Age of the micronektonic squid Pterygioteuthis gemmata (Cephalopoda: Pyroteuthidae) from the central-east Atlantic based on statolith growth increments

Abstract

Pterygioteuthis gemmata Chun, 1910 occurs worldwide in tropical oceanic waters except in the western Atlantic, Caribbean and Mediterranean Seas.This species is one of the smallest oceanic squids, with mature males and females attaining 25 and 32 mm mantle length (ML), respectively. Paralarvae, juveniles and adults of P. gemmata are most common in epipelagic and mesopelagic waters of the tropical Atlantic. Their abundance is approximately similar to that of the predominant epiand mesopelagic tropical squids, the micronektonic Abraliopsis atlantica (Enoploteuthidae), Onychoteuthis banksi (Onychoteuthidae) and planktonic cranchiids. The biology of P. gemmata is poorly-known. The hectocotylus of males lacks a chitinous hook which is found in the closely-related species Pterygioteuthis giardi; additionally in females only the right oviduct is functional.'Because of frequent occurrence of both mature females and small paralarvae (1.3-1.5 mm ML), spawning of P. gemmata has been considered to take place in upper epipelagic layers of the Gulf of Guinea. Juveniles (> 100 mm ML) and adults make diel vertical migrations, ascending to the epipelagic water layers at night and descending into mesopelagic layers during the daytime. Age and growth rates in micronektonic oceanic squids have been studied by statolith ageing techniques in the enoploteuthids Abraliopsis atlantica, Abraliopsis pfefferi and Enoploteuthis leptura from the tropical Atlantic-and Abralia tngonura from the North Pacific. The daily nature of growth increments within A. trigonura statoliths has been validated by statolith analysis in newly hatched paralarvae kept in aquaria. It has been found that all these enoploteuthids are rather slow-growing animals with life span of around 6 months.-The aim of this study was to estimate growth rates and maximum age of mature P. gemmata by statolith growth increment studies. Ninety-five Pterygioteuthis gemmata of 10-32 mm ML were caught during the biological surveys of the orange-back squid Sthenoteuthis pteropus (Ommastrephidae) in the central-eastern Atlantic carried out by Russian research vessels 'Prognoz' and 'Ocher' in June-September 1985 and in September 1988. Samples were taken in open waters of the western part of the Gulf of Guinea (latitudes 215'N-3°3O'S and longitudes 4°05'-15°W) off the Exclusive Economic Zones of African countries. Squid were caught at night (20-04 h) at depths of 25-300 m using the Russian zoological pelagic trawl RT/TM 33 type (vertical opening 8-10 m) equipped with a 6 mm mesh liner. Squid were identified using the key elaborated by Nesis. The dorsal mantle length was measured to the nearest 1 mm. Maturity stages were assigned from the scale developed for omnastrephid squids. Statoliths were removed from 12 maturing and mature females and 4 mature males with mantle lengths close to maximum known values for this species. Statoliths were then stored in 96% ethanol for further analysis in the laboratory using statolithageing techniques. Total statolith length (TSL) was measured after Clarke. From each pair, one statolith was ground, first the anterior side, then posterior side on a wet waterproof sandpaper (1500 grit) and embedded in Canada balsam. Growth increments were counted from the nucleus to the edge of the lateral dome by two observers using the eye-piece micrometer of a 'Biolam-R14' light microscope (magnification x400). Statolith growth increments were well-resolved and similar in appearance and microstructure to daily gTowth increments of the enoploteuthid A. trigonura. Assuming them to be daily, the total number of growth increments in the ground statolith of P. gemmata was considered to be the age of the squid in days. Total number of growth increments for each specimen was obtained as an average of count numbers, if deviation between the average and counts was less than 5%.