Diel patterns of vertical distribution in euthecosomatous pteropods of Hawaiian waters

2008·10.4 MB·English

by  Daniel T Nigro

THE VELIGER
© CMS, Inc., 2007
The Veliger 50(3):190-209 (October 1, 2008)
Diel Patterns of Vertical Distribution in Euthecosomatous Pteropods of
Hawaiian Waters
DANIEL T. NIGRO* and ROGER R. SEAPY
Department of Biological Science, California State University, Fullerton, California 92834, USA
(e-mail: [email protected])
Abstract. Nineteen species ofeuthecosomatous pteropods were identified fromepipelagicwaters offHawaii. Diel
patterns of vertical distribution, abundance, and shell size were assessed from 32 tows taken through five depth
intervals to 300 m during day and night periods using opening-closing 70 cm Bongo nets. Six species (Limacina
trochiformis, Creseis sp., C. virgula conica, Diacria danae, Diacavoliniaangulosa, and Diacria costata) were epipelagic
and showed limited to no diel differences. Thirteen species (L. inflata, Styliola subula, L. bulimoides, Creseis acicula,
C
Clio pyramidata, Cavolinia globulosa, L. lesueuri, inflexa, Cuvierina columnella, Hyalocylis striata, Diacria
maculata, Cavolinia gibbosa, and Diacria major) were either epipelagic/mesopelagic or mesopelagic, and showed an
increase in total mean abundance at night. Diel differences in vertical distribution are most parsimoniously
interpreted as upward nocturnal migrations. The results ofthis study are in general agreement with those obtained
for the same species in the North Atlantic Ocean and Caribbean Sea.
INTRODUCTION m
upper 140 during day and night periods by oblique
tows with continuously open and opening-closing nets.
The euthecosomatous (or shelled) pteropods are widely
Creseis virgula clava (Tesch, 1948), Clio pyramidata,
distributed in the world's oceans and the majority of Limacina inflata, and L. helicina (Phipps, 1774) were
species dwell in the epipelagic zone, although a few are captured in greater numbers at night than during the
mesopelagic or bathypelagic (Be and Gilmer, 1977). day, suggesting upward nocturnal migrations, while
Our understanding of their vertical distribution pat- Limacina trochiformis and Cavolinia longirostris (de
terns is based mainly on studies that have employed Blainville, 1821) were collected in comparable numbers
stratified-oblique or discrete-depth tows carried out in during the day and at night. Twenty other species were
the North Atlantic Ocean (Wormelle, 1962; Myers, identified, but no vertical distribution data were given.
1967; Haagensen, 1976; Wormuth, 1981; and Andersen In the second study (Tanaka, 1970), vertical distribu-
et al., 1997). The most comprehensive of these studies tionrecordsforindividual specieswerenotgivenexcept
(thWroorumguhthdi,scr1et9e81d)eptehmpilnotyerevdalssttroat1i0fi0e0dmtdouwrsingtadkaeyn f5o0rmLiamtacniingahtin(fbluatta,rawnhgiecdhdhoawdnwmaaxridmatol 5n5u0mbme)rsanadt
and night periods. Limacina inflata (d'Orbigny, 1836), had an even distribution between the surface and
L. lesueuri(d'Orbigny, 1836), Styliolasubula(Quoyand 400 m during the day.
Gaimard, 1827), and Clio pyramidata Linnaeus, 1767 The present study on the diel vertical distribution
were characterized as strong mimgrators, with peak and abundance of euthecosomes is the first from the
abundances at depths of200^400 during the day and Pacific Ocean based on replicated, opening-closing net
<100m at night. Limacina trochiformis (d'Orbigny, samples. Oblique tows were taken through five depth
1836), L. bulimoides (d'Orbigny, 1836), Clio cuspidata intervals in the upper 300 m of the water column
(Bosc, 1802), Creseis acicula (Rang, 1828), and C. during day and night periods, and vertical patterns of
virgula (Rang, 1828) showed highest daytime and distribution, density, and shell sizearecharacterizedfor
nighttime abundances at depths <100 m (i.e., they 19 species.
were shallow-water non-migrators).
