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Biologia 65/2: 301—308, 2010
Section Zoology
DOI: 10.2478/s11756-010-0024-8
Diel vertical migration and feeding of chaetognaths
in coastal waters of the eastern Mediterranean
George Kehayias & Dimitris Kourouvakalis
Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 301 00 Agrinio, Greece;
e-mail: [email protected]
Abstract: This study investigates the diel vertical distribution and the diet of the most important chaetognath species
found in the 0–50 m surface layer of a coastal area in the eastern Mediterranean during a 24-hour period in July 2004.
Among the recorded chaetognaths, Sagitta enflata was the most abundant species (41.6%), followed by S. minima (32.5%)
and S. serratodentata (20.8%). Those three species exhibited a “twilight migration” pattern, with only small differences
among them. Vertical separation was found between S. enflata and S. minima. Both species preyed mainly on cladocerans,
although copepods were the most abundant group in the zooplankton assemblage. The chaetognath species followed partially
the diel vertical migration of their prey. S. enflata showed high feeding intensity at different times in both day and night,
while S. minima fed more intensively at midday (12:00) and at dusk (20:00), and S. serratodentata in the morning (08:00).
It seems that in order to coexist in an area of low productivity the chaetognath species follow the basic ecological rules of
space, time and food-type separation, in order to reduce the inter- and intra-specific competition. The high preference of S.
minima and especially of S. enflata for the cladocerans made them probably the most important predators of cladocerans
during summer.
Key words: chaetognaths; vertical migration; feeding; eastern Mediterranean
Introduction
Chaetognaths are among the most abundant zooplankton predators which can often be an important component at the secondary consumer level in various marine
ecosystems (Feigenbaum 1991). In the oligotrophic waters of the eastern Mediterranean, the zooplankton is
mainly concentrated in the surface layers and especially
the 0–50 m depth strata (Mazzocchi et al. 1997). Various studies on the abundance and vertical distribution
of chaetognaths in this area reported that maximum
values were recorded in the 0–50 m depth layer and
decreased with depth (Mazzocchi et al. 1997; Batistić
et al. 2003; Kehayias 2004). The recent study of Kehayias & Ntakou (2008) has shown that in this surface
layer of the eastern Aegean Sea, the most abundant
chaetognath species follow the horizontal and vertical
distribution of their prey (copepods and cladocerans),
and occupy different niches in order to reduce the interspecific competition.
The diel vertical migration (DVM) of zooplankton
is a common phenomenon in all aquatic ecosystems,
and chaetognaths have also adopted this behavior at
least partly for feeding purposes (Pearre 1973; Stuart
& Verheye 1991; Gibbons 1992, 1994; Øresland 2000).
Despite the great importance of this phenomenon, there
are only two references about the diel migration of
chaetognath species such as Sagitta enflata (Grassi,
1881), S. minima (Grassi, 1881), S. setosa Müller, 1847
c
2010
Institute of Zoology, Slovak Academy of Sciences
and S. friderici Ritter-Záhony, 1911 in the Mediterranean (Pearre 1974; Andreu 1992), while there are no
attempts to relate this behavior to feeding. Moreover,
only a few studies have investigated the phenomenon of
DVM along with the feeding of chaetognaths on a finer
spatial and temporal scale within the surface layer of 0–
50 m (Pierrot-Bults 1982; Giesecke & Gonzalez 2004).
The present investigation comes as a sequel to the
study of Kehayias & Ntakou (2008) conducted in the
same coastal area of the eastern Aegean Sea. The aims
of this study were (a) to investigate the existence of
DVM patterns of the three most important chaetognath species during a 24–hour survey within the 0–50
m depth strata in July 2004, and (b) to relate this behaviour to feeding pattern, in order to contribute to the
understanding of the ecological role of chaetognaths as
the important zooplankton predators.
