population structure and diel vertical migration of euphausiid larvae in ...

Crustaceana 85 (6) 659-684

POPULATION STRUCTURE AND DIEL VERTICAL MIGRATION OF EUPHAUSIID LARVAE IN THE OPEN SOUTHERN ADRIATIC SEA (JULY 2003) BY ˇ C´ 1 ), MIRA MOROVIC ´ 2 ), IGOR BRAUTOVIC ´ 1 ) and BARBARA GANGAI1,3 ), DAVOR LUCI 1 ´ MARIJANA MILOSLAVIC ) 1 ) Institute for Marine and Coastal Research, University of Dubrovnik, Damjana Jude 12, P.O. Box 83, HR-20000 Dubrovnik, Croatia 2 ) Institute of Oceanography and Fisheries, Šetalište I. Meštrovića 63, HR-21000 Split, Croatia

ABSTRACT The composition, bathymetric distribution, and diel vertical migration of larval euphausiids in the oligotrophic southern Adriatic Sea were studied during the summer of 2003. Larvae of 11 species were identified. Of these, Thysanoessa gregaria is reported for the first time from the Adriatic Sea. Information is presented on the distribution and diel migrations of the five dominant species. Nematoscelis megalops furciliae had the widest bathymetric range, extending from surface to the bottom (1200 m); larvae of Stylocheiron maximum were found from 100 to 1200 m. Euphausia krohnii (calyptopes and furciliae), Stylocheiron abbreviatum calyptopes, and Stylocheiron longicorne furciliae ranged over 800 m. S. abbreviatum (furciliae) and S. longicorne (calyptopes) had more restricted bathymetric distributions. Different populations were associated with layers characterized by specific light intensities. Three migration patterns were observed: (i) nocturnal ascent to upper layers (E. krohnii, N. megalops, S. abbreviatum); (ii) scattered population through the water column (S. maximum); (iii) migration to upper layers at midday and night, and descent during the morning and evening (S. longicorne). Different stages of the same species showed different preferences for light intensity.

RÉSUMÉ La composition, la distribution bathymétrique et les migrations verticales des larves d’euphausiacés dans la zone sud oligotrophique de la mer Adriatique ont été étudiées au cours de l’été 2003. Les larves de 11 espèces ont été identifiées. Parmi elles, Thysanoessa gregaria est reportée pour la première fois dans la mer Adriatique. Des informations sont données sur la distribution et les migrations journalières des cinq espèces dominantes. La furcilia de Nematoscelis megalops présente le plus large champ bathymétrique s’étendant de la surface jusqu’au plancher (1200 m); les larves de Stylocheiron maximum ont été trouvées de 100 à 1200 m. Euphausia krohnii (calyptopes et furciliae), calyptopes de Stylocheiron abbreviatum et furciliae de Stylocheiron longicorne s’étendent sur 800 m.

3 ) e-mail: [email protected]

© Koninklijke Brill NV, Leiden, 2012

DOI:10.1163/156854012X643942

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BARBARA GANGAI ET AL.

S. abbreviatum (furciliae) and S. longicorne (calyptopes) ont une distribution bathymétrique plus restreinte. Différentes populations étaient associées à des couches caractérisées par une intensité lumineuse spécifique. Trois types de migrations ont été observées : (i) une ascension nocturne vers les couches supérieures (E. krohnii, N. megalops, S. abbreviatum) ; (ii) une population dispersée le long de la colonne d’eau (S. maximum) ; (iii) une migration vers les couches supérieures à midi et la nuit, et une descente le matin et en soirée (S. longicorne). Différent stages de la même espèce montrent des préférences pour la lumière intense différentes.

