Christopher A. Richardson

VERY LONG-LIVED MOLLUSKS CONFIRM 17TH CENTURY AD TEPHRA-BASED RADIOCARBON RESERVOIR AGES FOR NORTH ICELANDIC SHELF WATERS

Marine sediment records from the north Icelandic shelf, which rely on tephrochronological age models, reveal an average Δ R (regional deviation from the modeled global surface ocean reservoir age) of approximately 150 yr for the last millennium. These tephra-based age models have not hitherto been independently verified. Here, we provide data that corroborate Δ R values derived from these sediment archives. We sampled the youngest portion (ontogenetic age) of a bivalve shell, Arctica islandica (L.), for radiocarbon analysis, which was collected alive in 2006 from the north Icelandic shelf in ~80 m water depth. Annual band counting from the sectioned shell revealed that this clam lived for more than 405 yr, making it the longest-lived mollusk and possibly the oldest non-colonial animal yet documented. The 14C age derived from the umbo region of the shell is 951 ± 27 yr BP. Assuming that the bivalve settled onto the seabed at AD 1600, the corresponding local value of Δ R is found to be 237 ± 35 yr by comparison of the 14C age with the Marine04 calibration curve (Hughen et al. 2004) at this time. Furthermore, we cross-matched a 287-yr-old, dead-collected, A. islandica shell from AD 1601 to 1656 from the same site with the live-caught individual. 14C analysis from the ventral margin of this shell revealed a Δ R of 186 ± 50 yr at AD 1650. These values compare favorably with each other and with the tephra-based Δ R values during this period, illustrating that 14C from A. islandica can effectively record 14C reservoir changes in the shelf seas.

surface changes in the north Atlantic meridional overturning circulation during the last millennium

Despite numerous investigations, the dynamical origins of the Medieval Climate Anomaly and the Little Ice Age remain uncertain. A major unresolved issue relating to internal climate dynamics is the mode and tempo of Atlantic meridional overturning circulation variability, and the significance of decadal-to-centennial scale changes in Atlantic meridional overturning circulation strength in regulating the climate of the last millennium. Here we use the time-constrained high-resolution local radiocarbon reservoir age offset derived from an absolutely dated annually resolved shell chronology spanning the past 1,350 years, to reconstruct changes in surface ocean circulation and climate. The water mass tracer data presented here from the North Icelandic shelf, combined with previously published data from the Arctic and subtropical Atlantic, show that surface Atlantic meridional overturning circulation dynamics likely amplified the relatively warm conditions during the Medieval Climate Anomaly and the relatively cool conditions during the Little Ice Age within the North Atlantic sector.

Isotopic partitioning between scallop shell calcite and seawater: Effect of shell growth rate

Abstract The relationship between molluscan shell growth rate and skeletal δ18O and δ13C was investigated in a detailed field study for the scallop, Pecten maximus. Seasonal variation in shell growth rate was found to be a governing factor influencing shell δ18O and δ13C. At low shell growth rates, shell δ18O were more positive (of the order +0.4‰) and δ13C more negative (up to −2‰) as compared with predicted values for precipitation of inorganic calcite in isotopic equilibrium with seawater. The deviations in δ18O were hypothesized as reflecting possible differences in solution carbonate chemistry at the site of mineralization in the extrapallial fluid as compared with that of the external seawater medium. The deviations in shell δ13C were consistent with incorporation of isotopically depleted respiratory 13C (i.e., a metabolic effect). A trend toward more depleted shell δ18O and δ13C values occurred at higher shell growth rates, with negative δ18O values as compared with predicted equilibrium at shell growth rates above 0.13 mm per day. These simultaneous negative deviations in skeletal δ18O and δ13C were interpreted as resulting from a kinetic effect. The implications for environmental reconstruction from molluscan isotopic records are discussed in light of a model of isotopic behavior based on the findings of the study.

Experimental investigation into partitioning of stable isotopes between scallop (Pecten maximus) shell calcite and sea water

Stable isotopic compositions of bivalve shells have often been used for the reconstruction of high-resolution records of palaeotemperature and palaeoproductivity cycles. A major assumption in such studies is that isotopic equilibrium between shell carbonate and sea water is maintained at the time of precipitation. This assumption was tested in the laboratory for scallops, Pecten maximus, cultured over the temperature range 10–17°C. At the low shell growth rates exhibited (<0.1 mm day−1), deviations of shell δ18O from equilibrium were +0.6‰ over the experimental temperature range, a temperature equivalency of approximately −3°C. This is hypothesised as reflecting possible differences in the solution carbonate chemistry at the site of mineralisation in the extrapallial fluid (EPF) as compared to that of the external sea water medium, from which the EPF is isolated. Measured depletions of shell δ13C (of the order of −2.0‰) are interpreted as resulting from introduction of 13C-depleted respiratory CO2 into the EPF and subsequent incorporation into the shell.

Stereotypic and size-selective predation in Polinices pulchellus (Gastropoda: Naticidae) Risso 1826

Abstract Polinices pulchellus were size-selective in their choice of Cerastoderma edule. Large predators (12–15.9 mm shell length) selected both larger and a wider size range of cockles than smaller individuals (4–11.9 mm shell length). Considerable overlap occurred in the sizes of cockles frequently drilled by different size classes of snails, indicating that certain sizes of cockles may be most profitable to a wide range of predator sizes. Consumption rates were highest during July and August and were closely related to seawater temperature. Inner and outer drill hole diameters were both correlated with predator size, and the morphology of the drill hole was geometrically similar across a range of predator sizes. Polinices pulchellus showed no preference for either the left or right valve and drilled most cockles in the centre of the shell valve. The relationship between the distance of the drill hole from the umbo and prey size was unaffected by predator size, such that predators of different sizes were not found to drill cockles in different positions. When disturbed during drilling, incomplete drill holes were abandoned and, when drilling resumed, it occurred in new locations on the surface of the shell valve. The findings of this study highlight the stereotyped nature of drilling behaviour seen in the family Naticidae.