Material and methods
Zooplankton samples were collected from 19–20 July 2004
at station A (37◦ 21.17 N, 26◦ 44.13 E) with a depth of
55 m and situated in a coastal area of the eastern Aegean
Sea near Arkii Islands (Fig. 1). Vertically stratified samples
were taken every four hours in 10 m depth intervals from the
surface to 50 m, starting at 08:00 pm (local time). A WP2 closing net (mouth area 0.25 m2 , mesh size 200 µm) was
used (UNESCO 1968) to conduct vertical hauls at a speed of
15–20 m min−1 . A Hydrobios flowmeter fitted at the mouth
of the net was used for the estimation of the water volume
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302
Fig. 1. Study area with sampling station A.
filtered. The samples were fixed immediately after collection
and preserved in 4% formalin solution buffered with borax.
All chaetognath specimens were extracted from the
samples, identified to species level and classified into four
maturity stages according to Kehayias et al. (1999), as follows: Stage I, young without visible ovaries; Stage II, immature with visible ovaries but no visible seminal vesicles;
Stage III, seminal vesicles present, ova visible, a few large;
Stage IV, filled seminal vesicles and large ova. Specimens
containing food in their guts were dissected and the gut
contents were examined. Food items were identified to the
nearest taxonomical level.
In order to account for cod-end feeding, chaetognath
specimens in which the prey was found in the forward 1/3
of the gut were excluded from the counts (Øresland 1987).
The “Food Containing Ratio” (FCR), expressed as the ratio
of chaetognaths containing food to the total chaetognaths
(Feigenbaum 1991) was estimated for each species and depth
layer at each sampling time. For the estimation of the zooplankton abundance, the whole samples were subdivided using a Folsom splitter and all specimens were identified to the
level of zooplankton group. In cases where the samples were
less concentrated, the whole sample was analysed.
Pearre’s (1982) selectivity index (C) was used to investigate possible food preferences of the chaetognath species
and their developmental stages. This index is based on χ2
analysis on the number of prey found in the diet in comparison to the number of the same prey being available in the
environment, as follows:
(|ad be − ae bd | − n/2)2
C=±
abde
1/2
(1)
where C = the selectivity index, α and b represent the abundance of a given prey taxa and of all other prey taxa, respectively; subscripts d and e indicate the diet and the environment, and α = αd + αe , b = be + bd , d = αd + bd ,
e = αe + be , n = α + b + d + e. The selectivity index C
ranges from +1 indicating positive selection to −1 indicating prey avoidance, while the sign of C is given by inspection
of (αd be − bd αe ). The selectivity values were calculated from
the relative abundance (percentage) of each prey item in the
diet, in relation to the total prey standing stock (Giesecke
& Gonzalez 2008). The index was tested statistically using
χ2 test with one degree of freedom at the 0.05 significance
level.
The vertical distributions of the zooplankton groups
and chaetognath species were determined by converting the
numbers of specimens per sample to percentages of the total
specimens caught in the 0 to 50 m depth range. The mean
depth for each species was calculated as follows:
WMD =
(NTi × Ti )
NTi
(2)
where WMD = weighted mean depth, NTi = the abundance
in the depth i, and Ti = depth (m).
Regression analysis was performed between the abundance of copepods and cladocerans vs. the abundance of
the chaetognath species for all sampling times and depths.
The existence of significant differences between the vertical
distributions of: (1) the chaetognath species, (2) their developmental stages, (3) the cladocerans Evadne sp. and Penilia avirostris and (4) the adult copepods and copepodites,
was investigated using the non-parametric Kruskal-Wallis
and/or U-test applied on the mean depths.
Results
Sagitta enflata (Grassi, 1881), S. minima (Grassi, 1881)
and S. serratodentata (Krohn, 1853) were numerically
most abundant chaetognath species in the 0–50 m depth
layer, accounting for 41.6%, 32.5% and 20.8% of the
total chaetognaths, respectively. The immature stage
I specimens dominated the population of S. enflata
(68.3%), while stage III dominated the population of
S. minima (60.9%) and stage II prevailed in the population of S. serratodentata (54.5%).
Sagitta enflata, S. minima and S. serratodentata
exhibited a similar DVM pattern, showing only small
differences among them (Fig. 2). Their vertical migrations were characterized by the gradual sinking to
deeper layers from the morning (08:00) to the afternoon (16:00), and a massive ascent to surface at dusk
(20:00). At 24:00 there was a second sinking of their
populations, while close to dawn (04:00) a gradual ascent towards the surface layers was recorded (Fig. 2).