Vertical distribution data in the Pacific Ocean come MATERIALS AND METHODS
from only two studies. The first was conducted by
McGowan (1960) in the North Pacific and sampled the The plankton samples used here come from a previous
study designed to determine diel patterns of vertical
distribution and abundance of squid paralarvae
*Corresponding Author: Daniel T. Nigro, Current address:
DepartmentofLifeScience, Long BeachCityCollege,4901 E. (Young and Harman, 1985 and Harman and Young,
Carson Street, Long Beach, California 90808, USA, e-mail: 1985) and heteropod gastropods (Seapy, 1990). Plank-
[email protected], phone: [phone] ton tows were taken in waters southwest of the
D. T. Nigro & R. R. Seapy, 2007 Page 191
16i0° 15i8° 1516°
o -
- Kauai 22°-
e
OahuCh 50'
^^Molokai -
i
\pCr\,Maui
*
^y C\^Hawaii 100
20°-
; *
.
a 150
a
t— '• 1 -
\%\ \ 18°-
200
-
i 20Ki M Day Night
1
Figure 1. Hawaiian Archipelago and the location of the 300
sampling area offthe southwest side ofthe island ofOahu.
Figure 2. Depth ranges oftows taken during day and night
periods. Vertical lines represent depth ranges of individual
Hawaiian island of Oahu (Figure 1) aboard the R/V tows. Horizontal dotted lines indicate the adjusted depths of
KANA KEOKI ofthe University ofHawaii between 9 each sample depth interval. Asterisks mark the two tows that
and 15 April 1984. Thestudyareawaslocatedinwaters were not used in the analysis.
m
that averaged 2000 in depth and ranged from 11 to
20 km off shore (Figure 1). Samples were collected
m
with opening-closing, 70-cm diameter (0.385 2) Bon-
mm
go nets constructed of 0.5 Nytex cloth. The nets
were fished obliquely through six target depth intervals
(0 50, 50 100, 100-150, 150-200, 200-300 and 300-
400 m) comprising the epipelagic zone off Hawaii
(Young et al., 1980) for thirty minutes in each depth
stratum during day and night periods (Figure 2).
Unfortunately, difficulties encountered in completing
m
tows in the 300^400 interval resulted in only one
m
successful tow during the day between 280 and 380
(Seapy, 1990). Because there was no replication and no
m
nighttime samples, the 300^4-00 interval was omitted
from the present study.
Limacinabulimoides
Previous research (Snider, 1975; cited in Wormuth,
1986) has shown that collection of larvae and young
Creseisvirgulaconica
post-metamorphic individuals of Limacina inflata and
L. trochiformiswas greatly increased by using nets with
a mesh size of 0.183 mm instead of 0.505 mm. Shell
diameter at metamorphosis of L. inflata and L.
mm
retroversa (Flemming, 1823) is about 0.4 (dis-
cussed in Lalli and Gilmer, 1989). Since this study does
not include larvae, those post-metamorphic individuals
mm
of limacinids between 0.4 and 0.5 were undoubt-
edly undersampled. Based on Snider's (1975) size-
frequency plots (reproduced in Wormuth, 1986), a
mm
0.505 netcollects about halfasmany individuals in
mm mm
the size range of0.4-0.5 as a 0.183 net.
A Benthos Time-Depth Recorder was attached to Diacavoliniaangulosa
dtheeptBhsongfoorneetacfhramdee,ptahndstthreatsutmartwiengreanddetfeirnimsihniendg lFeinggutrhe, W3. =Ewuitdhtehc,o<s1>o=meposshtelelriomrorapnhgloem,etarnidesC.=Whcearued:alLfol=d
subsequent to the tows from resultant time-depth plots. width.
