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Grigor-Et-Al.-2017.Pdf Journal of Plankton Research academic.oup.com/plankt J. Plankton Res. (2017) 00(00): 1–20. doi:10.1093/plankt/fbx045 Growth and reproduction of the chaetognaths Eukrohnia hamata and Parasagitta elegans in the Canadian Arctic Ocean: capital breeding versus income breeding † JORDAN J. GRIGOR1,2* , MORITZ S. SCHMID1 AND LOUIS FORTIER1 TAKUVIK JOINT INTERNATIONAL LABORATORY AND QUÉBEC-OCÉAN, UNIVERSITÉ LAVAL, QUÉBEC, QUÉBEC GV A, CANADA AND CENTER FOR SCIENCE OUTREACH, VANDERBILT UNIVERSITY, PMB , APPLETON PLACE, NASHVILLE, TN , USA † PRESENT ADDRESS: CENTER FOR SCIENCE OUTREACH, VANDERBILT UNIVERSITY, NASHVILLE, TN 37212, USA *CORRESPONDING AUTHOR: [email protected] Received November 29, 2016; editorial decision August 10, 2017; accepted August 12, 2017 Corresponding editor: Marja Koski In Arctic seas, primary production and the availability of food for zooplankton are strongly pulsed over the short productive summer. We tested the hypothesis that Eukrohnia hamata and Parasagitta elegans, two similar and sympatric arctic chaetognaths, partition resources through different reproductive strategies. The two species had similar nat- ural longevities of around 2 years. Eukrohnia hamata, which occurred at epi- and meso-pelagic depths, spawned two distinct broods in autumn and spring. Offspring production coincided with drops in the frequency of E. hamata with visible lipid reserves, characteristic of capital breeders. Growth was positive from April to January and negative in February and March. Growth and maturation were similar for the two broods. Storage reserves contained in an oil vacuole may allow E. hamata to reproduce and grow outside the short production season. Parasagitta elegans produced one brood in summer–autumn during peak production in near-surface waters, characteristic of income breeders. In winter, P. elegans co-inhabited meso-pelagic waters with E. hamata, where it neither grew nor reproduced. As the Arctic warms, the development of an autumn phytoplankton bloom could favour the summer–autumn brood of P. elegans. KEYWORDS: Eukrohnia hamata; Parasagitta elegans; chaetognaths; reproduction; growth; annual routines; Beaufort Sea; Arctic Ocean available online at academic.oup.com/plankt © The Author 2017. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] Downloaded from https://academic.oup.com/plankt/article-abstract/doi/10.1093/plankt/fbx045/4344873/Growth-and-reproduction-of-the-chaetognaths by guest on 05 October 2017 JOURNAL OF PLANKTON RESEARCH j VOLUME j NUMBER j PAGES – j INTRODUCTION may depend on the seasonal partitioning of resources, which can be achieved through differences in vertical Arrow worms or chaetognaths form a highly successful distribution, diet and reproductive strategies. For instance, phylum of semi-gelatinous marine zooplankton repre- the reproductive strategies of the two dominant arctic sented by ~200 species worldwide, with biomass esti- herbivorous copepods differ markedly in ice-covered – et al. mated at 10 30% that of copepods (Bone , 1991). waters. The large capital-breeder Calanus hyperboreus They can represent a significant fraction of the diet of reproduces in winter, fuelling egg production with et al. many animals such as amphipods (Gibbons , 1992), lipids accumulated in summer (Conover, 1967; Hirche other chaetognaths (Pearre, 1982), seabirds (Mehlum and Niehoff, 1996; Pasternak et al., 2001), whereas its and Gabrielsen, 1993), and are the preferred prey of the fi fi congener, Calanus nmarchicus, does not reserve lipids to larvae of some tropical sh and commercially important fuel its reproduction (income breeding, Sainmont et al., et al. et al. decapods (Sampey , 2007; Saunders , 2012). 2014). Low amounts of wax esters (Connelly et al., Chaetognaths are hermaphrodites. Spermatogonia 2012; Connelly et al., 2016), and peak reproduction in bud off from testes parallel to the tail walls to produce – fi spring summer in the epipelagic P. elegans (Kramp, spermatocytes, which are transmitted to a conspeci cto 1939; Sameoto, 1971; Grigor et al., 2014), suggest income fertilize its oocytes (Alvarino, 1992; Bergey et al., 1994). breeding with the summer maximum in prey abundance Each chaetognath possesses one pair of seminal vesicles fuelling reproduction. By contrast, the accumulation of on the tail segment and one pair of seminal receptacles wax esters in an oil vacuole (Pond, 2012), and year-round at the posterior ends of the oviducts. Two individuals fi breeding in Gerlache Strait, Antarctica (Øresland, 1995), rst come into contact with each other, before sperm is suggest capital breeding fuelled by lipid reserves in passed from the seminal vesicles of one individual to the E. hamata. seminal receptacles of the other (Goto and Yoshida, We describe the life histories of E. hamata and P. ele- 1985). Hatching strategies vary amongst species. For gans over an annual cycle based on weekly sampling of Parasagitta elegans instance, releases buoyant eggs directly zooplankton