The most abundant species S. enflata and S. minima
were distributed in different vertical layers, with the
latter found always deeper, as evident from the comparison of their mean depths (U-test, P = 0.041). No
distinctive ontogenetic vertical distribution within the
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Diel vertical migration and feeding of chaetognaths
303
Fig. 2. Vertical distribution of S. enflata, S. minima and S. serratodentata at the six sampling times, as percentages (%) of total
specimens caught in water column sampled. Totals of specimens caught in the 0–50 m column are given and the weighted mean depths
(m) are indicated (dotted lines).
Fig. 3. Vertical distribution of copepods and cladocerans at the six sampling times, as percentages (%) of total specimens caught in
water column sampled. Totals of specimens caught in the 0–50 m column are given and the mean depths (m) are indicated (dotted
lines).
population of any of the three species was recorded on
the basis of the mean depths of their developmental
stages (Kruskal-Wallis test, P > 0.05).
The vertical distribution of cladocerans during the
24–hours seemed to have followed an analogous pattern
to that of the chaetognath species, while this was not
so apparent in the case of copepods (Fig. 3). Evadne sp.
was the dominant genus in the cladoceran community,
accounting for 80.5%, while Penilia avirostris (Dana,
1852) and Podon sp. accounted for 16.8% and 2.7%,
respectively. There were significant differences between
the vertical distributions of Evadne sp. and P. avirostris
(U-test, P > 0.05), with the former always found shallower than the latter.
Gut content analysis was performed on 762 chaetognath specimens present in the samples of which 147
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304
Table 1. Composition of the gut contents as percentages (%) in the whole water column (0–50 m) and total number of prey items of
the chaetognaths Sagitta enflata, S. minima and S. serratodentata at different sampling times.
Diet / time
08:00
12:00
16:00
20:00
24:00
04:00
24 hour
Sagitta enflata
Copepods
Evadne sp.
P. avirostris
Unid. food
Food items
16.7
27.8
11.1
44.4
18
24.0
32.0
8.0
36.0
25
11.8
52.9
17.6
17.6
17
15.4
61.5
15.4
7.7
13
15.4
38.5
7.7
38.5
13
30.8
38.5
7.7
23.1
13
19.2
40.4
11.1
29.3
99
Sagitta minima
Copepods
Evadne sp.
P. avirostris
Unid. food
Food items
0.0
0.0
50.0
50.0
2
0.0
54.5
18.2
27.3
11
50.0
0.0
50.0
0.0
2
50.0
50.0
0.0
0.0
2
40.0
20.0
20.0
20.0
5
14.3
42.9
28.6
14.3
7
17.2
37.9
24.1
20.7
29
Sagitta serratodentata
Copepods
Evadne sp.
P. avirostris
Unid. food
Food items
37.5
12.5
0.0
50.0
8
0.0
33.3
0.0
66.7
3
0.0
0.0
0.0
100.0
2
0.0
0.0
0.0
100.0
1
0.0
0.0
0.0
100.0
2
100.0
0.0
0.0
0.0
3
31.6
10.5
0.0
57.9
19
had food remains in their guts (mean total FCR =
0.193). Of all examined specimens, 334 were S. enflata
with 99 of them containing food (mean FCR = 0.296),
261 were S. minima (29 of them containing food, mean
FCR = 0.111) and 167 were S. serratodentata (19 of
them containing food, mean FCR = 0.114). The diet of
the three chaetognath species at all sampling times is
presented in Table 1. Although copepods were the dominant zooplankton group in the study area and cladocerans were second in abundance (the average densities
in the 0–50 m water column were 434.4 and 137.8 ind.
m−3 , respectively), the latter accounted on average for
62.0% and 51.5% in the diets of S. minima and S. enflata, respectively. Evadne sp. was the most important
prey.
