Abstract:
This paper presents a method of determining the relative abundance of
the siphonophore species Apolemia uvaria in relation to depth in the
Monterey Bay Canyon. This species of phynosect was found to exist intact
only at depths below 250 meters. Further, density increased with depth
below this level. In addition, it was found to be more abundant in habitats
close to the canyon wall as compared to one mile off the wall. The unique
resources of the Monterey Bay Aquarium Research Institute (MBARI) and its
remote operated vehicle (ROV) were used to study and collect these
organisms.
A gut content analysis and description of the general feeding pattern of
A. uvaria was also conducted with samples collected at depths of up to 500
meters. It was shown to have a novel feeding strategy in comparison to
other siphonophores studied. In particular, its numerous, large gastrozooids
allow for flexibility in prey size and percentage of gastrozooids containing
prey.
Introduction:
Quantifving the depth distribution of deep-water siphonophores has
proven difficult to accomplish. In particular, phynosects are extre mely
fragile and can’t be collected by closing net systems (Pugh 1984). As a
result, only depth ranges have been recorded for species of this suborder of
siphonophores (Mackey 1987). This paper attempts to quantify the depth
distribution of the phynosect A. uvaria. This species of giant apole mid has
been observed at lengths of up to thirty meters. It has been previously
sighted intact by the submersibles "Alvin" and Johnson-Sea-Link (Mackey
1987) and is often seen in fragments at shallow depths. The abundance
relative to habitat was also examined and quantified. The habitats compared
were defined by their proximity to the Monterey Bay Canyon wall.
In addition to addressing questions of distribution, a gut content analysis
was conducted by the collection and dissection of gastrozooids. The data
collected on dietary composition was compared to similar work done on
other siphonophore species to gain an understanding of general feeding
patterns. (Biggs 1977: Purcell, 198 1a, 1981b, 1981c, 1982. 1983: Purcell and
Kremer, 1983). It was also compared to limited work done on fragmented
colonies of A. uvaria that drifted into surface waters (Purcell, 1981a).
The resources of the Monterey Bay Aquarium Research Institute (MBARI)
provide a unique opportunity to observe these organisms in situ at depths
to four hundred and fifty meters. All dives by the remote operated vehicle
(ROV) have been recorded by a Sony Betacam DXC 3000 and stored as time
coded videocassettes in the archives. These tapes have been annotated for
all sightings of various organisms including A. uvaria. These annotations
include the date, time, video frame, depth, latitude, and longitude of the
sightings. This infor mation can be recovered and printed from the database.
Using the videotapes and database, the number of organisms sighted within
certain depth ranges and the time spent by the ROV in each of these depth
ranges was determined for a one year period. The density in a specific
depth range was defined as the ratio of these two numbers. Hence, the units
of density as calculated here are colonies sighted per unit time. This
technique assumes that the time spent by the ROV at certain depths is
related to the area sampled at that depth relative to other depths. Density
by depth was also calculated for two separate habitat types defined by their
closeness to the canyon wall. One set of dives is classified as being in the
vicinity of the canyon wall; the other set of dives takes place approximately
one mile off the canyon wall.
By use of the methods above, the relative abundance of A. uvaria was
determined by depth and habitat type. In addition, the general feeding
pattern and gut contents are also described.
Materials and Methods:
Distribution
The depth distribution of A. uvaria was examined using the archival
tapes and database of annotations for ROV dives in the year 3/20/89-
3/20/90. All noted citations for Apolemia in the archives were reviewed in
order to deter mine the number of colonies observed and the depth at which
observations were made.
In order to determine the relative abundance of A. uvaria over various
ranges of depth, it was necessary to formulate an accurate measure of the
area sampled by the ROV in each depth range. The amount of time spent in
each depth range was used to measure the approximate area sampled by the
ROV. In order to determine the time spent in each depth range, all depth
reports by the ROV's computer with a value falling within the given range
were counted over the year examined in this study. On average, these depth
reports are recorded at two second intervals (Bruce Gritton, MBARI database
manager). Therefore, the following equation was used to determine the time
spent in a particular depth range:
(of depth reports within depth range) X (2 sec/depth reportl- time in
depth
60 sec./min.
range
(min.)
This value was calculated over the following intervals: 0-25 meters,
26-100 m. 101-200 m. 201-250 m. 251-300 m. 301-350 m. 351-400 m and
401-450 m. Above twenty-five meters, most of the time is spent at or near
the surface launching or recovering the ROV. The visibility during these
operations is low and the sampling time is greatly reduced. Thus, this range
has been separated to give a more accurate picture of the top one hundred
meters. It is important to note that during the year studied, the ROV did not
dive deeper than 450 meters. From the above infor mation, relative density
was defined by the ratio of colonies observed in each depth range to time
spent in that depth range, as follows:
of colonies observed in depth range
density (colonies/minute)
time spent in each depth range (min.)
In addition to examining the general depth distribution of A. uvaria the
distribution within two different habitat types in the Monterey Bay Canyon
was examined. The two habitats sampled were described as being either in
the vicinity of the canyon wall or approximately one mile off the wall.
Twelve dives taking place in the year examined were classified as being
canyon wall dives; twenty-four dives were water column dives taking place
about one mile off the wall. The latitude and longitude of these dive sites
are marked in figure 1. For each of these dives, the time spent in the depth
ranges of 0-25 meters, 25-100 m. 101-200 m. 201-300 m. 301-400 m, and
401-450 meters was determined in the manner described above. Again, the
frequency of Apolemia occurrence in each of the two habitat types was
deter mined with the units of colonies observed per unit time. This allows a
comparison of relative abundance and depth distribution of Apolemia close
to the canyon wall and in a habitat further off the wall.
