (Fig. 5.4.8b). Their overall mean density was 482.4 m^-2. All developmental stages from metanauplie to furcilia 6 were found for T. macrura (Fig. 5.4.9b). Furcilia were clearly dominating over calyptopis, with furcilia 1, 2 and 4 as the most common stages. The mean developmental stage with time shows a large variation in dominating stage, but with a clear decreasing trend in the representation of younger calyptopis stages as the cruise progressed (Fig. 5.4.10b). The presence of all stages indicates an extended spawning period for T. macrura, and based on the presence of furcilia 6 in the samples they started their spawning in September (based on developmental times from Menshenina and Spiridonov 1991).

Fig. 5.4.10: Average developmental stage at a certain station against the day of sampling (day 1 at the 9 January 2011) for a) Euphausia superba, b) Thysanoessa macrura and c) E. crystallorophias.

The figure for E. frigida is not included due to the low numbers of larvae. N = Nauplius, MN = Metanuaplius, C = Calyptopis, F = Furcilia.

Larvae of the coastal species E. crystallorophias were mostly found at stations closest to shore at Gerlaiche Strait and at the southernmost transects of the grid (Fig. 5.4.8c).

They were found at 18 stations, and were comparably low in density with a mean density of 28.3 m^-2. Naupli to fucilia 1 stage were present in the samples, dominated by calyptopis 1 (Fig. 5.4.9c). Calyptopis 1 were continuously dominating throughout the sampling period, but with later calyptopis stages getting more represented with time (Fig. 5.4.10c). Given that furcilia 1 were found in the samples, E. crystallorophias may slowly have started their spawning already at the end of October (based on developmental time from Ikeda 1986).

E. frigida were only found at 10 stations, all offshore (Fig. 5.4.8d), and they had the lowest densities of all the species with a mean density of 1.5 m^-2. Larval developmental stages from metanaulius to furcilia 2 were found, dominated by calyptopis 2 (Fig.

5.4.10d).

Further work

For the genetic samples preserved in ethanol, two research activities will be carried out at the Moscow State University/N.K. Koltsov Institute of Developmental Biology in Moscow. First, a general phylogenetic relationship of Euhapusiid crustaceans will be done based on comparative analysis of sequences of mitochondrial gene COI - the first subunit of cytochrome oxidase c, which is commonly used in barcoding. DNA barcoding is a technique that uses a short gene sequence from a standardised region of the genome as a diagnostic ”biomarker” for species and subspecies. Secondly, population structure of krill is going to be revealed using microsatellites which are appropriate for the description of heterogeneity within groups of individuals. It may provide us information on isolation of krill larvae originated from different water masses.

Samples and data

See section on RMT 8 sampling.

References

Baker AdC, Boden BP, Brinton E (1990) A practical guide to the euphausiids of the world. Nat Hist Mus Pub, London.

Ikeda T (1984) Development of the larvae of the Antarctic krill (Euphausia superba Dana) observed in the laboratory. J Exp Mar Biol Ecol 75: 107-117.

Kirkwook JM (1982) A guide to the Euphausiacea of the Southern Ocean. ANARE Res Notes 1, 1-45 pp.

Menshenina LL, Spiridonov VA (1991) The developmental rate of the Antarctic Euphausiid larvae (by the field data). Okeanologiya 31: 621-627.

Siegel V (2000) Krill (Euphausiacea) life history and aspects of population dynamics. Can J Fish Aquat Sci 57 (3): 130-150.

van Guelpen L, Markle DF, Duggan DJ (1982) An evaluation of accuracy, precision, and speed of several zooplankton sup-sampling techniques. J Cons Int Explor Mer 40: 226-236.

Witek Z, Koronkiewicz A, Soszka GJ (1980) Certain aspects of the early life history of krill Euhapusia superba Dana (Crustacea). Pol Polar Res 1: 97-115.

5.4.2 Environmental transcriptomics of the Southern Ocean salp,