The use of Pearre’s selectivity index (C) revealed
statistically significant (χ2 , P < 0.05) positive selection
of S. enflata for cladocerans (+0.22 < C < +0.46) and
negative for copepods (−0.35 < C < −0.61), while the
same was recorded for S. minima: +0.20 < C < +0.58
for cladocerans and −0.10 < C < −0.80 for copepods
(χ2 , P < 0.05). Although copepods were the most abundant prey in the diet of S. serratodentata (31.6%), no
positive selection for this prey was recorded (χ2 , P >
0.05), possibly due to the small number of recognized
food items and the large proportion of unidentified
food remains (57.9%). No naupliar stages of copepods
were found as food items in any of the chaetognaths,
and high proportions of unidentified food remains were
present in their guts (Table 1). Among juveniles, stage
I of S. enflata consumed higher proportions of cladocerans than copepods in comparison to stage II. Considering Pearre’s selectvity index (C), stage I showed
positive values for cladocerans and always negative for
copepods, while stage II showed also slightly positive selection for copepods at particular sampling times and
depths (χ2 , P < 0.05). S. minima showed higher preference for Penilia avirostris in comparison to Evadne
sp. (+0.19 < C < +0.51 and −0.13 < C < +0.47,
respectively).
Fig. 4. Mean FCR values for S. enflata, S. minima and S. serratodentata calculated for the three prey categories (copepods, cladocerans and unidentified prey), plotted along with the weighted
mean depths of each chaetognath species and of copepods and
cladocerans at the six sampling times.
During the 24-hour period there was an alteration in the depth of occurrence, the feeding intensity and the diet preferences of chaetognaths, leading to a succession of feeding events. At 08:00 S. enflata (Fig. 2) was at the same depth layer with cladocerans and partially with copepods (Fig. 3), showing positive correlation with the former group (regression analysis, r2 = 0.797, P = 0.042). At this time
S. enflata showed a feeding intensity (FCR) of 0.327
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Diel vertical migration and feeding of chaetognaths
(Fig. 4), with its diet consisting of cladocerans and
copepods at a ratio of 5:2 (Table 1). In contrast, at
08:00 S. minima was distributed deeper than its prey,
and showed low feeding intensity. The great proportion
of unidentified food in the gut contents of both species
could indicate that the feeding must have started earlier. At the same time, S. serratodentata occupied the
surface layer (Fig. 2) and showed maximum feeding
intensity (Fig. 4), preying mainly on copepods (Table 1).
Between 08:00 and 12:00, copepods and especially
cladocerans, seemed to migrate from the surface to
greater depths, with the maximum of occurrence at
the 20–30 m depth layer (Fig. 3). S. enflata seemed
to follow partially this movement, descending down
to 10–20 m layer, while S. minima ascended slightly
and appeared also at the same depth stratum (Fig. 2).
Both species presumably took advantage of the vertical movement of prey to increase their feeding intensity (Fig. 4). Considerable proportions of cladocerans and copepods were found in the gut contents
of these two species in the 10–20 m depth layer at
12:00.
The period between 12:00 and 04:00 was characterized by the abrupt ascent of cladocerans towards the
0–10 m layer (Fig. 3). However, none of the chaetognath
species followed this migration of prey and descended
instead, reaching the 30–40 m depth layer (Fig. 2).
At 20:00 cladocerans and S. enflata had the maximum of occurrence at the same depth strata and
we recorded a positive correlation between their mean
depths (regression analysis, r2 = 0.907, P = 0.013).
The ascent of S. enflata in the surface layers, where
the maximum of cladocerans was also found (Fig. 3),
must have been beneficial for the intense preying
(Fig. 4). This suggestion is corroborated by the similarity between the presence of cladocerans in the
vertical axis and their proportions in the gut contents. Also, since the lowest proportions of unidentified food were recorded at this hour (Table 1), it
seems probable that it was the beginning of the intense nocturnal feeding. However, the small numbers
of specimens of the other two chaetognath species provide no evidence that their ascent in the upper layers at 20:00 was related to the presence of cladocerans.
