Harmful Algal Blooms (HABs) are a hazard for coastal environments worldwide; identifying screening thresholds of chlorophyll-a (chl-a) associated with increased risk of HABs is a management priority. Molecular surveillance of coastal phytoplankton and bivalve biotoxins could be used to link chl-a with HAB risk, but requires an understanding of whether the HAB risks increase uniformly as chl-a rises, or whether some taxa are disproportionately favored, and if these relationships vary by season. In this study, we present a novel use of molecular abundance data to investigate the scientific bases for estuarine chl-a thresholds protective against HABs. In San Francisco Bay (SFB), California, the relationship between molecular relative abundance (as measured by 18S metabarcoding) of nine different HAB taxa, absolute quantitative polymerase chain reaction (qPCR) abundance, and mussel toxin concentrations of a subset of the taxa were investigated for thresholds as a function of increasing chl-a. Our results show most HAB taxa did not increase in absolute or relative abundance during SFB's spring bloom interval, when chl-a levels were highest (>10 µg/L) but the assemblage was dominated by non-harmful diatoms. However, several flagellated, mixotrophic taxa did increase above their molecular baseline in fall, and the combined probability of any HAB occurring above baseline was elevated when chl-a reached ∼4.6 µg/L in the fall. This work demonstrates the promise of molecular approaches in disentangling the seasonally complex interplay between stressors and phytoplankton/HAB community responses and has the potential to provide clearer, more cost-effective monitoring and mitigation strategies for managers.

San Francisco Bay, California, typically has chlorophyll a (chl-a) concentrations below 10 µg L−1, despite nutrient loadings exceeding those in many estuaries with recurring harmful algal blooms (HABs). However, in August 2022 there was a Heterosigma akashiwo (raphidophyte) bloom with chl-a concentrations exceeding 450 µg L−1, resulting in widespread hypoxia and fish die-off. We used protist community (18S ribosomal ribonucleic acid gene metabarcoding) and environmental data collected 2–3 times per month from 2015–2023 to assess differences between 2022 and other years, and capture dynamics pre-, during, and post-bloom. The 2022 protist community clustered separately from other years in non-metric multidimensional scaling and hierarchical cluster analyses. A diatom bloom in June generated the highest median June chl-a concentration (15 µg L−1) recorded between 1977–2023 and drew down nutrients (nitrogen, silicate), leading to the establishment of a heterotrophic community in July with low dinoflagellate abundance (0.9

The chemical composition of mollusk shells is a useful tool in (paleo)climatology since it captures inter- and intra-annual variability in environmental conditions. Trace element and stable isotope analysis with improved sampling resolution now allows in situ determination of the composition of mollusk shell volumes precipitated at daily to sub-daily time intervals. Here, we discuss hourly resolved Mg / Ca, Mn / Ca, Sr / Ca, and Ba / Ca profiles measured by laser ablation inductively coupled plasma – mass spectrometry (ICP-MS) through shells of the photosymbiotic giant clams (Tridacna maxima, T. squamosa, and T. squamosina) and the non-photosymbiotic scallop Pecten maximus. Precise sclerochronological age models and spectral analysis allowed us to extract daily and tidal rhythms in the trace element composition of these shells. We find weak but statistically significant expressions of these periods and conclude that this cyclicity explains less than 10 % of the sub-annual variance in trace element profiles. Tidal and diurnal rhythms explain variability of, at most, 0.2 mmol mol−1 (∼ 10 % of mean value) in Mg / Ca and Sr / Ca, while ultradian Mn / Ca and Ba / Ca cyclicity has a median amplitude of less than 2 µmol mol−1 mol mol−1 (∼ 40 % and 80 % of the mean of Mn / Ca and Ba / Ca, respectively). Daily periodicity in Sr / Ca and Ba / Ca is stronger in Tridacna than in Pecten, with Pecten showing stronger tidal periodicity. One T. squamosa specimen which grew under a sunshade exhibits among the strongest diurnal cyclicity. Daily cycles in the trace element composition of giant clams are therefore unlikely to be driven by variations in direct insolation but rather reflect an inherent biological rhythmic process affecting element incorporation. Finally, the large amount of short-term trace element variability unexplained by tidal and daily rhythms highlights the dominance of aperiodic processes in mollusk physiology and/or environmental conditions over shell composition at the sub-daily scale. Future studies should aim to investigate whether this remaining variability in shell chemistry reliably records weather patterns or circulation changes in the animals' environment.