Page 192 The Veliger, Vol. 50, No. 3
Table
1
m
Water column densities (numbers ofindividuals beneath 100 2 ofocean surface) for day and night periods and
assignment ofspeciesto vertical groupings. To facilitatecomparisonsbetween species, the nighttimedensitiesarealso
expressed as percentages. Differences in watercolumn densities between day and night periods were assessed using a
X2 test (where: s = significant at P = <0.05 and ns = not significant). Each species was placed into either an
epipelagic (=1) or epipelagic/mesopelagic and mesopelagic (=2) group (see results). Vertical groups shown in
parentheses are hypothesized due to lack of statistical support.
Density
Vertical
Species Night Percentage Day Significance group
Limacina inflata 1514.9 34.5 93.2 s 2
Str/io/a subula 734.6 16.7 56.0 s 2
Limacina trochiformis 733.6 16.7 1014.4 s 1
Limacina bulimoides 409.2 9.3 209.1 s 2
Creseis acicula 233.6 5.3 57.7 s 2
Cliopvramidata 201.8 4.6 14.8 s 2
Cavoliniaglobulosa 180.4 4.1 142.5 s 2
Limacina lesueuri 153.4 3.5 3.9 s 2
Creseis sp. 63.3 1.4 57.5 ns 1
Cavolinia inflexa 46.6 1.1 23.0 s 2
Creseis virgula conica 45.1 1.0 57.6 ns 1
Diacria danae 18.9 0.4 21.5 ns 1
Cuvierina columnella 17.9 0.4 0.0 (2)
Diacavolinia angulosa 10.9 0.3 17.4 ns 1
Diacria costata 10.0 0.2 7.9 ns 1
Hyalocylis striata 7.7 0.2 0.0 (2)
Diacria maculata 3.5 0.1 0.0 (2)
Cavolinia gibbosa 2.0 0.1 0.0 (2)
Diacria major 0.5 <0.1 0.0 (2)
Total 4387.9 1776.5
The dates and times ofthe tows used here are given in depth interval was assigned to that interval, even
m m
Nigro (2002). Recorded ranges for the depth strata though it ranged 30 into the 140-200 interval.
were reasonably accurate in the 0-50 and 50-100 m Two plankton samples (port and starboard) were
targeted intervals, but progressively less so for the collected during most tows. During five tows, however,
deeper intervals (Figure 2). The cause ofthe inaccura- the port or starboard net became fouled and only the
cies at increased depth was that prior to each tow, we sample from the open net was available. Also, either
had to estimate the length of wire paid based on the the port or starboard sample was not used from four
wire angle (determined using a hand-held wire angle tows because ofpoor sample preservation. Altogether,
indicator) and the maximal target depth (by division of 55 plankton samples from the 32 tows were used in the
the target depth (m) by the cosine of the wire angle). analysis. At least four replicate samples each for the
Because the cable is progressively less likely to remain day and night periods were used from the first three
straight as depth increases, achieving the target depths depth intervals (0-45, 45-90, and 90-140 m). One and
by this method become progressively less accurate. two samples forthe day and night periods, respectively,
m
Based on the recorded depth ranges ofthe tows, five were used for the 140-200 depth interval. For the
m
sample depth intervals were established: 0-45, 45-90, 200-300 depth interval, two samples were used for
90-140. 140-200, and 200-300 m (Figure 2). These each diel period.
depth intervals were chosen to minimize the amount of The volume of water filtered during each tow was
overlapbetweenadjacent intervals, asmosttowsranged calculatedbasedon the numberofrevolutionsrecorded
somewhat outside their targeted depth interval. Tows by calibrated T.S.K. Model OI-210 Flow Meters
that ranged outside a depth interval but that fished mounted inside the frame of each net. The volume of
m
mostly within it were considered to have fished entirely water filtered duringeach tow averaged of2841 3 and
m
within that interval. Also, tows that ranged through ranged from 1142 to 6027 3 (Nigro, 2002). Expend-
only part of a depth interval but did not extend into able Bathythermograph (XBT) casts were made daily
another one were considered to have fished entirely during the cruise. There was little variability between
within it. We made one exception to this requirement; the results from day to day. Briefly, the average surface
the first oftwo daytime tows in the 200-300 m sample temperature was about 25°C and the mixed layer
&
D. T. Nigro R. R. Seapy, 2007 Page 193
0-45 m
40 (n=24)
80
Density(ind. 1000m"J)
2 4 6 8 10 50 (n=9)
45-90m
II
50 50 (n=69)
g 50, (n=48)
100 12.0
7/H 90-140m
150
50J
(n=35)
200 (n=4)
50
140-200m
300 Day Night 1.0 mm 3.5
0.5 SizeClasses
Figure 4. Creseis sp. (A) Mean densities (ind. 1000 m~3) in each depth interval during day (open bars) and night (hatched bars)
periods. At the midpoint ofeach depth interval, ranges ofdensities among replicated tows are indicated by horizontal bars. (B)
Percentofindividualsineachsizeclassfromeachdepthintervalduringtheday(openbarsabovethex-axis)andnight(hatchedbars
below the x-axis). The number ofspecimens measured is indicated in parentheses.
