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The Antarctic scallop Adamussium colbecki is a promising proxy for sea-ice persistence and can potentially resolve subannual seawater conditions characteristic of annual and multiannual sea ice. Alternating groups of widely- and narrowly-spaced striae (small ridges on valve surfaces) are thought to indicate seasonal growth differences: wide groups in summer, narrow groups in winter. Shell oxygen (δ18Os) and carbon (δ13Cs) in striae groups may therefore reflect seasonal seawater conditions. We expect lower δ18Os in wide summer striae groups under both annual and multiannual sea ice if glacial meltwater mixes through the water column. We also expect higher δ13Cs in wide striae groups under annual sea ice but not under multiannual sea ice, as phytoplankton blooms post seaice breakout enrich seawater δ13CDIC. Scallops were collected from two sites in western McMurdo Sound (Ross Sea) located ~30 km apart: Explorers Cove (EC) has multiannual sea ice and Bay of Sails (BOS) has annual sea ice. Adults were collected live by divers at 9–18 m depth in 2008 from EC and BOS. Additional juveniles (< 2 yrs) were collected from EC in 2016. Two adults each from EC and BOS and two 2016 juveniles were serially sampled for stable isotopes. δ13Cs decreases over ontogeny due to metabolic effects; the linear trend was removed to enable seasonal comparison. Detrended residuals are referred to as δ13Cs det. Mean δ18Os (~3.7‰) is not different in narrow and wide striae groups under either annual or multiannual sea ice, suggesting negligible glacial meltwater mixing at depth and minimal seasonal temperature change at both sites. δ18Os values are within expected equilibrium range and decrease over ontogeny, suggesting increased growth during warmer temperatures in older scallops. In contrast, mean δ13Cs det is ~1‰ higher in wide summer striae groups than narrow winter striae groups under annual sea ice at BOS, but not different between striae groups under multiannual sea ice in EC adults. δ13Cs det is also higher in wide summer striae groups from 2016 EC juveniles, however sea ice broke out at EC in 2015, so juveniles experienced annual-like sea-ice conditions. Seasonal differences in δ13Cs suggest that carbon isotopes coupled with striae width in A. colbecki may be a good proxy for sea-ice persistence in Antarctica both in modern and fossil assemblages. 

We studied the population and size distribution of the parasitic foraminifer Cibicides antarcticus living on the shell of the Antarctic scallop Adamussium colbecki within Explorers Cove, Western McMurdo Sound, Antarctica. Previous work examined populations and parasite load between two distinct geographic locations, but our study focuses on the population and size distribution of C. antarcticus within one embayment, Explorers Cove. We hypothesize that if A. colbecki are living in the same embayment and have one recruitment event, then populations and their size distributions should be similar; but, if they have differing populations and sizes, they likely are recruiting from very localized microhabitats with varying recruitment events. Live A. colbecki were collected from the Jamesway (water depth 24.4 m), Smallberg (9.1 m), and Anoxic Pit (9.1 m) sites in Explorers Cove. Five top valves were examined for C. antarcticus under 75x magnification. The foraminifera were counted, their spatial distribution noted, and their largest diameter was measured using ImageJ. All data from each site was pooled to compare the sites. Results indicate that all the sites had different populations of parasitic C. antarcticus. Smallberg had the most parasitic foraminifera (n = 663), followed by Jamesway (n = 319); the Anoxic Pit site had the fewest (n = 55). The largest size classes (0.70–1.30 mm) occurred at the Anoxic Pit and Smallberg sites, while the smallest size classes (0.18–0.70 mm) were found at Jamesway, the deepest site. The average size of Cibicides was also smaller at Jamesway (0.71 mm), compared to Smallberg (0.92 mm), and Anoxic Pit (0.94 mm). In general, C. antarcticus recruits to the youngest part of the scallop shell while larger adults are found on oldest part of the shell. The skewed size frequency distributions and differing population sizes suggest that C. antarcticus has localized microhabitat recruitment in Explorers Cove, rather than one synchronous recruitment event. 

