Notes on the Ascidian component of a Marine Benthos survey in Australian Antarctic Territory.
The Marine Benthos Program on Voyage 5 (BRAD) 1996/97 by Research Survey Vessel Aurora Australis aimed to quantify the abundance and diversity of benthic macroinvertebrates on different sediment types in four areas of Prydz Bay (Bardsley 1997). Unlike previous projects that collected marine invertebrates as bycatch from other programs (Zeidler 2015), this investigation sampled benthic invertebrates directly, and produced the specimens discussed in the article 'Three Antarctic ascidians from Four Ladies Bank' (Bathie and Pett 2019). The distribution of ascidians across different substrates 21 years ago led us to consider the possible impact on ascidians of glacier retreat, which has been associated with more frequent ice scour events and increased rates of sedimentation (Sahade et al. 2015). Although ascidians have been proposed as sentinel species able to signal effects of global warming on benthic composition (Segelken-Voigt 2016), our investigation indicates that ascidians in Australian Antarctic Territory have received little attention so far.
Prydz Bay on the east coast of Antarctica is a deep embayment where the giant drainage system of Lambert Glacier ends at the Amery Ice Shelf (Fig. 1, Bathie and Pett 2019). Prydz Channel is a broad trough, 400-500 m deep and up to 100 km wide and was probably eroded by a fast-flowing ice stream passing seaward, now between Fram Bank and Four Ladies Bank (O'Brien et al. 2016). From a depth of 700 m in the Amery Depression, the sea floor rises gently to Four Ladies Bank, a shelf bank 100-200 m deep, the edge of which probably marks the outer limit of the East Antarctic Ice Sheet at the Last Glacial Maximum (Mackintosh et al. 2014). Most areas of Prydz Bay shallower than 690 m show iceberg ploughmarks (O'Brien et al. 1997), some relict and some that may have been freshly made by large modern icebergs (O'Brien et al. 2016). Much of the sea floor consists of fine mud and biosiliceous ooze (O'Brien et al. 2014).
Materials and Methods
Working in conjunction with the 21-day marine geophysical survey and sediment sampling program (Harris et al. 1998), zoologists and biologists from Museum of Victoria (now Museums Victoria) and Deakin University were allocated 48 hours for sampling the benthos of Prydz Bay (Bardsley 1997; Quilty 1997).
Four sites were sampled to examine the distribution of benthic macro invertebrates on different sediment types: Fram Bank (67[degrees] 10' S, 70[degrees] 40' E, 290 m), Prydz Channel (67[degrees] 10' S, 72[degrees] 19' E, 550 m), Prydz Channel East (67[degrees] 10' S, 74[degrees] 30' E, 430 m), and Four Ladies Bank (67[degrees] 27' S, 76[degrees] 40' E, 320 m) (Bardsley 1997) (Fig. 1).
Five beam trawls were conducted at each site, deployed for up to 10 minutes per drag and towed at a speed of 1 to 1.5 knots, enabling calculation of the area trawled and hence the density of organisms (mass kg/[m.sup.2]) (Bardsley 1997). The metal trawl frame was 3 m long and 0.5 m high with runners at either end. The leading edge of the 8 m cone-shaped net (30 mm mesh) attached to a weighed chain formed a ground rope that curved back behind the top of the net to the beam so that mobile animals disturbed by the ground rope could not escape upwards.
The first trawls took place on 22 February 1997 in Prydz Channel, the last one failing when wire wrapped around the trawl frame. Deteriorating weather then stopped the project for a few days but the Voyage Leader recorded that morale was very high after pro ductive hauls (Quilty 1997). Trawling resumed on 4 March on the northern face of Fram Bank, followed by a revisit to Prydz Channel, that was fruitful despite the net being badly torn. A team worked overnight to repair it before the last trawls at 320 m on the western flank of Four Ladies Bank (Quilty 1997). Collected material was sorted to Class or Order, the mass and number of individual specimens recorded for each taxon. All material was to be lodged and curated at the Museum of Victoria (Bardsley 1997).
Successful hauls varied between 57 and 200 kg of material (Bardsley 1997). Prydz Channel sites (400-500 m) subject to streams carrying gravel and sand from inland (Harris et al. 1998) and carrying meltwater from the ice shelf (Williams et al. 2016), were dominated by Porifera (sponges), Holothuroidea (sea cucumbers) and Pennatulacea (sea pens). Only 5% of the total mass of Ascidiacea (sea squirts) came from the channel. The highest biomass on the thick but coarse sediment on the northern face of Fram Bank (300 m) (Harris et al. 1998) was again Porifera, but followed by Ascidiacea (72% of Ascidian mass). Trawls at Four Ladies Bank, in gravelly muddy sand (Harris et al. 1998) subject to ocean currents entering the bay also produced an abundance of sponges, but there were fewer sessile sea squirts than mobile sea cucumbers. Four Ladies Bank contributed 23% (10 kg) of the total mass of Ascidiacea (Fig. 2).