(n=60)
0-45m
Density(ind. 1000m"J)
40J
D 6 12
26
//
// 1
1
50
50- (n=13)
H
h
IT) 45-90m
O
100-
c (n=H)
? (0.1)
50
B
150-
Ouh
Q
90-140m
200- (n=2)
30
60
1.0 6.0
300 Day Night 1.0mmSizeClasses
Figure 5. Creseis virgula conica. Legend as for Figure 4.
Page 194 The Veliger, Vol. 50, No. 3
40 » (n=3)
iiai»«M m
0-45
Density(ind. 1000m"3) 40 (n=9)
80
0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
80-,
40
50 5.5 45-90m
100
90-140m
•§ 150
200
140-200m
300 Day
2.I0 1 1 1 1 4.15
B 0.5mmSizeClasses
Figure 6. Diacavolinia angulosa. Legend as for Figure 4.
extended to about 65 m, below which the temperature stereomicroscope fitted with a calibrated ocular micro-
decreased steadily to about 10C at about 300 m and meter. In a given sample, all individuals belonging to a
about 6°C at 500 m. species were measured ifthe number was less than 100.
Plankton samples were fixed in 4% buffered seawa- Forthose speciesthat occurredin numbersgreaterthan
ter-formalin for 48 hr, after which they were trans- 100 per sample, at least 100 randomly selected
ferred to 40% isopropanol. All euthecosomes were individuals were measured (see Nigro, 2002).
removed from the samples. Species identifications were Shell diameters were measured for most species in
based primarilyon Beand Gilmer(1977), van der Spoel the Family Limacinidae, as the shells in this family are
(1967), and van der Spoel et al. (1997); for details see coiled and increase in diameterwith growth. Diameters
Nigro (2002). One species of Creseis was not described were measured from the outside margin ofthe aperture
in the literature and is referred to here as Creseis sp. to outside of the last whorl with the shell apex in an
Each species was enumerated using a Wild M5A upward direction. For Limactna bulimoides, however,
stereomicroscope and the counts were converted to shell length was used instead of diameter because
m
density values expressed as individuals per 1000 3 lengthis greater than diameterin this species (Figure 3)
(ind. 1000 m"3). The species densities of the port and and, thus, is a better indicator ofshell size. For species
starboard nets for each tow were calculated separately in the Family Cavoliniidae, shell lengthsweremeasured
and then averaged. Day and night density data were from the posterior to the anterior margins ofthe shells
compared by means of Poisson regression analysis (Figure 3). For identification purposes, the posterior
usingthe SASversion 8.01 statistical package. Foreach angle for some Creseis spp. and caudal fold width for
species an interaction effect was assessedbetween depth Diacavolinia sp. was measured (Figure 3).
and diel period. For each species with a significant Size-frequency distributions were constructed within
interaction, differences between day and night densities each depth interval for each species. Mean sizes were
were compared within each depth interval. compared between day and night periods within each
ANOVA
Shell sizes were determined usingthe aforementioned depth interval by one-way using Minitab
.