At midnight (24:00) there was a slight decrease in
the relative abundance of cladocerans in the surface 0–
10 m which seemed to dispersed in deeper layers, while
the same was noticed for all chaetognath species. During this period the feeding intensities of S. enflata and
S. minima were lower than at 20:00, and an increase in
the proportions of the unidentified food in the gut contents was recorded (Table 1). An increase in abundance
of cladocerans in the surface layers at dawn (04:00) suggests their upward migration during the first hours after
midnight. At the same time, only a slight upward movement for all chaetognath species was recorded (Fig. 2).
There was a small increase in the FCR values for S.
enflata, S. minima and S. serratodentata (Fig. 4) and a
305
decrease in the proportions of unidentified food in their
guts (Table 1).
Discussion
The relative abundances of the three chaetognath
species S. enflata, S. minima and S. serratodentata in
the investigated area, as well as the ontogenetic structure of their populations, are in accordance with the
study of Kehayias & Ntakou (2008) conducted at the
same area and season and using the same sampling
gear. Kehayias & Ntakou (2008) discussed extensively
the possibility that the plankton net with a porosity of
200 µm, could have under-sampled the smaller chaetognaths, especially the younger specimens of S. minima,
thus leading to sampling bias. Though, it is well known
that the choice of the dimensions and of the mesh size
in plankton nets is a trade-off between the gain in larger
taxa (or/and ontogenetic stages) and loss of the smaller
ones. Chaetognaths are large zooplankters and their relatively scarce distribution in the water column justifies
the use of large mouth nets (such as the one used in
the present study) accompanied with large mesh size.
Besides, 200 µm mesh size is one of the most commonly
employed in the zooplankton research in the Mediterranean and allows for the comparison of the results
in the region. Moreover, the strong oligotrophy of the
area, characteristic of the eastern Mediterranean waters
(Mazzocchi et al. 1997), contributed to the extremely
low numbers of chaetognath specimens recovered from
the samples.
The results of the present study showed that the
numerically most important chaetognath species S. enflata, S. minima and S. serratodentata exhibited a pattern of DVM which resembles the “twilight migration”
in which the migrators approach the surface near dawn
and dusk (Hutchinson 1967). There are only a few studies concerning the DVM of chaetognaths investigated
in finer depth intervals within the surface layer of 0–50
m. Andreu (1992) found S. enflata to migrate through
the water column in the upper 0–70 m in the western
Mediterranean during the day, whereas S. minima did
not exhibit any appreciable vertical migration. PierrotBults (1982) in the central northwest Atlantic near
Bermuda found that both S. enflata and S. serratodentata ascended to surface layers at night, performing
vertical migrations within the 0–50 m, while S. minima undertook migration below this depth. In contrast,
Giesecke & Gonzalez (2004) in northern Chile did not
find any indication of vertical migration for S. enflata in
samples from three depth strata (0–25 m, 25–50 m and
50–100 m) and concluded that if short distance migration had occurred, it remained undetected due to the
sampling resolution.
Although copepods were the most abundant zooplankton group in the area, the two numerically most
important chaetognaths S. enflata and S. minima
showed a clear preference for cladocerans. In contrast,
S. serratodentata preferred copepods to cladocerans.
The few references concerning the diet of this species
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306
pointed out that it preys heavily on copepods (Kehayias
et al. 2005 and references therein; Kehayias & Ntakou
2008). Kehayias & Ntakou (2008) found S. enflata to
prey more intensively on copepods than cladocerans,
but only at certain sampling depths within the 0–50
m layer, while S. minima showed higher preference for
cladocerans. Kehayias et al. (2005) reported that during
summer and early autumn of 1997, cladocerans were the
most important prey for S. enflata in the north Aegean
Sea, concluding that this opportunistic predator probably responded to the high abundance and dominance of
cladocerans in the zooplankton community of this area
(Kehayias et al. 2005). In the present study, however,
copepods outnumbered cladocerans at a ratio of 3 to 1.
This indicates that they were preferred as prey on the
basis of the criteria other than the mere abundances in
zooplankton.