Maximum Shell Size, Growth Rate, and Maturation Age Correlate With Longevity in Bivalve Molluscs

Bivalve molluscs are newly discovered models of successful aging, and this invertebrate group includes Arctica islandica, with the longest metazoan life span. Despite an increasing biogerontological focus on bivalves, their life history traits in relation to maximum age are not as comprehensively understood as those in vertebrate model aging organisms. We explore the allometric scaling of longevity and the relationship between development schedules (time to maturity and growth rate) and longevity in the Bivalvia. Using a traditional nonphylogenetic approach and the phylogenetically independent contrasts method, the relationship among these life history parameters is analyzed. It is demonstrated that in bivalves, maximum shell size, development, and growth rates all associate with longevity. Our findings support the observations of life history patterns in mammals and fish. This is the first investigation into the relationship among longevity, size, and development schedules throughout this group, and the results strengthened by the control for phylogenetic independence.

Can shell scars on dog cockles (Glycymeris glycymeris L.) be used as an indicator of fishing disturbance

Abstract The use of shell damage records as an in situ indicator of past fishing disturbance was investigated using the dog cockle Glycymeris glycymeris L. Shell sections of dog cockles collected from four areas subjected to varying levels of fishing disturbance were examined for the presence of damage records or shell ‘scars’. Animals from a heavily fished area had significantly higher levels of scarring than those from three lightly fished areas. From an estimation of the age of the shells (from internal growth lines and dating of each line), the year in which scarring occurred was determined and this was compared to yearly records of fishing effort. There was a weak but significant positive correlation between the frequency of shell scars per year and the intensity of fishing effort. Our data suggest that whilst scarring in shells of G. glycymeris cannot accurately be used to estimate past fishing intensity on a year-by-year basis, it can be used to differentiate between severely impacted and lightly fished areas of the sea bed.

Evolutionary traits in Perna viridis (Linnaeus) and Septifer virgatus (Wiegmann) (Bivalvia: Mytilidae)

Perna viridis (Linnaeus) and Septifer virgatus (Wiegmann) are widely distributed mytilids but only rarely do their local distribution patterns overlap. Thus, whilst P. viridis occurs predominantly in sheltered environments in water which is often highly turbid and sometimes even heavily polluted, S. virgatus is more typically confined to cleaner wave-exposed habitats. S. virgatus possesses a broader more robust shell and stronger byssal attachment than P. viridis, characteristics well suited for life on high energy rocky shores; the shell of S. virgatus is also much flatter ventrally, a feature which in epibyssate taxa such as mussels is generally considered to enhance physical stability. Although weaker mechanically, P. viridis produces copious amounts of mucus and has unusually large labial palps with strong ciliary rejection tracts, features more appropriate for coping with the high sediment loadings often associated with sheltered, low energy habitats. P. viridis also possesses a larger more mobile foot, and crawls more readily than S. virgatus. In particular it exhibits a marked propensity to climb vertically upwards, a response which would effectively elevate this species above any accumulated sediment; S. virgatus, by contrast, does not exhibit this behavioural response. The production of byssus threads and their distribution around secured mussels held under calm and turbulent flow in laboratory aquaria were highly variable though significant interspecific differences are reported. These mytilids appear, therefore, to have evolved contrasting suites of structural and behavioural traits which correlate well with a presumed advantage for each species within its preferred habitat. Reciprocal transplantation of these mussels between shores of different wave exposure would help to establish whether or not these evolutionary traits do indeed have adaptive value.

Population structure and growth of Donax trunculus (Bivalvia: Donacidae) in the western Mediterranean

A comparison of shell growth in Donax trunculus (collected between 1988 and 1990 of Cullera, Spain) has been carried out using an analysis of cohort progression in monthly length frequency distributions, hyaline surface shell growth rings and internal microgrowth bands. In the Mediterranean there are two periods of recruitment of D. trunculus, one in the summer (July to September) and the other in winter (December to February). Clams recruited to the population in winter display a clear cessation in shell growth during the following summer which may possibly be correlated with spawning, whereas individuals of the summer recruited cohort show no growth cessation the following summer and continue to deposit shell during this period. The normally opaque shell of D.trunculus reveals the presence of translucent hyaline growth rings when the shells are backlit by a strong light source, and these have been shown to be laid down in the shell during summer months. Formation of a hyaline ring is accompanied by a narrowing of the microgrowth patterns present in shell sections. Both the hyaline rings and the length frequency distributions have been used to determine the age and growth rate of D. trunculus.

Spatial and temporal evolution of imposex in dogwhelk Nucella lapillus (L.) populations from North Wales, UK

Nucella lapillus imposex and organotin tissue contamination were assessed, during 2006, at twenty sites in North Wales, between Anglesey and Shell Island on the Lleyn Peninsula. Vas Deferens Sequence Index (VDSI), Relative Penis Size Index (RPSI) and the percentage of affected females (%I) were used to assess imposex levels which varied between 0.5 and 3.8 for VDSI, 0.0 and 11.5% for RPSI and 49 and 100% for %I. Tributyltin (TBT) and triphenyltin (TPT) concentrations in whole tissues varied between 0.8 and 39 and 0.4 and 2.1 ng Sn/g dry weight, respectively. TBT represented the higher fraction of butyltin compounds in the tissues, suggesting that TBT inputs continue to occur. Comparisons with nineteen years of data collected during previous studies demonstrated that there had been a significant reduction in imposex levels over the last two decades following the introduction of legislative restrictions in the U.K. regarding the use of organotin based antifouling paints.