&
D. T. Nigro R. R. Seapy, 2007 Page 195
0-45m
45-90m
90-140m
40-200m
(n=l) 200-300m
11111P(n=l)
50-
illllll
100
B ' 0.1 mmSizeClasses 1.0
Figure 7. Limacina trochiformis. Legend as for Figure 4.
version9.0. Tukey'soptionwasusedtocorrecttheerror and together represented 68% of the total nighttime
rate since multiple comparisons ofmeans were made. density. Eight species (Limacina bulimoides, Creseis
After completion of the present study, the sample acicula, Clio pyramidata, Cavolinia globidosa (Gray,
residues were transferred to the Marine Biodiversity 1850), Limacina lesueuri, Creseis sp., Cavolinia inflexa
Processing Center ofthe Los Angeles County Museum (Lesueur, 1813), and Creseisvirgula(Rang, 1828)conica
of Natural History where they are housed under Eschscholtz, 1829) had nighttime densities between 409
"Hawaiian Bongo Net Collection" (<http://collections. and 45 ind. 100 m~2 comprising 30% ofthe total. The
,
nhm org/collection.html?code=bongo>) last eight species had nighttime densities ofless than 19
.
ind. 100 m"2 making up the remaining2% ofthe total.
,
RESULTS The water column densities during day and night
periods were compared by x2 analysis (Table 1). Nine
Watercolumn densities (number ofindividualsbeneath of the eleven species with the highest total mean
100 m2 of ocean surface) were computed for each nighttime densities (>45 ind. 100 nT2) showed signif-
species during day and night periods (Table 1). The icant diel differences. Only one of those that were
combined nighttime density for all species (4388 ind. significantly different, Limacina trochiformis, was more
100 m~2) was nearly two and one-half times greater abundant duringtheday. A total offive species showed
than the total daytime density (1777 ind. 100 m 2). no significant diel differences, and five species were
Three species (Limacina inflata, Styliola subula, and L. collected only at night.
trochiformis) had densities of 1515 to 734 ind. 100 m"2 Based on the above results, each species was placed
,
Page 196 The Veliger, Vol. 50, No. 3
0-45m
Density(ind. 1000rrT)
o 1 3 4 5 H 1 45-90m
6.0
-/,
50-
100- 90-140m
? g ioo
a,
I 150-
u
Q 50
(n=l)
2004 140-200m
100
H
50
Day Night (n=l)
300
200-300m
.25 1.75
B 0.05mmSizeClasses
Figure 8. Diacria danae. Legend as for Figure 4.
into one of two vertical daytime groupings (Table 1, were restricted to the upper 200 m. For Creseis sp., all
last column). The first group (epipelagic) was com- but fourspecimens (in the daytime 140-200 m samples)
m
posed of those species present in comparable densities were collected in the upper 140 (Figure 4A).
m
during both day and night periods with the exception Individuals in the 0^45 interval were found only at
of one, Limacina trochiformis, which was captured in night, and a significantly greater mean nighttime
significantly greater numbers during the day. The density was found in the 45-90 m interval. The sum
second group(epipelagic/mesopelagicand mesopelagic) of the mean densities below 90 m was about twice as
is inferred from the results and, therefore, is hypothet- great during the day than at night, but the difference
ical because samples were not collected from the was not significant. Shell lengths ranged from 1.0 to
mm
mesopelagic zone. This group consisted ofspecies that 3.5 (Figure 4B), and no significant differences
were either present in the epipelagic zone in signifi- were found between diel periods.
cantly higher numbers during the night or were absent Creseis virgula conica was limited to the upper 90 m
from the daytime samples. Individual depth interval during the day and night (Figure 5A), except for two
means and tests for significance for each species in the individuals captured in the nighttime 90-140 m sam-
two groups are summarized in Nigro (2002). ples. Replicate variability was high above 90 m during
both diel periods. Higher daytime densities were
Epipelagic Species Group recorded from both the 0^5 and 45-90 m depth
intervals, but the differences were not significant. Shell
Three species (Creseis sp., C. virgula conica, and lengths ranged from 1.0 to 6.0 mm (Figure 5B). Mean
Diacavolinia angulosa (Eydoux and Souleyet, shell lengths were similar in the 0^-5 interval, but were
Ms.)(Gray, 1850)) had water column densities that significantly greater at night than during the day in the
m
were comparable between day and night (Table 1) and 45-90 interval.
