Age and Growth in Paralarvae of the Mesopelagic Squid Abralia Trigonura Based on Daily Growth Increments in Statoliths

MARINE ECOLOGY PROGRESS SERIES Vol. 82: 31-40,1992 Published May 14 Mar. Ecol. Prog. Ser. l Age and growth in paralarvae of the mesopelagic squid Abralia trigonura based on daily growth increments in statoliths Keith A. Bigelow* Department of Oceanography. School of Ocean and Earth Science and Technology (SOEST), University of Hawaii at Manoa. Honolulu, Hawaii 96822, USA ABSTRACT: Growth increments in statoliths were used to estimate the age of 55 Abralia trigonura (Berry, 1913),a small tropical mesopelagic enoploteuthid squid collected from Hawaiian waters in 1988 and 1989. Growth increments were examined with light and scanning electron microscopy. Daily periodicity of Increment formation and the statolith size at hatching were confirmed by rearing hatchlings in the laboratory. Mantle length (ML) at age data suggested that paralarvae grew exponentially at a rate of 5.9 741 ML d-' for ca 40 d post-hatching, while post-paralarvae exhibited slower growth. No significant difference in growth was noted between paralarvae collected from spatially or temporally separated samples, though estimated length differed at hatching. Estimated paralarval mortality was 7.0 d~'.Within statolith microstructures, a transition in increment widths occurred at an average of 41.5 increments, which corresponded to 10.9 mm ML. The transition probably represents the end of the paralarval period, which is marked by a shift from an epipelagic to a mesopelagic habitat INTRODUCTION differences in survival, which will relate to recruitment variability in adult populations (Houde 1987). Similar Oceanic squid (Teuthoidea: Oegopsida) generally relationships between larval survival and recruitment have planktonic young (paralarvae) that are typically presumably exist for squids. Paralarval growth rates confined to the near-surface waters (0 to 200 m) of the and duration of planktonic life, however, are un- epipelagic zone (Young & Harman 1988). Paralarval explored aspects of the early life history of squids. squid spend an unknown length of time in the plank- Determination of growth rates and duration of the ton and then typically descend to an adult habitat in paralarval stage require accurate age estimates. In the mesopelagic (400 to 1200 m) or bathypelagic exploited squid populations, age and growth have (> 1200 m) zone (Roper & Young 1975),although some sometimes been inferred from analysis of length- species (e.g. Thysanoteuthis rhombus) may remain in frequency distributions (e.g. Squires 1967, Summers epipelagic waters as adults. Variation in mortality rate 1971). Analysis of length-frequency distributions is during the planktonic period could have considerable usually futile for tropical and subtropical quid popula- influence on population abundance. Small declines in tions, which may spawn throughout the year, making paralarval growth rates may lead to a substantially length-frequency modes difficult or impossible to longer planktonic period over which high mortality detect. An alternative method for age estimation has rates are presumed to operate. In larval fishes, focused on statoliths, which contain microscopic intraspecific variability in growth rates can cause large growth increments. Statoliths have been successfully used for age analysis because the periodic features ' Present address: Honolulu Laboratory, Southwest Fisheries within the microstructure of statoliths can be related Science Center, National Marine Fisheries Service, NOAA, to actual time periods (see review by Rodhouse & 2570 Dole Street, Honolulu, Hawaii 96822-2396, USA Hatfield 1990). 0 Inter-Research/Printed in Germany 32 Mar. Ecol. Prog. Ser. 82: 31-40, 1992 During the late 1970s and early 1980s, various re- searchers (Spratt 1978, Lipinski 1978, 1980, Knstensen 1980, Rosenberg et al. 1981, Morris & Aldnch 1985) reported the occurrence of increments in statoliths of a variety of loliginid and ommastrephid species, but could not clearly demonstrate that increments were deposited daily. In more recent years, studies have attempted to establish the frequency of increment formation. Vali- dation procedures include correlating length- frequency data with statolith analysis (Radtke 1983, Natsukan et al. 1988), using known-age laboratory specimens (Yang et al. 1986) or placing marks on the statolith and rearing individuals for a known time period (Dawe et al. 1985, Lipinski 1986, Jackson 1989, 1990, Nakamura & Sakurai 1990).These 8 studies have provided validation of the frequency of increment formation in 2 orders of cephalopods (Teuthoidea and Sepioidea), which suggests daily increment formation Fig. I Location of study areas off (A) windward and in statoliths may be common to many cephalopod (B) leeward Oahu, Hawa~i species. Enumeration of daily increments from these studies provides longevity estimates of 4 to 12 mo, (ML) of each preserved specimen was measured to suggesting that squids are fast-growing, short-lived 0.1 mm by an ocular micrometer with a stereomicro- animals. scope. There was no attempt to account for shrinkage The objective of this paper is to assess the utility of due to tissue dehydration in ethanol. statolith microstructures in studying the early life Juvenile, subadult and adult squids were collected history of Abralia trigonura (Berry. 