Overall Appearance and Size
Krill are small, shrimp-like crustaceans that form a significant part of the ocean’s food web. These organisms are highly abundant in marine environments, serving as a primary food source for a diverse array of marine animals, ranging from fish to large whales. Understanding their physical attributes helps to appreciate their ecological importance and widespread presence across the world’s oceans.
Krill generally possess a translucent or semi-transparent body, which allows some light to pass through them. Their overall shape is elongated and streamlined, resembling a small shrimp or prawn. While their appearance can vary slightly among the 85 known species, this basic form is consistent.
Most krill species are relatively small, commonly measuring between 1 to 2 centimeters in length. However, some larger species, such as the Antarctic krill (Euphausia superba), can grow up to 6 centimeters long. Despite their individual small size, krill often aggregate in vast swarms that can span several kilometers.
These massive swarms can sometimes appear as a reddish or greenish patch in the water, visible from a distance despite their individual translucency. The collective coloration is influenced by the phytoplankton they consume.
Key Body Structures
The body of a krill is distinctly divided into two main sections: the cephalothorax and the abdomen. The cephalothorax is formed by the fusion of the head and the thorax, and it is covered by a protective exoskeleton called a carapace. This rigid covering shields the krill’s vital organs and the bases of its thoracic appendages.
Attached to the cephalothorax are several important appendages, including a pair of long, sensory antennae. These antennae extend forward from the head and are used by krill to detect changes in their environment, aiding in navigation and sensing.
Beneath the cephalothorax are the thoracic legs, also known as pereiopods. Krill possess five pairs of these legs, specialized for feeding. These appendages have fine bristles that help them filter and capture microscopic plankton from the water, their primary food source.
The second major body section is the abdomen, which is segmented and extends backward from the cephalothorax. This part of the body is often referred to as the “tail section.” Along the underside of the abdomen are five pairs of pleopods, also known as swimmerets. These paddle-like appendages beat rhythmically to propel the krill through the water, enabling their characteristic swimming motion.
At the very end of the abdomen is the tail fan, composed of a central telson and two pairs of uropods. This fan-like structure can be rapidly fanned to create a powerful thrust, allowing the krill to perform quick escape maneuvers and evade predators.
Unique Characteristics
One of the most striking features of krill is their ability to produce light, a phenomenon known as bioluminescence. This light production occurs through specialized organs called photophores, located on various parts of their body, including the eyes, the coxae (base segments) of the pereiopods, and the pleopods along the abdomen.
The light emitted by krill is a soft, blue-green glow. The functions of this bioluminescence are varied, potentially serving as camouflage against predators, aiding in schooling behavior, or playing a role in mating displays. The specific purpose can vary by species and environmental context.
Another distinctive anatomical feature of krill is their visible, feathery gills. Unlike many crustaceans that have gills hidden within a gill chamber, krill’s gills are externally exposed and located under the carapace on the thoracic segments. This external placement helps distinguish krill from other similar-looking marine organisms.
While krill are described as translucent, their overall coloration can exhibit subtle variations. This can be influenced by the species and their diet. For example, krill that have recently consumed large amounts of reddish phytoplankton may appear to have a reddish or pinkish hue.
Similarly, if their diet consists more of greenish phytoplankton, they might take on a slightly greenish tint. This dietary influence on their body color reflects their recent feeding activities.