Systematics and zoogeography of the squid genus Illex (oegopsida; cephalopoda)

Lu, Chung-Cheng (1973) Systematics and zoogeography of the squid genus Illex (oegopsida; cephalopoda). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A study of the systematics and the distribution of the squid genus Illex Steenstrup, 1880 was made, utilizing specimens and data from the collections of various institutions throughout the world. The thesis begins with an extensive survey of the literature on the four species of this ommastrephid genus. Three species, Illex illecebrosus (Lesueur, 1821), I. coindetii (Verany, 1837) and I. argentinus (Castellanos, 1960) are here described and illustrated in detail, together with a modified description of I. oxygonius Roper, Lu and Mangold, 1969. Neotypes of I. illecebrosus and I. coindetii are established. A selection of morphometric characters are studied to determine the growth pattern of each species. The growth patterns are expressed as linear regressions between the mantle length and various measurements and indices standard in teuthoid systematics. The taxonomic and morphometric characters of all four species are compared. Strong sexual dimorphism in the head dimensions, arm lengths and sucker sizes exist in I. coindetii, I. oxygonius I argentinus. Sexual dimorphism involving these characters in I. illecebrosus is not apparent. The bathymetric range of I. illecebrosus is 0-500 m. During its inshore migration in Newfoundland waters it is in relatively shallow water of 15-30 m, in the Chesapeake Bay region it is concentrated in the upper 150 m. The bathymetric range of I. coindetii is 0-1080 m, but concentrated at 200-500 m in the Gulf of Mexico and at 400-600 m in the Caribbean Sea. In the eastern Atlantic, I. coindetii is found at 160-320 min the Blanes region of Spain, 200-250 m along the Algerian coast, 40-500 m in the PortVendres and Las Rosas region, and at 37-485 m in the Gulf of Guinea. The bathymetric range for I. argentinus is 0-800 m and that of I oxygonius is 50-555 m with greatest concentration at 50-300 m in the Chesapeake Bay region. All four species appear to exhibit diel vertical migration in that they are closely associated with the bottom during the daylight hours and disperse at night. The relationship between the occurrence of Illex and basic hydrographic conditions, e.g., temperature, salinity, and density are analyzed insofar as the available data permits. I. coindetii and I. oxygonius inhabit warmer, more saline water, while I. illecebrosus and I. argentinus are distinctly boreal and antiboreal species, inhabiting less saline cold water. Noting the amphi-Atlantic distribution of I. coindetii, yet the lack of collections from the mid- Atlantic region, it is postulated that the distribution is associated with the gyre of water circulation in the northern Atlantic. The most important limiting factor appears to be temperature. All four species are valid, although some characteristics commonly associated with cephalopod taxonomy are felt to be of little importance in delineating the species of this genera. It is further suggested that greater importance be given to some characters not commonly employed.

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