&
D. T. Nigro R. R. Seapy, 2007 Page 197
Density(ind. 1000m"'')
2 3 4 1 m
0-45
50
100 45-90m
•S 150-
200 90-140m
100-,
H
50
Day Night (n=l)
300U
200-300m
.
-i 1 1 1 1 1 1 1
2.7
mm
0.1 SizeClasses
B
Figure 9. Diacria costata. Legend as for Figure 4.
Diacavolinia angulosa was recorded in waters above night (Figure 8A). Maximal densitieswere foundabove
m m
200 during both day and night periods (Figure 6A). 45 during both day and night periods. Variability
Mean densities were low (<2 ind. 1000 m~3) with high was high among replicates, with no significant differ-
replicate variability and no significant differences be- ences in density between diel periods. Shells ranged
mm
tween diel periods. Shell lengths ranged from 2.0 to narrowly between 1.25 and 1.75 (Figure 8B). The
4.5 mm and did not differ significantly between day and largest individuals (>1.60 mm) were found only in
nightperiods,althoughthelargestindividuals(>3.5 mm) night samples, but diel differences were not significant.
m m
were taken only from the upper 45 (Figure 6B). Diacria costata ranged from the surface to 300
m
Three species (Limacina trochiformis, Diacria done during the day and was restricted to the upper 90 at
van der Spoel, 1968, and D. costata Pfeffer, 1879) had night (Figure 9A). During the day, the majority of
m
daytime water column densities that were similar to individuals were found in the 45-90 interval, and in
m
their nighttime densities except for L. trochiformis, the 0—45 interval at night. Nighttime densities were
which had greater densities during the day (Table 1), significantly higher compared to the day in the 0-45 m
and extended to 300 m. Limacina trochiformis was depth interval. Although daytime densities in the 45-
m
most abundant above 90 m, with lower densities in the 90 interval were about three times higher than at
m
45-90 interval and low to very low densities night, the difference was not significant. Shell sizes
extending down to 300 m (Figure 7A). Replicate ranged from 1.8 to 2.7 mm (Figure 9B) and were
variability was high with daytime densities higher in comparable between diel periods in the 0-45 and 45-
m
all depth intervals above 200 m, but the diel differences 90 depth intervals.
were not significant. Shell diameters ranged narrowly
between 0.6 and 1.0 mm (Figure 7B). A significantly Epipelagic/mesopelagic and Mesopelagic
4la5rgmerimnetearnvals,hebllutditahmeetsemralwlas(0.f0o2unmdma)t ndiifgfhetreinnctehewa0s- Species Group
probably biologically meaningless. Eight species {Limacina inflata, Styliola subitla, L.
Diacria danae was found from the surface to 300 m bulimoides, Creseis acicula, Clio pyramidata, Cavolinia
m
during the day and was limited to the upper 90 at globulosa, L. lesueuri, and Cavolinia inflexa) had
I
Page 198 The Veliger, Vol. 50, No. 3
60 -,
30
0-45m
30
(n= 1022)
60
Density(ind. 1000rrT)
100
50 100 150 200 50 100 150 200
50A
256 (n=l)
45-90m
50- (n=707)
50
(0.2)
100
-i
100
° 50
(0.1) C5
(n=l)
H 150 90-140m
(n=393;
50
200 140-200m
H
—
300J Day Night 200-300m
0.I6 1 1 1 1 i i 1.I3
mm
B 0.1 SizeClasses
Figure 10. Limacina inflata. Legend as for Figure 4.