INTRODUCTION

Euphausiids comprise a significant portion of the oceanic biomass and play an important role in the pelagic food web (Mauchline & Fisher, 1969; Casanova, 2003; Timofeev & Selifonova, 2005; Harvey et al., 2009). Their diel, seasonal, and vertical migrations transport organic matter from the epipelagic zone to deeper layers, thus functioning as a component of the ocean’s “biological pump” (Longhurst & Harrison, 1989; Longhurst et al., 1989). Euphausiid vertical migrations depend on many factors: light intensity, ambient temperature, the presence of a thermocline, as well as on salinity, density, viscosity, nutrititional demands, and their moulting cycle (Spiridonov & Casanova, 2010). The Challenger Expedition (1874-1876) provided the first detailed data on euphausiids. Since then, several authors have presented data on developmental stages in the Antarctic sea (Mackintosh, 1973; Makarov, 1977, 1979, 1983; Fevolden, 1980; Brinton & Townsend, 1984; Ikeda, 1984; Ross et al., 1987; Men’shenina, 1988, 1992; Daly, 1990; Makarov et al., 1991; Makarov & Men’shenina, 1992; Vallet et al., 2011), the Pacific Ocean (Boden et al., 1955; Hirota et al., 1984; Paul et al., 1990; Suh et al., 1993), and the Atlantic Ocean (see Brinton, 1962; Mauchline & Fisher, 1969). A literature review shows that information on Mediterranean euphausiids is largely restricted to older papers (e.g., Ruud, 1936; Trégouboff & Rose, 1957; Brinton, 1962; Mauchline & Fisher, 1969; Franqueville, 1970, 1971; Boucher & Thiriot, 1972; Wiebe & D’Abramo, 1972; Casanova, 1974). Euphausiids were recorded for the first time in the Adriatic by Ruud (1936), based on collections of the Danish “Thor” expedition. Gamulin (1948) and Hure (1955, 1961) later confirmed these findings. Šipoš (1977), studying the horizontal distribution of Adriatic euphausiids, found 12 species, 92.3% of those known in the Mediterranean (see Mavidis et al., 2005). The biology and ecology of the developmental stages of Mediterranean euphausiids have even less been studied. Guglielmo (1979) reported total specimen numbers per sample (Isaacs-Kidds midwater trawl net) of some euphausiid larvae in the deep southern Adriatic. Casanova (1974) provided similar data: 279 specimens in the Tyrrenian Sea, 582 in the Ligurian Sea, and 116 in the Levantine basin.

VERTICAL MIGRATION OF EUPHAUSIID LARVAE

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Brancato et al. (2001) investigated diversity and vertical migration of adults and juveniles of euphausiids across the Strait of Messina. Using the BIONESS electronic multinet, they noted 25.1 ind. 1000 m−3 as the higher value of total adults and juveniles in the Ionian Sea. McGehee et al. (2004), using a WP2 plankton net, studied the summer diel distribution of euphausiid larvae in two layers: 0-150 m and 150-600 m at three Ligurian Sea stations. The maximum, 1.6 ind. m−3 , was found during the day in the upper layer. The present work reports the first detailed data for euphausiid larvae in the Adriatic Sea based on collections over 96 h at an open-water station in the oligotrophic, deep southern area. Emphasis has been placed on describing the species’ (i) composition, (ii) abundance, and (iii) diel vertical migration (DVM), especially as these relate to environmental variables (temperature and light intensity) and ontogenetic factors. Study area The southern Adriatic Sea is a semicircular, oligotrophic basin with depths to about 1200 m (fig. 1). It is influenced by water masses from the Ionian Sea, via the Strait of Otranto. The general pattern of water circulation between the Adriatic and Ionian is bimodal, oscillating between the cyclonic and anticyclonic upperlayer Ionian circulations (Orlić et al., 1992; Gaˇcić et al., 2002). On the other side, the southern Adriatic is influenced by the shallow middle and northern Adriatic areas, in which North Adriatic Dense Water (NadDW) and Middle Adriatic Dense Water (MadDW) are formed (Vilibić & Orlić, 2002). In addition, there is a yearround cyclonic gyre in the southern Adriatic (Gaˇcić et al., 2002). The interaction of these patterns strongly influences the distribution of southern Adriatic plankton (Kršinić & Grbec, 2002; Civitarese et al., 2010). The southern Adriatic has generally stable environmental conditions throughout the year (Benović et al., 2005). Thermocline formation starts in May and, by July and August, that is found at ca. 15 m (Morović et al., 2006). The temperature below the thermocline is rather constant, varying between 15° and 13°C (Luˇcić et al., 2009). Salinity is generally higher than 38 psu in the whole water column (Morović et al., 2006). Changes in the upper 100 m are related to seasonal and atmospheric influences; those in deeper layers are governed by inter-annual changes in regional circulation patterns.