in the Amundsen Gulf (Beaufort Sea) from into the water (Kotori, 1975; Hagen, 1999), whereas the August 2007 to July 2008, complemented by collections young of Eukrohnia hamata are retained for some time in fi from other areas of the Canadian Arctic in autumn of the folded lateral ns of the adults (Alvarino, 1968 and the 2 years. In particular, growth of size cohorts, repro- references therein). duction and oil reserves are tracked to explore the In contrast to the high diversity of chaetognaths in hypothesis that E. hamata is a capital breeder that et al. other seas (De Souza , 2014), only three species are spawns during most of the year, while P. elegans is an frequently reported in Arctic plankton surveys. income breeder, reproducing during periods of high Parasagitta elegans is a neritic species, often peaking in prey availability. abundance in epipelagic waters; E. hamata is abundant in meso-pelagic and deep waters; and Pseudosagitta max- ima, growing much larger (up to 90 mm) than the other species (40–45 mm), is typically bathy-pelagic but may METHOD also occur near the surface in the Arctic (Bieri, 1959; Alvarino, 1964; Terazaki and Miller, 1986; Sameoto, Study area 1987). A fourth species, Heterokrohnia involucrum has also Sampling was conducted on-board the CCGS Amundsen, been recorded at bathy-pelagic depths, so far only in the primarily from October 2007 to August 2008 in the Arctic Ocean (Dawson, 1968). While their contribution Beaufort Sea (Fig. 1). To complete the annual cycle of to higher trophic levels is poorly documented, arctic observations, additional collections from the Labrador chaetognaths may consume an important fraction of Sea, the Canadian Archipelago and Baffin Bay were copepod biomass. Although admittedly little better than included in the analyses (Fig. 1). In addition to the 27 a guess, Welch et al. (1992) estimated that chaetognaths stations sampled at weekly or higher resolution in the in Lancaster Sound ingest 51% of the copepod biomass Amundsen Gulf region (69–72°N, 121–131°W) from − − annually (164 of 319 kJ m 2 yr 1). November 2007 to August 2008, two stations were At high arctic latitudes, resource availability is sampled in Nachvak Fjord in August 2007, two stations strongly pulsed seasonally, with maximum phytoplank- in Parry Channel in October 2007 and two stations in ton and zooplankton biomass occurring during the short northern Baffin Bay in September 2008 (Fig. 1 and ice-free season in late summer and autumn (Falk- Supplementary Table 1). Amundsen Gulf, our main Petersen et al., 2007; Falk-Petersen et al., 2009;;Søreide sampling region, is a 400 km long, 170 km wide channel et al., 2010). Here, the co-existence of similar species with a maximum bottom depth of ~630 m that connects Downloaded from https://academic.oup.com/plankt/article-abstract/doi/10.1093/plankt/fbx045/4344873/Growth-and-reproduction-of-the-chaetognaths by guest on 05 October 2017 J. J. GRIGOR ET AL. j ANNUAL LIFE CYCLES OF TWO ARCTIC ARROW WORMS Fig. 1. Bathymetric maps of the Canadian Arctic Ocean indicating the regions and stations (black circles) where chaetognaths were sampled from August 2007 to September 2008. Station IDs provided. the south-eastern part of the Beaufort Sea to the water column at single stations in Nachvak Fjord, Parry Canadian Archipelago. Amundsen Gulf is typically cov- Channel and Baffin Bay were obtained using a ered with sea ice from October to early June (Barber conductivity-temperature-depth (CTD) system. We also and Hanesiak, 2004). Three water masses are detected collected seawater samples from 9–10 depths in the in the Amundsen Gulf (Geoffroy et al., 2011 and refer- upper 100 m, using the CTD rosette equipped with 24 ences therein); the Pacific Mixed layer (PML; 0–60 m), 12 1 Niskin-type bottles (OceanTest Equipment). the Pacific Halocline (PH; 60–200 m) and the Atlantic Immediately after sampling, subsamples (500 ml) for the layer (AL; >200 m). Nachvak Fjord is a pristine 45-km determination of total chl a were filtered onto 25 mm long sill-fjord in Nunatsiavut, Northern Labrador, with Whatman GF/F filters (nominal porosity of 0.7 μm). a maximum depth of 210 m (Bell and Josenhans, 1997; Chl a was measured using a Turner Designs 10-AU Simo-Matchim et al., 2016). The deep semi-enclosed fluorometer, after 24 h extraction in 90% acetone at basin of Baffin Bay lies further north, and is connected 4°C in the dark without grinding (acidification method via Parry Channel to the Beaufort Sea in the west of Parsons et al., 1984). (Fig. 1). Two similar samplers were used to collect zooplank- ton: a square-conical net with a 1 m2 opening area, 200 μm mesh and a rigid cod-end, and a Hydrobios® Sampling Multinet with nine individual opening and closing nets In the Amundsen Gulf, estimates of weekly profiles of with 0.5 m2 openings, 200 μm mesh and rigid cod ends. temperature and salinity between November 2007 and At 12 stations, the 1 m2 net was deployed cod-end first July 2008 are from Geoffroy et al.
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