Euphausiidae Family of  Euphotic Krill

North Pacific Kill, Euphausia pacifica. A representative example of a Krill from the Euphausiidae Family.

Phylogeny: Euphotic Krill of the Euphausiidae Family are members of the phylum Arthropoda, meaning that they have jointed legs. They are in the subphylum Crustacea, the superclass Multicrustacea, the class Malacostraca, the subclass Eumalacostraca, the superorder Eucarida, and the order Euphausiacea. The Euphausiidae Family is one of two families in this order- Benteuphausiidae and Euphausiidae. These families differ in that only Euphausiidae krill are bioluminescent. There are ten genera in this family, and around eighty-one species. The name Euphausiidae comes from the Greek words meaning “good lighting” or “well illuminated”, referring to their bioluminescence. The  name krill is Norwegian, meaning whale food.

Morphology: Euphotic Krill are small, shrimp-like animals with a cephalothorax (the carapace fused to the thoracic segments and covered with a carapace), an abdomen, and a tail fan. They have ten swimming appendages (swimmerets) on their abdomen. They have several pairs of thoracic legs (pereiopods or thoracopods) which are used for feeding and grooming. Unlike true decapods, these pereiopods are not used for locomotion. They have two antennae and stalked compounds eyes. Unlike true shrimp, krill have externally visible gills. Their outer shell is chitinous, and usually transparent. Their light-emitting photophores use luciferin in a chemical reaction to produce light. It is thought that they obtain the necessary chemicals from dinoflagellates in their diet.  Most Krill measure only around 2 cm (0.8 inches) in length, but some can reach 15 cm (5.9 inches).

Habitat and Distribution: Euphotic Krill are abundant in the open waters of temperate and polar seas, often comprising the greatest biomass in these areas. They can form dense (up to 10,000/ cubic meter) shoals that can stretch for miles. Most species in the family engage in vertical migration, spending the day 100-200 meters (328 – 656 feet) deep and ascending to surface waters during the night. This behavior helps them avoid shallow water predators and still feed in the nutrient rich surface waters. They can be found as deep as 2000 m (6560 feet). Euphotic Krill are found worldwide, though they are primarily cold water species. At least 15 species from this family are found in Mexican waters.

Reproduction: Euphotic Krill are gonochoric (male or female for life) and reproduce sexually through indirect fertilization. The male deposits a sperm packet (spermatophore) on the female’s abdomen. As the female releases her eggs they are fertilized. She may release up to 10,000 eggs at one time. The eggs hatch into planktonic larvae.

 Ecosystem roles: Euphotic Krill are usually omnivorous suspension feeders, though some are carnivorous predators. They feed on planktonic algae, diatoms, copepods, fish larvae, and other zooplankton. Krill are vitally important food for baleen whales, seals, fish, squid, and birds. They are an essential part of the food web in many regions. They are also essential to many fisheries.

 

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