MATERIAL AND METHODS

Zooplankton was sampled at a single station (∼1200 m depth) in the southern Adriatic Sea (41°44 N 17°52 E) from 22 July 2003 to 28 July 2003. The sampling

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Fig. 1. Location of the sampling station (shaded circle) in the southern Adriatic, July 2003.

programme was interrupted from 25 July midday to the morning of 27 July, owing to inclement weather (table I). Nineteen sample series (152 vertical hauls) were collected with a Nansen opening-closing net (200 μm mesh, 113 cm diameter) at the following depth intervals: 0-15 (above the thermocline), 15-50, 50-100, 100200, 200-400, 400-600, 600-800, and 800-1200 m. Five sample series were taken during the morning, four at midday, four in the evening, and six at night. Identification of the various larval stages of the euphausiids was performed according to the classifications of Mauchline & Fisher (1969), Casanova (1974), and Brinton et al. (2000). The major larval phases are: calyptopis, furcilia, and postlarvae.


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TABLE I Time and frequency of mesozooplankton sampling, July 2003 Date 22 July 23 July 23 July 23 July 23 July 23 July 24 July 24 July 24 July 24 July 25 July 25 July 27 July 27 July 27 July 28 July 28 July 28 July

Sampling time (h) 20:25-23:50 02:42-03:45 07:25-10:15 13:20-15:50 16:45-18:40 21:05-23:05 01:25-05:00 07:00-10:00 15:40-16:00 19:30-21:01 02:15-03:30 07:05-09:15 00:50-04:25 06:45-10:00 14:45-17:05 01:15-04:10 06:50-09:00 14:30-17:15

In the calyptopis phase, three stages are recognized: Ca I have functional mouthparts, as well as the first thoracic leg (= maxilliped); in Ca II, the abdomen starts to be segmented; in Ca III, the uropods are beginning to develop and the telson has separated from the 6th abdominal somite. There are no pleopods in the various stages of calyptopis. The furcilia phase is the most elaborate, and the numbers of stages are variable among species and sometimes even within certain species. During the first two stages, the pleopods appear and become functional, while in later stages the development of the thoracic limbs, the appearance of the abdominal photophores, and the further shaping of the telson take place. The state of development of the pleopods and the number of terminal spines on the telson determine the developmental substages distinguished in the furciliae: In furcilia I (F I) the pleopods are not developed, from stage F I 0 until stage F I 5. In furcilia II (F II) the pleopods start to develop. From stage F II (1 + 2), where only one pair of pleopods is functional and the other two are not, until stage F II (4 +1), where four pleopod pairs are functional and only one is not yet operational. In furcilia III (F III) all pleopods are functional and the terminal spines (t) of the telson as well as its lateral spines (l) become reduced. From phase F III (7t +

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3l), with the full complement of seven terminal and three lateral spines, up to and including F III (1t + 2l), with one terminal and two lateral spines left. In the postlarval stages, all thoracic somites are further formed, the telson is reduced, the oral appendages develop, and the secondary sexual characters begin to appear. The average hauling speed of all tows was 0.5 m s−1 . Samples were preserved in a 2.5% formalin-seawater solution buffered with CaCO3 . Species identification was performed with an Olympus SZX-9 stereomicroscope. Euphausiid larvae were counted from the total samples and the abundance for each developmental stage was expressed as the number per 100 m3 . Light was measured daily to 90 m at 0600, 1200, and 1800 h (local time) with a profiling radiometer (PRR800 Biospherical Instruments Inc.) at 14 wavelengths (340-710 nm) in addition to measurements of PAR (Photosynthetic Available Radiation) attenuation (400-700 nm). The Idronaut 316 CTD probe was used to 1200 m depth and the SeaBird OC25 probe, equipped with a Wetlabs FLUO sensor, above 200 m. Chlorophyll concentrations were calculated from fluorescence with software provided by SeaBird. The probes are accurate to 0.01°C, 0.003 salinity, and 0.5 m depth. Representative species of euphausiid larvae within a depth layer were determined according to their frequency of occurrence (percentage) and relative abundance. The weighted mean depth (WMD) of all representative species was calculated as: WMD =

(ni zi di )/

(ni zi )

where di is the midpoint of the depth interval of a sample i; zi is the thickness of the stratum; and ni is the number of individuals within each depth layer (ind. 100 m−3 ). Euphausiid larvae were identified using the relevant keys available for Mediterranean and North-Atlantic species (Mauchline & Fisher, 1969; Casanova, 1974; Brinton et al., 2000). Of each individual, its developmental stage was determined in order to obtain basic insight into the reproductive biology of the euphausiids. Voucher specimens of each species are preserved in the collection of the Institute for Marine and Coastal Research, Dubrovnik.