Apart from the prey abundance, the size of prey,
and/or prey-predator co-occurrence particularly along
the vertical axis could explain the prey selection by
chaetognaths (Pearre 1973; Kimmerer 1984; Feigenbaum 1991). Considering the prey size, cladocerans
seemed to have been more attractive for the smaller
specimens of S. enflata (stage I). This has been also reported by Kehayias & Ntakou (2008) for the same area
and by Pearre (1976) for the western Mediterranean. A
great number of juvenile cladocerans found in the guts
of stage I specimens of S. enflata in the present study,
suggests that their small size made them more suitable
prey for the juveniles. Kehayias et al. (2005) also found
that mostly juvenile stages of cladocerans accounted
for a high proportion in the diet of stage I specimens of
S. enflata. The higher preference of S. minima for Penilia avirostris compared to Evadne sp. which was by
far more abundant in the area, could also be attributed
to the co-occurrence along the vertical axis.
The feeding intensity expressed as the mean FCR
values for all chaetognath species in the present study
were among the highest recorded in the Mediterranean
and in other parts of the world (Pearre 1974; Feigenbaum 1991; Dur & Saiz 2000; Batistić et al. 2003; Kehayias 2003; Giesecke & Gonzalez 2004; Kehayias et
al. 2005). The main factor leading to this high feeding intensity of chaetognaths in the area was the presence of cladocerans, which have been heavily preyed
upon, especially by S. enflata. The mean abundance
of the total zooplankton in the surface 0–50 m layer
recorded in the present study, was far lower compared
to other coastal areas of the Mediterranean basin (e.g.,
Saronicos Gulf: Siokou-Frangou 1996; Gulf of Naples:
D’Alcalà et al. 2004; Evoikos Gulf: Ramfos et al. 2005;
north Aegean Sea: Isari et al. 2006). Reports from
the various Mediterranean areas have shown that the
maximum abundance of cladocerans is found in July
and diminishes in the autumn period (Siokou-Frangou
1996; D’Alcalà et al. 2004). It could be suggested that
chaetognaths as opportunistic predators, in this area of
low productivity, utilized the seasonal peak of cladocerans. This was perhaps more crucial for the young
generation of the dominant species in the area (S. en-
G. Kehayias & D. Kourouvakalis
flata) which was at the beginning of its major breeding
period (Kehayias et al. 2005). In this way, the juveniles of S. enflata that came from the summer’s breeding batch, requiring instantly available and considerable
amount of energy for their development, directed their
diet to cladocerans, possibly because of their greater
nutritional value in comparison to copepods (Persson
& Vrede 2006). Also, S. minima, even if it was not in
the middle of reproductive period, seemed to take the
advantage of the new food supply produced by the increase of the cladoceran community. Giesecke & Gonzalez (2008) in the northern Chile found that S. enflata
reproduces in the period of the maximum abundance of
copepod nauplii probably in order to benefit from the
increased preying of its offspring on nauplii.
The vertical distribution and abundance of prey
during the 24–hour period seemed to have influenced
both the vertical migrations and the feeding intensity
of the chaetognath species. S. enflata showed high feeding intensity at various times in both day and night,
S. minima preferred to feed more intensively at midday (12:00) and at dusk (20:00), and S. serratodentata in the morning (08:00). Several references pointed
out the preference of S. enflata for the night feeding
(Pearre 1974; Szyper 1978; Kimmerer 1984; Kehayias
et al. 1996; Marazzo et al. 1997; Øresland 2000). Conversely, Giesecke & Gonzalez (2004) reported no differences between day and night and Terazaki (1996)
found its feeding activity to be higher between sunrise
and noon. Pearre (1974) reported that S. minima fed
most intensively at 22:00 and at 06:00, while there are
no references concerning the diel feeding rhythm of S.
serratodentata.
There were at least two periods during the 24 hours
when the three chaetognath species migrated to deeper
layers, one early in the afternoon (16:00) and the other
at midnight (24:00). Satiation as the major factor contributing to the midnight sinking of chaetognaths has
been reported for several species (Pearre 1973, 1974;
Stuart & Verheye 1991; Gibbons 1994). Our results on
the vertical distribution, the feeding intensity and the
presence of unidentified food, point that satiation was
probably among the reasons why chaetognaths left the
surface to disperse and distribute deeper at midnight:
after the intake of large quantities of food at 20:00,
the specimens of S. enflata sank to deeper layers to digest their food (24:00) and this was confirmed by the
increase in the proportion of the unidentified food remains in the gut contents. Moreover, satiation could
also act as the crucial factor leading to the afternoon
sinking (16:00), considering that after the intense prey
consumption at 12:00, chaetognaths would have needed
the time and space for the digestion of their food. We
conclude that the afternoon downward movements of
chaetognaths have as instigator neither the light stimulus nor the migration of prey, but their satiation.
