Bay barnacle

Balanus improvisus

Family Balanidae - barnacles (crustacea)

Shell conical, made of 6 triangular plates (parietes) fused together with walls (radii) between plates; walls (radii) narrow at top, disappear at base with plates (parietes) touching or nearly so; plates are whitish, smooth; no horizontal striations; base plate is calcareous, usually stays on hard substrate when barnacle is removed; base plate with radial patterns; aperture diamond-shaped; opercular plates at the aperture with radial grooves; internal plate tergum with long spur (rounded extension), less than 1/4 the width of the basal margin.
Similar Species
May be confused with B. subalbidus. The walls (radii) of B. subalbidus are usually more narrow, sometimes almost nonexistent, at least in the conical specimens. B. improvisus has a radially grooved base. The best way to distinguish these two is by looking at the terga. B. improvisus has a long spur, and B. subalbidus has a short, wide spur. Though sometimes found together, B. improvisus prefers less saline waters than B. subalbidus.
Bay, in lower salinities
Maximum Size
2 cm (3/4 in), usually less than 1 cm
Other Common Names
Previous Scientific Names
Amphibalanus improvisus