RESULTS

Hydrographical conditions After a rapid decrease over the first 5 m, irradiance declined steadily to 90 m depth. PAR intensities at the surface ranged from 1.6 × 10−1 to 1.6 × 10−2 μ E cm−2 s−1 . In the layer immediately below the thermocline, it varied from 6.6 ×


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Fig. 2. A, vertical profiles of temperature from 0 to 200 m; and, B, in the whole water column; in the southern Adriatic, July 2003. Data represent average values from all measurements: the thin line indicates the minimum; the thick line indicates the mean; the dotted line indicates the maximum.

10−2 to 4 × 10−3 μ E cm−2 s−1 . At 50 m depth, PAR varied from 9 × 10−3 to 5 × 10−4 μ E cm−2 s−1 ; at 90 m depth, from 1.2 × 10−4 to 6 × 10−5 μ E cm−2 s−1 . Vertical temperature profiles differed little throughout the study. The highest temperature (27°C) occurred at the surface. Temperature decreased gradually below the thermocline to 50 m, at which depth the average was 18.55 ± 0.77°C (fig. 2). A small temperature decrease was further observed from 100 m depth (∼14°C) to the bottom (∼13°C).

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Fig. 3. A, vertical profiles of salinity from 0 to 200 m; and, B, in the whole water column; in the southern Adriatic, July 2003. Data represent average values from all measurements: the thin line indicates the minimum; the thick line indicates the mean; the dotted line indicates the maximum.

Salinity generally was high throughout the entire water column: from 38.23 at the surface, 38.95 at the thermocline, to 38.64 recorded in the bottom layer (fig. 3). Chlorophyll concentrations averaged 0.49 ± 0.10 mg m−3 at the surface and 0.56 ± 0.02 mg m−3 at the thermocline. They increased to a pronounced maximum of 1.26 mg m−3 at 74 m depth (average: 1.06 ± 0.12 mg m−3 ), below which they steadily decreased to 0.50 ± 0.02 mg m−3 at 200 m depth (fig. 4).


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Fig. 4. Vertical profile of chlorophyll concentration from 0 to 200 m in the southern Adriatic, July 2003. Data represent average values from all measurements: the thin line indicates the minimum; the thick line indicates the mean; the dotted line indicates the maximum.

Species composition and abundance of euphausiid larvae Larvae of eleven species of euphausiids were collected: Thysanopoda aequalis Hansen, 1905, Nyctiphanes couchii (Bell, 1853), Thysanoessa gregaria G. O. Sars, 1883, Euphausia krohnii (Brandt, 1851), Euphausia brevis Hansen, 1905, Euphausia hemigibba Hansen, 1910, Nematoscelis megalops G. O. Sars, 1883, Stylocheiron suhmi G. O. Sars, 1883, Stylocheiron longicorne G. O. Sars, 1883, Stylocheiron abbreviatum G. O. Sars, 1883, and Stylocheiron maximum Hansen, 1908. The most frequent euphausiid stages were S. longicorne furciliae, E. krohnii

47 73

8

7 27

15-50

8

8

0-15 8 17 33

Legends: Ca, calyptopis; F, furcilia; Pl, postlarva.

Euphausia krohnii (Brandt) Ca Euphausia krohnii F Euphausia krohnii Pl Nematoscelis megalops G. O. Sars Ca Nematoscelis megalops F Stylocheiron longicorne G. O. Sars Ca Stylocheiron longicorne F Stylocheiron abbeviatum G. O. Sars Ca Stylocheiron abbreviatum F Stylocheiron abbreviatum Pl Stylocheiron maximum Hansen Ca Stylocheiron maximum F

Species

50 7

86

50-100 7 36 36

38 6 6 13

25 25 56

100-200 19 25 31

200-400 6 56 63 6 63 25 63 100 6 13 6 6

Depth layer (m)

14 7

79 7 21 64

7 36

400-600

8 8

69

600-800 15 8

22 33

56

89

11 11

800-1200

TABLE II List of dominant euphausiid larvae and their frequency of occurrence (%) in the various depth strata of the southern Adriatic Sea, July 2003

