Arctica islandica

Overview

Arctica islandica is a bivalve whose shell growth records enable studies of the ocean. Its form can be explored by following a nearly circular outline and concentric external ridges.^[1]

Key facts

External concentric ridges and internal shell growth lines are different structures. Observation deepens from viewing outward patterns to reading an internal record of time.^[1]

Time and place

It is a bivalve still living today. A 1997 study described its distribution across continental shelves on both sides of the North Atlantic, in Europe and North America, and compared material from the North Sea and neighboring waters.^[1]

The time range describes source context, not the complete duration of this species.

Size, form and skeleton

The umbo, the shell’s apex, lies toward the front. The external periostracum is yellowish brown in young individuals and black in older ones. The rounded outline contains differences in orientation and growth.^[1]

Shell height can reach about 10 cm. This is the shell’s height, distinct from its front-to-back length.^[2]

Habitat and reproduction

It generally lives buried in bottom sediment, with short siphons at the sediment–water boundary. It prefers fine sediment but is also recorded in coarse sand and gravel.^[1]

Fossil discovery

Rob Witbaard’s 1991 survey even included a cake wager. After a promise to provide cake if more than ten were caught, the final net haul brought up many clams, requiring the promise to be kept.^[1]

References

1. Witbaard (1997) — Reading the sea through shell growth lines
2. Tree of the sea: The use of the internal growth lines in the shell of Arctica islandica for the retrospective assessment of marine environmental change
3. Maxwell1969 — Middle Tertiary Mollusca from North Otago and South Canterbury
4. Treatise — Shell features of Venericardia

Research questions

Does growth rate differ among seas even within the same species? Researchers compared growth records from the North Sea and neighboring seas. Small shells provide evidence for reading changes in marine environments.^[1]

Observation

The illustration shows the exterior of a single left valve. Move your gaze from the anteriorly placed umbo along the concentric ridges toward the margin.^[1]