Valvometry, the electronic measurement of bivalve shell opening and closing, has been demonstrated to be a valuable biomonitoring technique in previous ecological and environmental studies. Valvometric data has been shown to relate significantly to pollution, predation, animal stress and feeding activity. However, there is a need for valvometric techniques applicable to coral reef environments, which may provide critical insights into reef resilience to ocean warming and acidification. Giant clams are endemic to coral reefs and hold great promise as valvometric recorders of light availability, productivity and other environmental variables. Despite this promise, prior valvometric work on giant clams has been limited by specialized hardware less accessible to developing countries where many coral reefs are found. Here we report on an open-source approach that uses off-the-shelf components to monitor smooth giant clam (Tridacna derasa) valve opening behavior, and tests this approach in the simulated reef environment of the Biosphere 2 Ocean. Valvometric data corroborates the influence of light availability on diurnal behavior of giant clams. The clams basked during daylight hours to expose their photosymbionts to light, and adopted a partially-closed defensive posture at night. The animals showed variations in the frequency of complete closures, with most occurring during night-time hours when the animals prioritize filter-feeding activity, clapping their valves to expel pseudofeces from their gills. Closure frequency showed a significant relation to pH and a significant lagged relationship to chlorophyll-a productivity, which are both a function of algal productivity in the Biosphere 2 Ocean tank. These results suggest that the animals fed on phytoplankton following periodic bloom events in the Biosphere 2 Ocean during the experiment. We propose that giant clams exhibit behavioral plasticity between individuals and populations, and advocate for the more widespread use of valvometry to enable comparative studies of reef environment and animal health.

Giant clams are reef-dwelling bivalves that reach unusual sizes through a partnership with photosymbiotic algae. To date, no shell-based biogeochemical proxy has been found which directly records the photosymbiotic development and health of these animals. We present new results showing a size-related decline in nitrogen isotopic values of shell-bound organic matter from the hinge layers of giant clams from the Northern Red Sea. In three of four tested shells, δ15N values decline from >+4‰ at the juvenile stage to between 0 and −2.5‰ at maturity. These trends are consistent with a transition from heterotrophic nutrition early in the bivalve's life to receiving most of their nutrition from photosynthetic symbionts and external dissolved inorganic nitrogen at maturity. We find more muted declines or no change within the outer shell layer, with more inter-individual variability, which is likely related to the greater influence of the symbionts in the adjacent siphonal mantle of the animals. We use a von Bertalanffy-linked trophic model that uses δ15N of nitrate and particulate organic matter to corroborate and explain the trophic transition in the ontogeny of the clams, and propose that high-resolution δ15N measurements in bivalves could be used as a proxy for photosymbiosis and reef paleoenvironmental conditions in the fossil record.

The health of reef-building corals has declined due to climate change and pollution. However, less is known about whether giant clams, reef-dwelling bivalves with a photosymbiotic partnership similar to that found in reef-building corals, are also threatened by environmental degradation. To compare giant clam health against a prehistoric baseline, we collected fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red Sea. After calibrating daily/twice-daily growth lines from the outer shell layer, we determined that modern individuals of all three species (Tridacna maxima, T. squamosa and T. squamosina) grew faster than Holocene and Pleistocene specimens. Modern specimens also show median shell organic δ15N values 4.2‰ lower than fossil specimens, which we propose is most likely due to increased deposition of isotopically light nitrate aerosols in the modern era. Nitrate fertilization accelerates growth in cultured Tridacna, so nitrate aerosol deposition may contribute to faster growth in modern wild populations. Furthermore, colder winter temperatures and past summer monsoons may have depressed fossil giant clam growth. Giant clams can serve as sentinels of reef environmental change, both to determine their individual health and the health of the reefs they inhabit.

Bivalve shells have a long-standing reputation as archives for high-resolution (seasonal scale) (paleo)climate variability due to their incremental growth, yielding accurate shell chronologies, and their abundance, diversity, and high preservation potential in the fossil record (Schöne and Surge, 2012). Capitalizing on innovations in geochemical techniques, high-resolution sclerochronology can now resolve changes in bivalve shell chemistry beyond the daily resolution (e.g. Sano et al., 2012; Warter et al., 2018). When applied on fossil shells, these ultra-high-resolution records have the potential to bridge the gap between climate and weather reconstructions and yield unprecedented information about bivalve paleobiology, extreme weather events in past climates and even astronomical cycles (Warter and Müller, 2017; de Winter et al., 2020; Yan et al., 2020). However, studies of sub-daily scale shell chemistry are almost exclusively limited to giant clams (Tridacna spp.), due to their high growth rates. It is hitherto unknown if and how such diurnal cycles in chemistry differ in other genera across the bivalve clade and/or whether they are exclusive to photosymbiotic clams. In addition, it is not clear whether the daily cycles are formed in response to environmental conditions (e.g. light or temperature sensitivity) or reflect circadian rhythms. To answer these questions, we combine ultra-high-resolution (hourly scale) Laser Ablation ICP-MS trace element profiles through shells of various tridacnid species from the tropical Gulf of Aqaba with profiles through the giant scallop (Pecten maximus) from the temperate Atlantic coast of northwestern France. We observe trace element cycles on in the daily frequency domain in both tridacnids and pectinids. This shows that these diurnal cycles are formed regardless of shell mineralogy (aragonite vs. calcite), living environment (tropical inter-tidal vs. temperate sub-tidal) and occur in highly unrelated bivalve taxa. Our data helps the interpretation of similar records from fossil shells in terms of past (extreme) weather events, climate, and shell growth. References de Winter, N. J. et al. Paleoceanography and Paleoclimatology 35, e2019PA003723 (2020). Sano, Y. et al. Nature Communications 3, 761 (2012). Schöne, B. R. & Surge, D. M. Treatise Online 24, Volume 1, Chapter 14 (2012). Warter, V., Erez, J. & Müller, W. Palaeogeography, Palaeoclimatology, Palaeoecology 496, 32–47 (2018). Warter, V. & Müller, W. Palaeogeography, Palaeoclimatology, Palaeoecology 465, 362–375 (2017). Yan, H. et al. PNAS 117, 7038–7043 (2020).