668 BARBARA GANGAI ET AL.


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postlarvae, and S. abbreviatum furciliae (table II). Among the less numerous but frequently occurring (>80%) larvae were N. megalops furciliae, S. abbreviatum calyptopes, and S. longicorne furciliae (table II). The highest number of euphausiid larvae, 626 ind. 100 m−3 , occurred in the 15-50 m layer, followed by 276 ind. 100 m−3 in the 50-100 m layer (fig. 5). Euphausia krohnii (Brandt, 1851) E. krohnii calyptopes (Ca) made up 67.34% of the Ca II stages. There were higher numbers in the 100-200 m layer, with a maximum of 90 ind. 100 m−3 (fig. 6). At midday, all calyptopes were aggregated in the 600-800 m layer (table III). At night, they migrated to the upper layers, and in the morning they were spread from the surface to 800 m, with the bulk of the population above 200 m. According to the WMD and PAR data, E. krohnii calyptopes are founded in low-light-intensity layers (fig. 8). F III (1t + 2l) furciliae dominated (87.5%) and were found throughout the water column. There were higher densities above 100 m, with a maximum of 120 ind. 100 m−3 (fig. 7). The highest dispersion of the population was found in the morning, from 200 m to the bottom. According WMD data (table III), furciliae aggregated mostly at 300 m. Although there was a slight migration toward the surface at night, part of the population remained in the deeper layers. Compared to the calyptopes, PAR data and the bathymetric distribution of E. krohnii furciliae indicated that they are limited by the higher wave-length spectrum. Nematoscelis megalops G. O. Sars, 1883 Two-thirds of the two calyptopis stages in the samples were Ca III. These were only found in the 200-400 m layer during the night, with 3 ind. 100 m−3 . Furciliae were more abundant. By far, the most frequent furcilia stage (88.36%) was F III (1t + 2l). Furciliae were found from 100 m to the bottom. A higher abundance was found in the 100200 m layer, with a maximum of 32 ind. 100 m−3 and an average of 15 ind. 100 m−3 (fig. 9). Furciliae migrated daily. In the morning and afternoon, they stayed in the 720-780 m layer, while in the evening and at night they rose to about 330 m. Their distribution was centered at 566 m (table III). PAR data showed that individuals preferred reduced light intensity, arriving in the surface layers only during the night (fig. 10). The obvious differences in abundance between morning, day, sunset, and night samplings clearly indicate vertical migration. The differences are not only between species, but also between developmental stages within a species. Changes in light intensity are the main factor controlling vertical displacement of larvae and adults (Mauchline & Fisher, 1969).

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Fig. 5. Vertical distribution of total numbers of euphausiid larvae (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50-100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

Fig. 6. Vertical distribution of Euphausia krohnii (Brandt, 1851) calyptopes (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

Euphausia krohnii (Brandt) Ca Euphausia krohnii F Nematoscelis megalops G. O. Sars Ca Nematoscelis megalops F Stylocheiron abbreviatum G. O. Sars Ca Stylocheiron abbreviatum F Stylocheiron longicorne G. O. Sars Ca Stylocheiron longicorne F Stylocheiron maximum Hansen Ca Legends: Ca, calyptopis; F, furcilia.

Species

WMD 184 369 — 779 324 104 300 108 —

SRD 0-800 200-1200 — 200-1200 200-600 15-400 200-400 0-600 —

morning WMD 700 300 — 723 384 82 150 89 241

SRD 600-800 200-400 — 50-1200 200-1200 15-200 100-200 50-400 100-1200

midday WMD — 293 — 332 465 62 430 165 452

SRD — 200-400 — 50-800 200-1200 15-200 100-600 15-600 200-1200

evening

Time of day

WMD 148 127 300 339 333 48 177 69 —

SRD 50-200 0-800 200-400 0-1200 200-1200 0-200 100-400 15-800 —

night

WMD 217 250 300 566 362 109 203 104 341

all data

TABLE III The weighted mean depth (WMD) and sampling depth ranges (SDR) of the most common euphausiid larvae at different times of the day, indicated in meters, July 2003


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Fig. 7. Vertical distribution of Euphausia krohnii (Brandt, 1851) furciliae (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50-100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

Fig. 8. Extrapolated average depth PAR light intensities at midday and evening, and weighted mean depth (WMD) positions of Euphausia krohnii (Brandt, 1851) calyptopes and furciliae, July 2003. Arrows indicate the direction of movement to nighttime WMD. Horizontal lines indicate the range of midday light intensities at which the species occurred in the water column.


