Illex

Not to be confused with Ilex, the plant genus for hollies.

Illex
Illex coindetii
Illex illecebrosus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Order:	Oegopsida
Family:	Ommastrephidae
Subfamily:	Illicinae Posselt, 1890
Genus:	Illex Steenstrup, 1880
Type species
Loligo illecebrosa Lesueur, 1821
Species
See text

Illex, commonly known as shortfin squids, is a genus of squid in the family Ommastrephidae and the only member of the subfamily Illicinae. It contains four species:

• Illex argentinus, Argentine shortfin squid
• Illex coindetii, southern shortfin squid
• Illex illecebrosus, northern shortfin squid
• Illex oxygonius, sharptail shortfin squid

Molecular systematics

A 2006 study evaluated the taxonomy of the squid genus Illex using molecular data from the cytochrome c oxidase subunit I (COI) gene. Traditionally, the genus has included four recognized species: Illex illecebrosus, Illex coindetii, Illex oxygonius, and Illex argentinus. I. argentinus is the most morphologically distinct and occurs in the South Atlantic Ocean, while the three North Atlantic species overlap geographically and display significant morphological variability, making them difficult to distinguish based on physical characteristics alone.

The molecular analysis revealed four distinct genetic clades within Illex, supporting the existence of four separate species. However, the study also found mismatches between some specimens' traditional morphological identifications and their genetic groupings, particularly among the North Atlantic species. This suggests that current morphological traits are not always reliable for identifying Illex species and that a combination of genetic and morphological data is needed to accurately resolve their taxonomy.

Diet and feeding

Illex squids are active carnivorous predators. They use their specialized arms and tentacles, which are equipped with suckers, to capture prey. These squids employ a combination of ambush and active hunting strategies, often utilizing stealth and camouflage to blend into their surroundings. Their large eyes enable them to detect prey, primarily small fish such as herring and sand lance, as well as crustaceans like krill and shrimp.

Illex squids are opportunistic feeders, adjusting their diet based on the availability of prey in their environment. They primarily feed at night, often coming to shallower waters for hunting. Once prey is captured, the squid uses its sharp beak and radula to break down the food before digestion. Overall, Illex squids are nocturnal hunters that rely on speed, stealth, and efficient feeding mechanisms to secure prey in a variety of marine habitats.

Synapomorphies

The Illex squids, like other members of the family Ommastrephidae, share several key synapomorphies that define their group. These include a streamlined, fusiform body shape that aids in fast swimming, a feature common to many squids for minimizing drag in the water. They possess lateral fins running along the sides of their bodies, which help with stabilization and maneuverability. Illex squids also have two long tentacles used for capturing prey, distinct from their eight shorter arms, and capable of rapid extension and retraction. Their skin contains chromatophores (pigment cells) and iridophores (light-reflecting cells), allowing them to change color and blend into their environment, a crucial adaptation for both hunting and avoiding predators. Additionally, Illex squids have large eyes, an adaptation that enhances their ability to detect prey in low-light conditions during their nocturnal hunting. Finally, their sharp beaks, adapted for breaking down fish and crustaceans, further characterize them as efficient carnivorous predators. These shared traits distinguish Illex squids from other squid families and highlight their evolutionary adaptations.