The long-lived clam Arctica islandica, a new model species for ageing research

Ridgway I, Richardson, C.A., Scourse, J.D., Wanamaker, A.D. Jr., & Butler, P.

School of Ocean Sciences, College of Natural Sciences, Bangor University

Introduction

The Ocean Quahog, Arctica islandica (Linnaeus 1767), is the oldest non-colonial animal known to science, attaining an age in excess of 400 years [1]. Funded by Research into Ageing^TM we seek to establish A. islandica as a new model ageing species.

Arctica islandica

Also known as the Iceland Cyprina, Ocean Quahog and Mahogany Clam, A. islandica is a long-lived suspension feeding bivalve mollusc. It lives burrowed into the top 5cm of sand and muddy substrates (figure 1) in suitable habitats around the shelf seas of North European and North American continents, ranging from the Bay of Biscay in the south to the sub-Arctic waters off Iceland, commonly at depths between 25 & 80 m. The species is dioecious (separate sexes), with larval development taking between 30 to 60 days depending on seawater temperature, and sexual maturity is reached at between 7 and 13 years of age. 

Figure 1.  View of Arctica islandica buried just below and also on the sediment surface.

The shells of A. islandica contain an ontogenetic record of shell growth in the form of wide annual summer growth increments separated by narrow growth lines.  Counting the number of lines gives an accurate estimate of the age of the clams and measurement of increment width an estimate of inter-annual growth rate (see figure 2). The discovery of what appeared to be an extremely old, long-lived A. islandica clam in the 1980s, far older than was thought previously possible, was the stimulus to investigate the periodicity of the increments. On the basis of seasonal changes in stable oxygen isotope profiles [2] and from field mark-recapture experiments [3] it was shown that the increments were deposited annually. Specimens with ages of between 100 and 200 years old are documented in the literature with dead collected northern North Sea and Icelandic specimens attaining ages of 268 and 374 years old respectively [7, 4]. However it was not until relatively recently that the documented maximum lifespan of A. islandica increased dramatically, from 268 years in 2003 [5, 7] to 374 years in 2005 [4], with the latest oldest reported specimen being at least 405 years old [1].  It is still to be demonstrated if these exceptionally old clams demonstrate evidence of physiological ageing, except for the accumulation of age pigment [6].

The anatomy, behaviour, physiology and more recently the ecology of A. islandica have been extensively studied because of their commercial importance along the American east coast; 150,000 tonnes are collected globally each year, principally by hydraulic clam dredges. This exceptionally long-lived clam has been termed the ‘tree of the sea' as the growth increment series measured from the shells could be used retrospectively to reconstruct marine environmental change. The School of Ocean Sciences, Bangor University is a world leader in the field and has been working toward constructing a 1000-year master chronology for the marine environment using the growth increment series in A. islandica shells [1, 7].

Figure 2. Idealized preparation of an A. islandica shell for sclerochronological studies (from Scourse et al., 2006). (A) External valve face, showing line of section through the left valve used to generate acetate peel replicas. (B) Acetate peel replica cross-section of valve showing annual growth band increments along shell margin (arrows) and within the hinge plate. The axis of growth through the hinge plate used to generate increment data is indicated by the dashed white line. (C) Numbered annual growth bands are measured from the earliest growth bands to the most recent along the axis of growth. Juvenile early bands are wide and reflect the ontogenetic growth curve of the individual. (D) Narrow senescent late bands from the outer part of the hinge plate axis.

What is known about ageing in A. islandica?

Despite interest in this clam's longevity and the measurement of growth increment series, little research into how this species has apparently managed to defy the onset of the ageing processes has been conducted. Earlier studies into the responses to stress with age were undertaken from a bio‑monitoring/eco‑toxicology angle, however in the past 5 years there has been an increasing number of papers documenting age associated trends in bivalves, normally focussing on the ‘free radical theory of ageing'. To date there has only been one publication on ageing in A. islandica which documented accumulation of lipofuscin with age, but showed no accumulation of protein carbonyl in the gill tissues [6]. Comparative biology of ageing in bivalves has demonstrated that better preservation of mitochondrial and antioxidant enzyme activities and the avoidance of waste accumulation may have enabled longer-lived individuals to live longer [8] than shorter lived species.  Shorter-lived bivalve species show a more pronounced decrease in mitochondrial function with ontogeny [9] and long-lived cold water dwelling species possess antioxidant activity that is higher than in shorter-lived species [10].

Why study Arctica islandica?