Antarctic ascidians and the effects of climate change
The authors have alluded elsewhere to material from previous Prydz Bay surveys (1991, 1993) awaiting examination at Museums Victoria (Bathie and Pett 2019). After viewing the collection, we judge it to include several specimens of two species we discussed: Pyura discoveryi (Herdman, 1910), often found anchored in sediment (Herdman 1923), and Bathypera splendens Michaelsen, 1904, which probably prefers vertical or sloping flat surfaces subject to currents inhibiting the buildup of sediment (Young and Vasquez 1995). Whereas our small sample included Cnemidocarpa pfefferi (Michaelsen, 1898), the unregistered material appears to be dominated by C. verrucosa (Lesson, 1830), said to be the most abundant and conspicuous Cnemidocarpa in Antarctic waters (Brueggeman 1998), and known to survive periods of turbulent sediment by increasing the rate of squirting (by which it repels matter likely to block branchial apertures) (Torre et al. 2014).
The authors have no information about the relative abundances of these species, but note that populations can change rapidly. Sea ice that melts in summer is proposed as a proxy for availability of food for ascidians in the Antarctic (Segelken-Voigt et al. 2016; Bathie and Pett 2019), but drifting icebergs of glacial origin can have a devastating effect on sessile communities. Ice scour, which can modify the substrate while seasonally removing the bottom fauna of large areas (Tatian et al. 1998), has been studied intensively as a key structural force shaping benthic communities (Torre et al. 2017). Ascidians with high growth rates are known to colonise quickly after a change to the substrate (Kowalke et al. 2001), and one species may be rapidly replaced by another in a complex interplay of limiting factors. New silt barriers can prevent larval dispersal, or directly kill juveniles (Young and Chia 1984). Sediment dwellers are less likely to re-establish on newly bare rocks. A reduction in light levels (caused by suddenly proliferating algae) deterring larval settlement, or competition from other sessile invertebrates (like the sponges dominating assemblages in Prydz Bay), could produce local conditions conducive to the spread of C. pfefferi where C. verrucosa previously had been dominant, or vice versa. It is evident that in conditions of high sedimentation and frequent ice scour, abundant, diverse and changing benthic communities, including ascidians, have flourished in Antarctica (Sahade et al. 2015). This fast response to environmental modification makes ascidians potential indicators of environmental change (Segelken-Voigt 2016).
Recently attention has turned to the effects of climate change. In western Antarctica, Sahade et al. (2015) reported major shifts in assemblage structure attributable to increased sedimentation rates associated with glacier retreat. Communities in which the dominant filter-feeders had been ascidians in 1998, were dominated by sponges and seapens by 2010-evidence that sediment-tolerant assemblages had reached a critical threshold of sediment sensitivity after which the whole ecosystem had shifted to an alternative equilibrium (Sahade et al. 2015)--although this change was observed in habitat closest to the glacier, rather than on outer shelf banks. Also, at Potter Cove, it was hypothesised that in a location newly exposed by glacier retreat, known ascidian pioneer species such as Molgula pedunculata (Herdman, 1881) would dominate; in fact, the most abundant was C. verrucosa (Lagger et al. 2017) with its ability to increase the rate of squirting. It is likely that M. pedunculata, commonly observed with a stalk holding it above the sediment of the substrate (Torre et al. 2014), has been disadvantaged in a changing environment. In laboratory conditions of increasing sedimentation, Torre et al. (2014) measured no changes in filtration or ingestion which might mitigate the risk of suffocation.
It remains to be seen whether the glacial melt observed in western Antarctica, with increased rates of sedimentation and more frequent episodes of ice scour (Torre et al. 2017) will also be observed in Australian Antarctic Territory. The Collaborative East Antarctic Marine Census (CEAMARC) (2007-2008) was a multinational effort to survey the waters north of Terre Adelie, and George V Land with the intention of producing a benchmark against which future changes to marine life in eastern Antarctica can be studied (Terre Adelie is the French claim between segments of Australian Antarctic Territory) (Fig. 1, Bathie and Pett 2019). After RSV Aurora Australis sampled benthic communities of the continental shelf and slope down to 2000 m (Hosie et al. 2011), a French team produced the report on ascidians off Terre Adelie (Monniot et al. 2011). There, M. pedunculata (15% of solitary ascidians) had outnumbered the genus Cnemidocarpa (6.5%, which included five specimens of C. pfefferi and ten of C. verrucosa). Pyura discoveryi also represented 15% of solitary ascidians, and the genus Bathypera, 8.1% (Fig. 3).
To date, this is the only information available to us for future comparisons of the ascidian component of eastern Antarctic benthic assemblages. As was anticipated at the time of the CEAMARC survey the Mertz Glacier Tongue of George V Land broke off in February 2010 (Hosie et al. 2011). The effects of the expected ice scouring of the sea floor, along with changes to sedimentation and the sea ice regime, are yet to be investigated directly by sampling of benthic invertebrates (Jansen et al. 2018).
We thank Museums Victoria Collection Manager Melanie Mackenzie and Honorary Associate Mark O'Loughlin for stimulating our curiosity about Antarctic benthic investigations. We appreciate the assistance of Kelly Merrin (Museums Victoria Marine Sciences) and staff of the Marine Invertebrate Laboratory who answer our questions and take an interest in our projects. We are grateful to the editors of The Victorian Naturalist for their practical advice on the manuscript.
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Carol Bathie and Janet Pett
Marine Research Group, Field Naturalists Club of Victoria Volunteers, Marine Invertebrates, Museums Victoria Email: email@example.com; firstname.lastname@example.org
Received 2 February 2018; accepted 15 November 2018
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|Author:||Bathie, Carol; Pett, Janet|
|Publication:||The Victorian Naturalist|
|Date:||Feb 1, 2019|
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