1913), a small on 2 occasions with 2 types of open nets: a 100 m* (<45 mm mantle length), muscular enoploteuthid rope trawl and a 40 m2 rectangular midwater trawl squid. As an adult, this species occupies a mesopelagic (RMT-40).Both nets were equipped with an acoustic habitat in the day and undertakes die1 vertical migra- depth telemeter to monitor and adjust trawl depth. tion at dusk and dawn. This study examines the spatial Trawl depth was confirmed with a Benthos TDR. In and temporal variation in growth rates of paralarvae April 1988, oblique rope trawl samples to 250 m were and estimates mortality within the early life history taken at night off windward Oahu over a depth of through enumeration of growth increments in stato- about 700 m (Fig. 1). Oblique RMT-40 samples were liths. Also, a change in increment width in the stato- taken in October-November 1989 off leeward Oahu liths appears to mark the transition to the mesopelagic over a depth of about 800 m (Fig. 1).In each sampling habitat. program, squids were sorted from the catch and preserved in 95 % ethyl alcohol. Validation of ageing methodology. Reared para- MATERIALS AND METHODS larvae of Abralia trigonura were used to determine the size of the statolith at hatching, to ascertain when the Collection of squid for age analysis. Plankton first growth increment was formed and to validate if samples were taken in November 1988 off windward increments were deposited daily. Squid eggs with Oahu, Hawaii, over bottom depths between 700 and developing embryos were caught in plankton samples 1500 m (Fig. 1).Oblique tows to 150 m were taken with taken in the October-November 1989 sampling off a 4 m2 ring net. The net was equipped with 505 pm windward and leeward Oahu (Fig. 1). Eggs were mesh, a TSK flowmeter and a Benthos time-depth sorted from the plankton on board ship using stereo- recorder (TDR). Plankton samples were also taken in microscopes. Each egg was placed in 1.5 m1 of 0.2 pm November 1989 off windward and leeward Oahu filtered seawater within tissue culture trays. After (Fig 1). For the 1989 samples, a 0.8 m2 ring net with hatching, paralarvae were transferred to 1 1 glass jars 505 pm mesh was towed obliquely to 100 m, over a of filtered seawater, placed on a plankton rotator, and bottom depth of about 500 m. In both years, paralarval reared under a 12 h light 12 h dark photoperiod at a squid were sorted from the plankton on board ship temperature of 22 to 24 "C. Throughout the rearing with a Zeiss stereomicroscope and were immediately experiment, no attempt was made to feed the para- preserved in 95 '16 ethyl alcohol. Dorsal mantle length larvae. After isolation, paralarvae were sequentially Bigelow: Age and growth of squid paralarvae 33 sacrificed by fixing in 95 %, ethyl alcohol at 24 h inter- light (translucent) ring. The region between the focus vals over an 8 d period. The correlation between the and the first dark ring is the nucleus. The natal ring number of statolith growth increments and the rearing is the dark ring corresponding to statolith size at period was described by type I linear regression hatching. (ANOVA, Sokal & Rohlf 1981).To test the hypothesis of Statolith dimensions were determined with an image daily increment formation, the slope of the regression analysis system (Campana 1987) interfaced with a equation was compared to an expected value of 1.0 Zeiss compound microscope. The digitized micro- using a Student's t-test (Sokal & Rohlf 1981). scopic statolith images (400 X) enabled the measure- Laboratory procedures. Statoliths were removed ments of focus-dorsal dome length, focus-rostra1 from the specimens with fine dissecting needles and length, total length between the dorsal dome and forceps under a stereomicroscope. First, the funnel was rostrum and increment widths along a transect from removed and the statocyst was torn away from the the natal ring to the dorsal region of the lateral dome cephalic cartilage. Statoliths could be seen as paired, (Fig. 2). white, opaque structures within the statocyst. The Microstructures were examined in whole mounted statocyst was pulled apart, allowing the statoliths to or ground preparations with light and scanning fall onto a microscope slide, where they were rinsed in electron microscopy (SEM). Under a compound micro- distilled water and allowed to dry. scope (400 X), increments were counted from the natal Terms used to describe the external morphological ring to the outer margin in the direction of the lateral features of a statolith follow Clarke (1978). The dome. I conducted counts on 2 separate occasions and statolith consists of 4 basic parts: dorsal dome, lateral averaged for a mean increment value. All increment dome, rostrum and the wing or attachment area.

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