daytime water column densities that were significantly Styliola subula was present in low to very low
m m
lower than nighttime densities in the upper 300 numbers above 200 during the day, and it was
m
(Table 1). These species are hypothesized to be daytime restricted to the upper 200 at night (Figure 11A). It
occupants of the epipelagic/mesopelagic and mesope- exhibited high replicate variability at night and
m
lagic zones. increased from low numbers at 140-200 to a
Limacina inflatawascaptured from surface waters to maximum in the upper 45 m. Significantly greater
m
300 during both diel periods (Figure 10A), except densities were found only at night in depth intervals
that it was absent from 140-200 m during the day. At above 140 m. The mean nighttime density at 140-
depths above 200 m it was recorded in very low 200 m was higher than during the day, but the
numbers during the day. At night replicate variability difference was not significant. Shell lengths ranged
was high, and significantly greater numbers were broadly from 1.0 to 9.0 mm. No significant differences
captured above 140 m with a maximum in the 0- in shell sizes (Figure 1 IB) were found, although
45 m depth interval. No significant diel differences individuals larger than 5.0 mm were taken only from
were found below 140 m, although the mean daytime night samples.
density in the 200-300 m interval was 17 times greater Limacina bulimoides was captured from surface
m
than at night. Shell diameters ranged from 0.6 to waters to 300 during the day with density increasing
1.3 mm, and were not significantly different in the 0- with depth, and itwasmainly found in the upper 140 m
m <
45 interval (Figure 10B). Small sample sizes (n 2) at night (Figure 12A). Nighttime densities were signif-
in deeper intervals from either day or night periods icantly greater compared to the day in depths above
prohibited statistical comparisons. 140 m. Replicate variability was high at night with
&
D. T. Nigro R. R. Seapy, 2007 Page 199
0-45 m
(n=655)
40
Density(ind. 1000m"J) loo-.
20 40 60 80 100 20 40 60 80 100
50
(0.2) 45-90m
50- i (n=516)
(0.1)
100 ,-, 90-140m
(n= 129)
Q.
Q
X (n=4)
200 140-200m
(n=9)
H
300 Day
200-300m
1 1 1 1 I
1.0 9.0
mm
B 1.0 SizeClasses
Figure 11. Styliola subula. Legend as for Figure 4.
m m m
densities increasing from a low at 140-200 to a 1000 3) between 140 and 300 during the day
maximum at 0^-5 m. The mean daytime density (Figure 14A). At night, this species was present only
between 140 and 200 m was higher than at night, but above 200 m with a maximal density in the 90-140 m
the difference was not significant. Shell lengths ranged interval. The total mean nighttime density was 20 times
mm mm
narrowly from 0.6 to 1.6 and were signifi- greater than during the day. Shell sizes varied widely
m mm
cantly larger at night in the 90-140 depth interval from 1.0 to 13.0 (Figure 14B). Only the smallest
(Figure 12B). In waters above 90 m, the largest shells (1.0-2.0 mm) were recorded from day tows.
individuals (>1.4 mm) were taken only at night. Large shells (>7.0 mm) were all recorded from night
Creseis acicula was present in low numbers from the tows in the upper 90 m.
surface to 300 m during the day and was restricted to Cavolinia globulosa was present down to 140 m
m m
waters above 200 at night, although it was only during the day and to 300 at night (Figure 15A).
abundant above 90 m (Figure 13A). High replicate Mean nighttime densities were significantly greater
variability was found among the night tows above than daytime densities in the 0^45 m interval and
90 m. Nevertheless, nighttime densities were signifi- significantly less in the45-90 m interval. No significant
cantly greater than daytime densities in the 0-45 and differences were found below 90 m. Shell lengths
45-90 m intervals. Most (80%) of those individuals ranged broadly from 0.5 to 6.5 mm and were
m
captured at night were in the 0^45 interval. Shell significantly greater at night in all depth intervals
mm m
lengths ranged broadly from 2.0 to 9.0 (Fig- above 140 (Figure 15B). Large individuals (>3.5
ure 13B) with no significant diel differences, except in mm) were collected only at night from tows above
the 45-90 m interval where the mean length was 200 m, except for one shell (5.5 mm) collected during
significantly larger at night. the day from the 9CM40 m interval.
Clio pyramidata was absent from waters above Limacina lesueuri was present in very low numbers
140 m and was scarce (mean abundances < 1.0 ind. (<0.3 ind. 1000 m"3) between the surface and 300 m