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Fig. 9. Vertical distribution of Nematoscelis megalops G. O. Sars, 1883 furciliae (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50-100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

Fig. 10. Extrapolated average depth PAR light intensities at midday and evening and weighted mean depth (WMD) positions of Nematoscelis megalops G. O. Sars, 1883 (furciliae), July 2003. Arrows indicate the direction of movement to nighttime WMD. Horizontal lines indicate the range of midday light intensities at which the species occurred in the water column.

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Fig. 11. Vertical distribution of Stylocheiron longicorne G. O. Sars, 1883 calyptopes (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50-100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

Stylocheiron longicorne G. O. Sars, 1883 There was only one calyptopis stage of S. longicorne (Ca III). These were found in small quantities from 100 to 600 m. Maximum abundance, 50 ind. 100 m−3 occurred between 100 and 200 m (average: 20 ind. 100 m−3 ) (fig. 10). WMD values suggest two ascents (noon and night) and two descents per day. According to PAR data, calyptopes prefer low light intensity (fig. 13). The most abundant furcilia phase was F II (1 + 4), with 29.61%, followed by F I 1, with 25.88%. The tendency of distribution for all furciliae phases combined was to show its centre at 104 m depth. The peak of distribution was nonetheless recorded in the 15-50 m layer, with a maximum of 400 ind. 100 m−3 and a mean value of 200 ind. 100 m−3 (fig. 12). Calyptopes and furciliae followed the rhythm of two daily ascent/descent cycles. Their central depth distribution was between 69 and 165 m (table III). Stylocheiron abbreviatum G. O. Sars, 1883 The Ca III stage represented 97.5% of all S. abbreviatum calyptopes, which were always found below 200 m. They were most abundant between 200 and 600 m, with a maximum of 13 ind. 100 m−3 (fig. 14). Slight migrations in both


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Fig. 12. Vertical distribution of Stylocheiron longicorne G. O. Sars, 1883 furciliae (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50-100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

Fig. 13. Extrapolated average depth PAR light intensities at midday and evening and weighted mean depth (WMD) positions of Stylocheiron longicorne G. O. Sars, 1883 calyptopes and furciliae, July 2003. Arrows indicate the direction of movement to nighttime WMD. Horizontal lines indicate the range of midday light intensities at which the species occurred in the water column.

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Fig. 14. Vertical distribution of Stylocheiron abbreviatum G. O. Sars, 1883 calyptopes (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50-100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

directions were noted (table III, fig. 16). PAR data showed that calyptopes prefer low light conditions (fig. 16). The most abundant furciliae (30%) were FI1, while FII (1 + 2), FI0, and FII (3 + 2) accounted for 28.4%, 27.0%, and 5.5%, respectively, of all furciliae. The maximum recorded was 28 ind. 100 m−3 and the average was 16 ind. 100 m−3 . Unlike calyptopes, most furciliae inhabited the upper layers and were most abundant between 15 and 200 m (fig. 15). The highest dispersion was at 400 m in the morning (table III). During the night, S. abbreviatum furciliae migrated toward the surface, passing through the thermocline. PAR data suggest that the furciliae are situated in higher light intensity layers (fig. 16). Stylocheiron maximum Hansen, 1908 The Ca III stage dominated (97.5%) total S. maximum calyptopes, which were present in low abundance. S. maximum larval stages were found only in a few samples at depths of 100-1200 m. Among the three recorded furcilia stages, FI0 was represented with 56.16%, F III (1t + 2l) with 40.5%, and F II (3 + 2) with 3.31%. Only one high value, 18 ind. 100 m−3 was found; this was in the 100-200 m layer (fig. 17). In the morning, the population was dispersed from 100 m to the bottom. By midday, the entire


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Fig. 15. Vertical distribution of Stylocheiron abbreviatum G. O. Sars, 1883 furciliae (no. individuals 100 m−3 ) in the southern Adriatic, July 2003. Depth ranges (m): 1 = 0-15; 2 = 15-50; 3 = 50-100; 4 = 100-200; 5 = 200-400; 6 = 400-600; 7 = 600-800; 8 = 800-1200.

population was found in the 800-1200 m layer (table III). S. maximum migrated to the upper layers during the night and then aggregated within the 200-400 m layer. Rare and less numerous euphausiid larvae were those of Nyctiphanes couchii, Thysanopoda aequalis, Euphausia hemigibba, Euphausia brevis, Stylocheiron suhmi, and Thysanoessa gregaria. N. couchii calyptopes were found in only two samples, with a maximum of 194 ind. 100 m−3 in the 15-50 m layer in the evening. Values in a second sample were very low,