Abstract The Gulf of Aqaba is home to three giant clam species with differing ecological niches and levels of photosymbiotic activity. Giant clams grow a two‐layered shell where the outer layer is precipitated in close association with photosymbiont‐bearing siphonal mantle, and the inner layer is grown in association with the light‐starved inner mantle. We collected 39 shells of the three species (the cosmopolitan Tridacna maxima and T. squamosa, as well as the rare endemic T. squamosina) and measured carbon and oxygen isotope ratios from inner and outer shell layers, to test for differences among species and between the layers of their shells. T. squamosina records higher temperatures of shell formation as determined by oxygen isotope paleothermometry, consistent with its status as an obligately shallow‐dwelling species. However, the known negative fractionation imparted on tissue carbon isotopes by photosymbiotic algae did not produce measurable offsets in the carbonate δ13C values of the more symbiotic T. squamosina and T. maxima compared to the more heterotrophic T. squamosa. Across all species, outer shell layers recorded mean growth temperatures 1.8°C higher than corresponding inner layers, which we propose is a function of the high insolation, low albedo microenvironment of the outer mantle, and potentially the activity of the symbionts themselves. Population‐wide isotopic sampling of reef‐dwelling bivalve shells can help constrain the ecological niches of rare taxa and help reconstruct their internal physiology.

Horizon scanning is an increasingly common strategy to identify key research needs and frame future agendas in science. Here, we present the results of the first such exercise for the field of sclerochronology, thereby providing an overview of persistent and emergent research questions that should be addressed by future studies. Through online correspondence following the 5th International Sclerochronology Conference in 2019, participants submitted and rated questions that addressed either knowledge gaps or promising applications of sclerochronology. An initial list of 130 questions was compiled based on contributions of conference attendees and reviewed by expert panels formed during the conference. Herein, we present and discuss the 50 questions rated to be of the highest priority, determined through an online survey distributed to sclerochronology community members post the conference. The final list (1) includes important questions related to mechanisms of biological control over biomineralization, (2) highlights state of the art applications of sclerochronological methods and data for solving long-standing questions in other fields such as climate science and ecology, and (3) emphasizes the need for common standards for data management and analysis. Although research priorities are continually reassessed, our list provides a roadmap that can be used to motivate research efforts and advance sclerochronology toward new, and more powerful, applications.

(1) University of Haifa, Leon Charney School of Marine Sciences, Marine Geosciences, Israel (daniel.e.killam@gmail.com), (2) Israel Oceanographic and Limnological Research, Haifa, Israel (timor.katz@mail.huji.ac.il), (3) Institute of Marine Sciences, University of California Santa Cruz, Santa Cruz, California, USA (apaytan@ucsc.edu), (4) University of Haifa, Leon Charney School of Marine Sciences, Marine Geosciences, Israel (apaytan@ucsc.edu)

Sclerochronology uses shell growth lines or bands for the construction of environmental time-series and the measurement of organism growth, but more study is needed to constrain the triggers of the dark cessation bands observed in many bivalve groups. We constructed a database of direct observations of modern growth seasonality across the class Bivalvia and compared the occurrence of seasonal growth bands to environmental data including latitude, temperature, and chlorophyll-a concentration. Bivalves with cold-season (winter) cessations are more common towards the poles, with logistic regression showing that temperature, followed by latitude of occurrence, displays the strongest relationship with occurrence of winter cessation. Remotely sensed and directly measured chlorophyll-a concentration show no significant relationship. Summer cessations are sparse and only weakly associated with environmental controls but are concentrated at the subtropical latitudes among temperate bivalves at their equatorial extremes. The rarity of summer cessations can be explained by the limited annual ranges of temperature in the tropics, combined with the exponential relationship of metabolic rate to temperature leading to a narrow window between normal functioning and mortality at high temperatures. This data suggests that, unless annual temperatures have low variability like in equatorial or polar regions, the season of growth cessation across bivalves is primarily a function of temperature tolerance through restriction of scope for growth. At most latitudes, growth bands can be interpreted as being primarily triggered by temperature stress, rather than seasonal starvation.

The Natural History of Red Sea Giant Clams Dan Killam PhD Candidate Earth and Planetary Sciences