The present data confirmed the reports of Kehayias & Ntakou (2008) about the relation between
the vertical distribution of prey and predator, leading to the conclusion that the chaetognath species preUnauthenticated
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Diel vertical migration and feeding of chaetognaths
ferred to feed at the strata of maximum prey abundance. Gibbons (1994) found chaetognaths feeding at
the copepod maximum which was near the surface at
night, thus explaining the observed nocturnal feeding
rhythms. Chaetognaths are predators which utilize an
ambush tactic; they prefer to wait for a prey passing
close enough to grasp it. Thus, during the 24-hour period they either have to follow their prey to an area
of high concentration, so as to increase the possibilities of capture, or take advantage of its movement during a migration process waiting for the prey to pass
by. There are several references suggesting that feeding does not occur during migration but only on arrival
in the copepod-rich upper layers (Szyper 1978; Øresland 1987). The results of the present study, however,
showed that both of the above strategies could be implemented by at least one species. Thus, although S.
enflata does not directly follow the vertical displacement of prey during the daylight, it seems to benefit
from this movement of potential food, perhaps from
the increase of both encounter rates and the ability to
detect its prey.
Although there are several references which explore
differences in the diet and vertical distribution among
sympatric species of chaetognaths, there was no substantial interpretation of these findings in terms of resource partitioning (Gibbons 1994). The small number
of chaetognath specimens found in the present study
(due either to the strong oligotrophy of the area or the
sampling bias) does not permit the drawing of strong
conclusions about resource partitioning, however there
are some indications of the existence of this ecological adaptation. The two numerically most important
chaetognaths S. enflata and S. minima, which had almost the same food preferences during the 24-hour period, showed a distinct vertical separation within the
surface 0–50 m water layer, possibly in order to reduce
the inter-specific competition. In addition, the lack of
significant differences in the vertical distributions of
the two ontogenetic stages of S. enflata means perhaps that there was another way of reducing the intraspecific competition through food-type partitioning: the
younger stage I specimens based their diet on cladocerans, while stage II showed preference to copepods,
as previously reported by Kehayias & Ntakou (2008)
for S. enflata. Those authors did not find differences in
the respective vertical distributions of S. enflata and S.
minima, but their data came only from daylight samplings. Kehayias et al. (1994) suggested that in oligotrophic seas like the eastern Mediterranean the vertical distribution of chaetognath species and/or ontogenetic stages in separate layers could be an adaptation
to reduce the inter- or intra-specific competition. This
could also be an attempt to reduce cannibalism (as both
inter- and intra-specific predation). This possibility was
mentioned in Pearre (1974) and supported by Brodeur
& Terazaki (1999) for populations of Sagitta elegans.
The lack of even a single incidence of cannibalism in
the present study could be an indication of a successful
implementation of the above strategy.
307
In conclusion, although based on small numbers
of collected specimens in one 24–hour period during
summer, the present paper provides useful hypotheses
about the ecological role of chaetognaths in oligotrophic
seas like the eastern Mediterranean. Coexisting within
the most productive surface 0–50 m depth layer, the
numerically most important chaetognaths seem to follow the basic ecological rules of space, time and foodtype separation in order to reduce the inter- and intraspecific competition. However, repeated observations in
different seasons and the study of greater numbers of
specimens of chaetognaths, along with their prey, are
required to provide a more solid basis for the interpretation of these hypotheses.
Acknowledgements
We wish to thank the organization “Archipelagos” and also
the captain and crew of R/V “Penelope” for their help in
the collection of the samples. We are especially grateful to
Dr. S. Pearre Jr. for his thorough review of the first version
of this manuscript and for his valuable suggestions.
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Received May 4, 2009
Accepted October 25, 2009
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