One of the most fundamental changes predicted to occur under warming scenarios for Antarctica is the invasion of durophagous (shell-breaking or peeling) predators—like decapod crustaceans—which were last common in Antarctic waters during the warmer Eocene Period, over 30 million years ago. Since then, Antarctica’s shallow-water benthos developed Paleo- zoic (or deep-sea-like) ecosystems dominated by epi- benthic echinoderms. Despite the looming predatory carnage, little is known about how predators structure shallow subtidal communities in Antarctica, especially in regard to predation on shelled prey. We therefore need to have a baseline of shell repair—if it occurs— prior to the initial invasion of crabs. Here, we assess whether the shell of the Antarctic Scallop, Adamussium colbecki, living in the shallow subtidal under sea ice, records an ontogenetic history of shell repair. Shells of A. colbecki(n=623 valves; ~ 0.50 mm thick) were collected from shallow depths (6–24 m) within western McMurdo Sound, Ross Sea, from the coldest waters on Earth (-1.97 °C): Four sites in Explorers Cove (EC) with semi-permanent (decadal or more) sea ice and a Ferrar Glacier site (located ~30 km south of EC) with annual sea ice and icebergs. All sites were composed of fine sediments interspersed with glacial erratics that were more common at Ferrar than EC. Ju- venile (≤ 50 mm) and adult portions of the shells were examined under a dissecting scope for shell repair. Results indicate that repair did occur and was consistent with predatory damage: 1) valves had ste- reotypic damage patterns, both in style and spatial distribution; 2) there were five styles of repair rang- ing from typical crab-like (jagged) repair to elongate repair; 3) scallops living under ice scour regimes (Ferrar) did not have significantly different repair frequencies than those living under semi-permanent sea ice (EC sites); and 4) none of the shells had shell repair consistent with ice scour as described previ- ously for Laternula elliptica, an Antarctic burrowing bivalve. Frequency of repair varied between 0.04 and 0.26 for the five sites and depths (mean 0.10) and adults had the highest frequency of repair. The mean repair frequency is similar to infaunal Laternulafrom other semi-permanent sea ice sites in McMurdo Sound, but higher than those reported for epifaunal brachio- pods from the Antarctic Peninsula where ice scour does occur. We posit that shell repair can be used as an indicator of durophagy in Antarctica: The forensic agents are unexpectedly sea stars and possibly fish. In a warming world, this scallop may not survive long withboth an increase in ice scour and the putative ar- rival of shell-breaking crabs at ~1 °C. 

Ecosystem engineers such as the Antarctic scallop (Adamussium colbecki) shape marine communities. Thus, changes to their lifespan and growth could have far-reaching effects on other organisms. Sea ice is critical to polar marine ecosystem function, attenuating light and thereby affecting nutrient availability. Sea ice could therefore impact longevity and growth in polar bivalves unless temperature is the overriding factor. Here, we compare the longevity and growth of A. colbecki from two Antarctic sites: Explorers Cove and Bay of Sails, which differ by sea-ice cover, but share similar seawater temperatures, the coldest on Earth (-1.97°C). We hypothesize that scallops from the multiannual sea-ice site will have slower growth and greater longevity. We found maximum ages to be similar at both sites (18–19 years). Growth was slower, with higher inter-individual variability, under multiannual sea ice than under annual sea ice, which we attribute to patchier nutrient availability under multiannual sea ice. Contrary to expectations, A. colbecki growth, but not longevity, is affected by sea-ice duration when temperatures are comparable. Recent dramatic reductions in Antarctic sea ice and predicted temperature increases may irrevocably alter the life histories of this ecosystem engineer and other polar organisms. 

To understand whether or how climate change will drive changes in Antarctic marine benthic ecosystems, one must first understand baseline population dynamics of the system. Species abundances may change seasonally, annually, or on decadal scales, for example, or due to rare cataclysmic events unrelated to a changing climate. We used SCUBA to collect foraminiferan protists in Explorers Cove, McMurdo Sound in austral spring in 1986 to 2016. Our research involved the cell biology and molecular evolution of large (>1mm), earlyevolving, agglutinated members of this assemblage, but during the course of this work we also charted changes in their populations from bulk surface sediment collection and semi-quantitative 0.25-m2 to 1-m2 quadrat sampling. We focused on two species of Astrammina, two species of Crithionina, Notodendrodes hyalinosphaira, larger calcareous species (Pyrgo peruviana, Cornuspira sp., Glandulina sp.), as well as Gromia cf. oviformis. During the 1990s, we noted that relative species abundances fluctuated substantially on an inter-annual basis. For example, Astrammina rara was very abundant in 1990 (75.9% of the total assemblage), dipped in 1993 and 1994 (54.9% and 58.7%, respectively), and rebounded in 1998 and 1999 (65% and 67%). By contrast, Astrammina triangularis abundances were low in 1990 (0.3%), peaked in 1993 (18.3%) and declined to 6.5% of the total assemblage in 1998. During the 2000s, we began tracking numericaldensities quantitatively by taking 7.4cm-diameter cores and wet-picking specimens recovered from the top cm of sediment. Similar fluctuations were observed in target species. Most notable was the rapid increase in a “silver saccamminid” species, first recognized at low abundance in 1998. In 2005 there were 412/m2 and since that time their numbers have increased to become the dominant species in the area (186,732/m2 in 2016). Over our study period, we also noted changes in meio- and macrofauna. In particular, we noted a dramatic increase in the numerical density of small epifaunal and infaunal tunicates (360/m2 in 2005 to 11,379 in 2016). We also observed a dramatic, qualitative reduction in the population of the Antarctic scallop Adamussium colbecki along the Explorers Cove ice wall, prompting us to examine the extent of their decrease by re-sampling the six stations reported by Stockton in 1982 using his methods. The results were surprising: The average scallop population had decreased 74%. Similar results were obtained in 2015 and 2016. Although the cause of the reduction remains unknown, we noted new recruits on the seafloor in 2016, indicating initial recovery from this event. Clearly, the Antarctic benthos is anything but static. Standardized, long-term environmental monitoring is necessary to uncover changes attributable to climate change. Explorers cove, with its proximity to McMurdo Station and the Taylor Dry Valley LTER site, is a prime candidate for such an endeavor.