Calanoida

Calanoida is an order of copepods, small crustaceans that form a major component of the zooplankton in marine and freshwater ecosystems. They are predominantly free-swimming, planktonic organisms found in oceans worldwide, from polar seas to tropical waters. Some freshwater calanoids occur in lakes and reservoirs, especially in the family Diaptomidae.

Morphology and life cycle: Calanoids typically have an elongated, cylindrical body with a distinguishable cephalothorax and a slender abdomen. The first pair of antennae (antennules) are often long, particularly in males. They possess five pairs of swimming legs used for locomotion and feeding, and their mouthparts are adapted for filter-feeding on phytoplankton and microzooplankton. Their life cycle includes six naupliar stages (N1–N6) followed by six copepodid stages (C1–C6) before reaching maturity.

Ecology and role: Calanoids are central to pelagic food webs. They feed on phytoplankton and microzooplankton and, in turn, serve as essential prey for fish larvae, larger zooplankton, and marine mammals. Many species perform diel vertical migration, rising toward the surface at night and descending during the day, contributing to carbon cycling in the water column. Notable marine genera include Calanus, Metridia, and Centropages; freshwater representatives include several diaptomid taxa.

Taxonomy and significance: Within copepods, Calanoida encompasses multiple families and numerous species. Calanus finmarchicus and Calanus glacialis are among the best-known in northern waters. Calanoids are widely studied for their ecological importance, biogeography, and responses to oceanographic change, making them key indicators of marine ecosystem health.