Main results of long-term worldwide studies on tropical nektonic oceanic squid genus Sthenoteuthis: an overview of the soviet investigations

Abstract

Large-scale ecological investigations of oceanic squids of the genus Sthenoteuthis were carried out in the open waters of tropical zone of all three oceans during 1961-1990. They were focused on S. pteropus in the Atlantic and S. oualaniensis in the Indian and Pacific (mainly eastern Pacific) Oceans. The quantitative distribution of squids is not uniform; the areas of dense concentrations coincide with dynamically active zones of divergences and convergences and hydrological fronts. Planktonic paralarvae live in the epipelagic zone near pycnocline. Juveniles and adult squids are interzonal animals, rising to the surface during the night-time for feeding, and descend to the depths of 200-1000 m at dawn. The spatial range structure is complicated and consists of several areas with high squid abundance, which are isolated geographically and encompass different ecological parameters of the squid populations. Genetic exchange exists between these areas, thus they represent the superpopulational level of intraspecies structure. Both species, especially S. oualaniensis, are in process of active adaptive radiation. Five intraspecific forms of S. oualaniensis and two forms of S. pteropus were found. Sthenoteuthis spp. are very fast-growing animals: they can reach ML 55-60 cm and BW 8-9 kg at the age of 300 d. They are typical r-strategists with small eggs (0.75-1.0 mm), high fecundity (up to 10-22 million oocytes), monocyclic 1-yr life cycle, fast replacement of generations, quick variations of size structure and high productivity. Paralarvae and early juveniles eat mainly crustaceans while main food items of adults are micronektonic lantern fishes (Myctophidae) and squids. Adult Sthenoteuthis do not play an important role in the feeding of large oceanic top-predators. In the epipelagic ecosystems the main food chain is: crustaceans → nyctoepipelagic myctophids → Sthenoteuthis. A high degree of individual and social behavioral organization is characteristic of Sthenoteuthis squids. This is one of the most important prerequisites for their ecological progress. Squid biomass and productivity were preliminarily assessed. These squids are of great interest as important functional elements of oceanic ecosystems and as potential fishery resources.