Polyteny and Size Variation in the Copepod Pseudocalanus from Two Semi-Landlocked Fiords on Baffin Island

Abstract

Two forms of the copepod Pseudocalanus, which differ only in size, coexist in two semi-landlocked fiords on Baffin Island. The large form possesses the same number of chromosomes as the small but its chromosomes are conspicuously larger and contain about seven times as much DNA. Due to polyteny, the large form has a greater cell size and a slower development rate than the small form. The large form, having an essentially annual life cycle, matures later than the small form. Two generations of the small form may be produced in a summer with only the first reaching maturity. As the fiord waters are warmer, the small form is smaller and develops faster than its counterpart in nearby seas. The large form may represent an evolutionary attempt to restore normal arctic size and development rate in the fiords. In the cold waters of the sea, the slow development rate of the large form might lead to an excessively long life cycle, perhaps biennial. This may explain the apparent absence of the large form outside the two fiords. The polytene form may be a new species but, as the taxonomy of the genus is indefinite, a new name is deferred.