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1. POPULATION AND SIZE DISTRIBUTION OF CIBICIDES ANTARCTICUS ON ADAMUSSIUM COLBECKI WITHIN EXPLORERS COVE, ANTARCTICA

   https://doi.org/10.1130/abs/2020AM-352890  

   Farmer, K. ; Walker, S.E. ; Bowser, S.S. ( April 2020 , Geological Society of America Abstracts with Programs)

   null (Ed.)

   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. 

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2. MOLLUSK FORENSICS IN ANTARCTICA: DO EPI- BENTHIC SCALLOPS EXHIBIT PREDATORY SHELL REPAIR?

   WALKER, S. ; BOWSER, S.S. ( June 2019 , PaleoBios)

   null (Ed.)

   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. 

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3. HIGH VARIABILITY OF SUBANNUAL GROWTH BANDS IN THE ANTARCTIC SCALLOP ADAMUSIUM COLBECKI

   https://doi.org/10.1130/abs/2018AM-324369  

   CRONIN, K. E ; WALKER, S. E ; BOWSER, S. S ( October 2018 , Geological Society of America Abstracts with Programs)

   null (Ed.)

   The Antarctic scallop Adamussium colbecki may be a crucial paleoenvironmental proxy for Antarctic sea ice during the Holocene. Sea ice can melt annually or persist for multiple years, with implications for the diet and growth of this ecosystem engineer. Subtle growth variations under each sea ice regime could be analyzed using striae (surficial concentric ridges) that putatively form fortnightly in juveniles. Previous work described alternating groups of widely spaced striae (summer) and narrowly spaced striae (winter). Each group may have 12 striae, or a pair of wide and narrow groups (cycle) may have ~ 28; both scenarios suggests approximate tidal (lunar) periodicity in striae formation. However, consistency of striae formation (total striae per valve and group) must be assessed in different environments, as factors such as sea ice or temperature could affect striae growth. We examined striae number, groups, and cycles in juvenile growth (< 50 mm) using scallops collected from two sites in western McMurdo Sound, Antarctica, that differ by sea-ice cover: Explorers Cove (EC) and Bay of Sails (BOS). Both sites have similar summer temperatures (-1.97°C), but EC has multi-annual sea ice whereas BOS has annual sea ice. We predict that annual melt and subsequent phytoplankton blooms likely induce a stronger environmental control than lunar periodicity. Thus, BOS scallops should have equal striae in wide and narrow groups, whereas EC should have fewer striae per wide group and fewer total striae as summer food availability would be greater at BOS and EC valves may cease growth in lower nutrient conditions. Median striae per wide or narrow group was similar at both sites (~12) and median total striae did not differ significantly between sites (EC: 188.5; BOS:183), suggesting striae formation is unaffected by sea ice. Similar median cycles per valve (~5), corroborate previous work that A. colbecki are ~ 5 years old at 50 mm shell height, and ~ 12 striae per group supports lunar periodicity of formation. However, striae per group varied widely (EC: 3–41; BOS 3–38) and 55% of valves had > 182 total striae and 30% had > 208, indicating ages of 7+ and 8+ yrs assuming fortnightly striae formation. Individual striae and group/cycle data contradict each other, calling into question consistent fortnightly striae formation in juvenile A. colbecki. 

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4. Nitrogen and oxygen isotopes in the shell of the Antarctic scallop Adamussium colbecki as a proxy for sea-ice cover in Antarctica.

   https://doi.org/10.1130/abs/2019AM-338279  

   Camarra, S. ; Puhalski, E. ; Gillikin, D. ; Cronin, K. ; Walker, S.E. ( October 2019 , Geological Society of America Abstracts with Programs)

   null (Ed.)

   Adamussium colbecki is a large thin-shelled scallop common in Antarctic waters and well represented in the fossil record. Shell oxygen (δ18Os) and nitrogen isotopes in carbonate bound organic matter (δ15NCBOM) have the potential to record sea ice state over time. To test this hypothesis we will analyze A. colbecki shells from Explorers Cove and Bay of Sails, western McMurdo Sound, Antarctica. These sites have different sea ice states: persistent (multiannual) sea ice at Explorers Cove and annual sea ice (that melts out every year) at Bay of Sails. Six adults shells collected at these sites in 2008 (3 from each site) and two juveniles collected in 2016 from Explorers Cove will be serially sampled for δ18Os values from the growing shell margin to the umbo. We hypothesize that melting glacial ice will pulse freshwater with low δ18O values into the system, which will be recorded in as larger amplitude pulses in shells from the Bay of Sails, but as dampened pulses in Explorers Cove. Carbonate bound organic material will be sampled for δ15NCBOM values. Recent studies illustrated that δ15NCBOM values provide a similar proxy as soft tissue δ15N values (Gillikin et al., 2017, GCA, 200, 55–66, doi: 10.1016/j.gca.2016.12.008). The organic content of the shell is low and the shells are thin, so δ15NCBOM values will be more time averaged than δ18Os values. Nevertheless, sea-ice organic N should have higher δ15N values compared to open water organics due to nitrate draw down in the ice (Fripiat et al., 2014, Global Biogeochem. Cycles, 28, 115–130, doi:10.1002/2013GB004729). Thus we expect large differences between Explorers Cove with persistent sea ice cover and Bay of Sails where the sea ice melts out every year. We posit that oxygen and nitrogen isotopes in A. colbeckishells have a high potential to record sea ice cover. 

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5. Population dynamics in the Antarctic benthos: Inter-annual fluctuation of foraminiferal, tunicate, and scallop abundances in Explorers Cove, western McMurdo Sound.

   Bowser, S.S. ; Walker, S.E. ; Cziko, P. ( April 2019 , PaleoBios)

   null (Ed.)

   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. 

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