Significant advances in our understanding of the processes involved in ageing have been made using classical model organisms of biogerontological research (e.g. yeasts, fruit flies, nematodes and rodents). Despite the advantages of these organisms, where their basic biology and genome are well known, and their short life spans enable cheap and quick longitudinal studies and experimental manipulations to be conducted, they have been primarily chosen for convenience, rather than for specific features pertinent to human ageing [11]. Long-lived organisms may be more appropriate models to compare with human ageing as the most intensively used animal models used in biogerontological research lack the very trait needed to emulate longevity, i.e. the ability to live long and therefore they have poorly developed defences against the destructive processes of ageing [12].

This alternative approach investigates the nature of exceptionally effective defences against the destructive ageing processes that biological evolution has designed. If evolution has produced a model of successful resistance to the damage of ageing, it might be possible to learn from the investigation of that model [12]. Despite being identified as a potential area of study for ageing research over 70 years ago, the study of long-lived animals has advanced only recently. Comparative ageing research based on the longest-lived non-colonial animal known, Arctica islandica offers a unique opportunity to remedy this situation. The term "negligible senescence" has been coined by Caleb Finch at USC to describe very slow or negligible ageing [13]. He listed several animals with this characteristic, including vertebrates and invertebrates and specifically named A. islandica as a potential organism.

Of the organisms identified by Finch [13] as having slow or negligible ageing it is believed that A. islandica is the most suitable for establishment as a long-lived model organism for biogerontological research. All other organisms identified (e.g. sturgeons, bowhead whales, turtles, rockfish and possibly lobsters) are either aggressive, too large to maintain in captivity in aquaria or prohibitively expensive to study. These logistical problems associated with documenting exceptionally long-lived species have resulted in few ageing studies specifically focusing on long-lived animals.  A. islandica offers a unique opportunity to study a long-lived organism as it is possible to collect large numbers of individuals and to maintain them in laboratory aquaria where they are relatively cheap to grow and study. Clams are anatomically simple, but have a germ line/soma distinction, unlike hydra - another marine species documented with extreme longevity.

What do we aim to do?

Through a 12 month programme of research we aim to provide the basis for future ageing research in A. islandica. This is a collaborative project between marine biologists at Bangor University and biogerontologists from Brighton University, Cardiff University and University College London. A large thrust of the research will be to ascertain when the species demonstrates signs of physiological ageing; loss of organ structure/function, incidence of neoplasms, and decline in cell proliferation rates. Through field sampling and interrogation of a 10,000 A. islandica shell archive housed in the School of Ocean Sciences it is anticipated that accurate estimations of the species maximum lifespan potential (MLSP) can be obtained for discreet populations. Current estimations are based on single specimen finds rather than demographic analysis of the populations. Before research into A. islandica can proceed a few key questions need to be answered: 1) do they exhibit signs of physiological ageing? 2) Can we establish cell cultures from A. islandica tissues 3) Are there age associated changes in fecundity? and finally 4) what is the MLSP of the species?

Knowledge of the ageing process in this clam will be obtained from studies of age-related changes in the rate of cellular proliferation, the ability of the cells to culture and cellular resistance to stress and histopathology of organ structures. Establishing cell cultures from marine invertebrates has been problematic, but in the last 5 years there has been some success. In vitro applications are alternative and exceptionally important tools for animal experimentation, for biotechnological applications and pathological investigations [14]. The use of cell cultures also optimises the use of animals and allows ethical testing of resistance to stress.

Conclusion

In order for A. islandica to be established as a model for ageing research there is a need for more demographic data on wild clam populations and for some basic research to be conducted into its biology and cellular biology, and how these factors change with age. Holmes [15] concluded that the adoption of new and unusual animals for research exploring basic ageing processes need not be difficult as long as it involves careful collaborations between biogerontologists and zoologists well versed in evolutionary principles and judicious application of the comparative method. We believe this Research into Ageing^TM funded project, is a prime example of the kind of collaboration which will enhance our understanding of the ageing processes in invertebrates.

Figure 1.  View of Arctica islandica buried just below and also on the sediment surface.

Figure 2. Idealized preparation of an A. islandica shell for sclerochronological studies (from Scourse et al., 2006). (A) External valve face, showing line of section through the left valve used to generate acetate peel replicas. (B) Acetate peel replica cross-section of valve showing annual growth band increments along shell margin (arrows) and within the hinge plate. The axis of growth through the hinge plate used to generate increment data is indicated by the dashed white line. (C) Numbered annual growth bands are measured from the earliest growth bands to the most recent along the axis of growth. Juvenile early bands are wide and reflect the ontogenetic growth curve of the individual. (D) Narrow senescent late bands from the outer part of the hinge plate axis.