Charles W. Thayer

SrCa and 13C12C ratios in skeletal calcite of Mytilus trossulus: Covariation with metabolic rate, salinity, and carbon isotopic composition of seawater

Minor element and isotopic compositions of marine bivalve calcite are frequently used as proxy records of seawater temperature and salinity. Although molluscan calcite is secreted at or near oxygen isotope equilibrium, the influence of metabolic activity (i.e., vital effects) on skeletal SrCa ratios and δ 13C values is not well known. We present measurements of skeletal chemistry from (a) consecutive samples milled in chronological order from the organisms final year of growth and (b) adjacent samples within three separate growth bands of the marine mussel Mytilus trossulus to investigate chemical disequilibrium effects among different parts of the shell. Seawater temperature and salinity were monitored for one year at Squirrel Cove, British Columbia. At the end of the year, a young, rapidly growing mussel (mussel A) and an old, slowly growing mussel (mussel B) were harvested from this site. Growth bands within the shells were sampled to provide a chronological record of shell carbonate chemistry. Results from consecutive samples show (1) a significantly higher average SrCa ratio in mussel A than that in mussel B and (2) δ 13C values that correlate well with salinity in mussel A but not in mussel B. Results from time-equivalent, adjacent samples show (1) higher SrCa and δ 13C values near the ventral margin than at time-equivalent regions on lateral margins of the shell and (2) little or no variation in δ 18O values. These observations suggest that skeletal chemistry (SrCa and δ 13C) is primarily controlled by rate of mantle metabolic activity and secondarily modified by variation in seawater salinity. Because metabolic activity in the mantle at the site of carbonate precipitation varies with shell curvature, the composition of calcite secreted along lateral margins is influenced to a greater extent by metabolic activity than calcite secreted coevally at the central margin of the shell. Hence, the chemistry of calcite secreted at the ventral margin is precipitated in near-equilibrium with seawater and allows accurate estimation of seawater SrCa ratios and carbon isotopic composition. In contrast, shell precipitation along lateral margins is dominantly controlled by metabolic activity, and calcite chemistry is not in equilibrium with ambient seawater. In this context, we present a biomineralization model where variations in skeletal SrCa and δ 13C values are explained in the context of mantle metabolic activity and seawater salinity.

Biological Bulldozers and the Evolution of Marine Benthic Communities

During the Phanerozoic, the diversity of immobile suspension feeders living on the surface of soft substrata (ISOSS) declined significantly. Immobile taxa on hard surfaces and mobile taxa diversified. Extinction rates of ISOSS were significantly greater than in other benthos. These changes in the structure of benthic communities are attributed to increased biological disturbance of the sediment (bioturbation) by diversifying deposit feeders.

Bivalve skeletons record sea-surface temperature and δ18O via Mg/Ca and 18O/16O ratios

Paleotemperatures have been widely deduced from skeletal 18 O/ 16 O ratios, but these are also dependent on salinity. Without an independent measure of salinity, 18 O/ 16 O ratios cannot provide accurate data on past temperature and climate. We grew marine mollusks (bivalves) in the field while real-time data on local seawater temperature and chemistry (Mg, Ca, δ 18 O, and salinity) were gathered. Here we show that for Mytilus trossulus, skeletal Mg/Ca ratios provide an accurate measure of temperature and that weekly sea-surface temperatures may be estimated with an apparent accuracy of approximately ± 1.5 °C. Thus, with analyses of both Mg/Ca and δ 18 O from the same specimen, it is possible to determine seawater temperature and δ 18 O.

Brachiopods versus Mussels: Competition, Predation, and Palatability.

Unlike other shell-enclosed marine invertebrates, articulate brachiopods are repellent to predators. Fish, sea stars, snails, and crabs all prefer bivalve molluscs such as mussels to articulates. The mussels tested are mobile and out-compete immobile articulates when space is limited. In subtidal field experiments, mussels alone and predators alone each reduced the survivorship of articulates. However, adding mussels to articulates in the presence of ambient predation increased brachiopod survivorship by diverting predation from the brachiopods to the mussels. Competition from mussels (or mussel-like bivalves) is a plausible cause of the post-Paleozoic decline of articulates.

Mass extinctions: Ecological selectivity and primary production

If mass extinctions were caused by reduced primary productivity, then extinctions should be concentrated among animals with starvation-susceptible feeding modes, active lifestyles, and high-energy budgets. The stratigraphic ranges (by stage) of 424 genera of bivalves and 309 genera of articulate brachiopods suggest that there was an unusual reduction of primary productivity at the Cretaceous/Tertiary (K/T) boundary extinction. For bivalves at the K/T, there were (1) selective extinction of suspension feeders and other susceptible trophic categories relative to deposit feeders and other resistant categories, and (2) among suspension feed-ers, selective extinction of bivalves with active locomotion. During the Permian-Triassic (P/Tr) extinction and Jurassic background time, extinction rates among suspension feeders were greater for articulate brachiopods than for bivalves. But during the K/T event, extinction rates of articulates and suspension-feeding bivalves equalized, possibly because the low-energy budgets of articulates gave them an advantage when food was scarce.

Energy content and chemical defense of the articulate brachiopod Liothyrella uva (Jackson, 1912) from the Antarctic Peninsula

Energy levels (x ± 1 SD) of whole soft body tissues and shells of the antarctic brachiopod Liothyrella uva (Jackson, 1912) were 9.0 ± 1.0 and 1.1 ± 0.4·kJ·g−1 dry wt, respectively. The majority of the energy in the soft body tissues was attributable to NaOH-soluble protein (17.4% dry wt) and in the shell to lipid (2.6% dry wt). An intact individual with a shell length of 3.4 cm and a wet and dry wt of 5.8 and 2.9 g, respectively, contained a total of 9.6 kJ. As dense populations of L. uva occur in the Southern Ocean, this species represents a considerable potential energy resource for predators, including sea stars and fish (ca 103kJ·m−2 in some areas). Total crude extracts of whole brachiopod soft tissues caused significant retraction of sensory tube-feet in 6 species of sympatric sea stars. Control assays employing whole crude extracts of the fresh soft tissues of the antarctic limpet Nacella concinna (Strebel, 1908) or seawater alone did not elicit significant sustained tube-foot retraction. Bioactivity was diminished in frozen extracts of brachiopod soft tissues exposed to the sensory tube-feet of the sea star Odontaster validus (Koehler, 1906), but was not diminished in the tube-foot retraction response of the sea star Neosmilaster georgianus (Studer, 1885). Lyophilized brachiopod soft tissues ground into a fine powder and embedded at a concentration of 2% (wt:vol) in agar pellets containing 5% krill caused significant feeding deterrence in an allopatric fish (the sheepshead minnow Cyprinidon variegatus Lacepede 1803). These findings suggest that, similar to temperate zone and tropical brachiopods, body tissues of this antarctic brachiopod are unpalatable to potential predators.

Quick-release cages and repetitive censusing of sessile epifauna

Abstract Quick-release cages of Vexar (plastic) are attached by Velcro to frames (anchored with Rawl-studs and sealed with epoxy) and placed over in situ populations on rock of any orientation. These cages are strong, economical, and easily removed for cleaning and repeated censusing of the same quadrats with minimal disturbance to the biota. Rawl-studs and quick-release cages permit in situ studies in water depths approaching the limits of SCUBA. The projecting studs provide repeatable registration of the grid used for close-up photocensusing. Similar cages are used for transplanted epibenthos.