Collection of Samples
The phynosect A. uvaria was collected using two different techniques.
Three individuals were collected using the ROV suction sampler. This
consists of a tube mounted on a poseable arm that uses suction to place
samples into separate, sealed plastic cylinders. Using this technique,
gastrozooids can be collected intact and preserved later at the surface to stop
digestion. The colony itsesf is broken apart by this procedure. Individuals
were collected on two separate daytime dives. Those ROV samples collected
on 5/1/90 were preserved in 802 ethanol; those collected on 5/7790 were
preserved in 102 solution of saturated formalin in seawater. These samples
were preserved from three to five hours after collection. All ROV samples
were collected at the C-4, C-5 canyon wall dive site as shown in figure 1.
The second technique used to obtain gastrozooids involved the use of a
sediment trap to fix live colonies in situ Cynthia Pilskaln of MBARI
provided these samples. The detailed construction and operation of this
apparatus is described by Honjo and Doherty (1987) and the discussion
below is intended as an overview. A diagram of the trap is included as
figure 2. The trap is suspended in the water column and collects samples
passively in its conical portion for a period of two weeks. As the samples are
funnelled into the collecting cylinders they are im mediately preserved with
a buffered for maldehyde solution. Thus, the colonies swim into the trap and
are immediately fixed, halting digestive processes. The exact number of
individuals captured by this technique is unknown because only the
gastrozooids are recovered intact. The three sets of trap samples were each
collected in two week intervals beginning 9/6/89 and ending 10/4/89. The
collections took place 100 meters above the canyon floor at a depth of 500
meters. The latitude and longitude of the collection site is shown in figure 1.
It is important to note that this is a site near the canyon wall.
Gut Content Analysis
The contents of the 189 gastrozooids total collected from both techniques
were isolated by dissection under 1.7-30 times magnification. The data
recorded included the length of the longest side of each gastrozooid, prey
description, prey size, and percentage of gastrozooids containing prey. This
data is similar to that gathered by others on several other siphonophore
species, making comparisons of general feeding patterns convenient
(Purcell, 198 1a, 1981b, 198 1c, 1982, 1983; Purcell and Kremer, 1983; Biggs
1977).
Results:
Distribution
The data used to calculate the depth distribution of A. uvaria are shown
in Table 1. Abundance appears to increase with depth below 250 meters.
Apolemia was not sighted above 250 meters during the year studied. A
graphical representation of the relationship of density to depth range is
included (Figure 3). Similar data for the canyon wall dives (Table 2) and the
dives one mile off the wall (Table 3) are also recorded. The abundance of A.
uvaria was found to be significantly higher in the canyon wall habitat.
Distribution by depth in both of these two habitats followed the pattern
established above, with abundance increasing with depth and no sightings
above 250 meters. A comparison between the two habitats of the density by
depth range is shown graphically (Figure 4).
Gut Content Analysis
The data collected on gastrozooid contents are summarized in Table 4.
The samples gathered by the sediment trap showed a higher percentage of
gastrozooids with prey compared to those collected using the ROV's suction
sampler. This may be a consequence of the fact that the ROV samples were
preserved hours after being collected. Digestive processes and egestion of
contents may have occurred as a result of the delay.
The trap specimens all had sergestids as a significant part of their diets.
It is important to note that the unidentified digested material was most
likely not sergestid soft body parts since examination of the material under
30 times magnification revealed no exoskeletal parts. Gastrozooid length
along the longest side varied from 1.95-16.25 millimeters. Both the size and
variation of size of the gastrozooids are large in comparison to other
siphonophore species (Purcell, 198 1a). Further, the number of gastrozooids
per individual was determined by videotape observations to exceed two
hundred gastrozooids per colony in some organisms. Among siphonophores,
this is considered to be a numerous amount (Purcell, 198 1a).
Discussion:
The unique resources of MBARI offer an opportunity to study organisms
below the epipelagic zone with greater ease than previously available. The
videotape archive provides an accessible body of infor mation and
observation for many organisms. Combined with the ROV's sampling
capabilities, it provides the tools to observe and collect otherwise elusive
organisms. In this paper, I have presented a technique by which these tools
can be used to quantify distributional questions. Using this technique, it was
found that A. uvaria has a depth range starting below 250 meters, with
density increasing with depth. The deep end of the depth range is currently
unknown because the ROV tether only allows it to reach maximum depths of
about 450 meters. This capability will soon be increased significantly to
1000 meters. At that time, it would be useful to examine the relative
ab undance at these deeper depths. Twelve other siphonophores have been
shown to have distribution tails beginning at about the same depth by
Margulis (1980) and extending to deep waters approaching 4000 meters. No
estimate of abundance by depth range was calculated in that study.
Further, taxonomical and morphological data are not available for
comparison given the previously stated difficulties of studying deep water
siphonophores.
The distribution by habitat type indicated a greater abundance close to
the canyon wall as opposed to one mile off the wall. In fact, several intact
individuals can often be seen in close proximity to one another in areas near
the canyon wall. This correlates with the high percentage of sergestids
found during gut content analysis. Visual observation seemed to indicate
that sergestids were more abundant in areas closer to the canyon wall.
Unfortunately, plankton tows haven't yet been collected and analyzed to
confirm this observation. It is important to note that colonies can be
observed in very close interaction with the canyon wall. Some colonies were
recor ded on videotape being caught up on the rocky substrate and
fragmenting. The gut content analysis carried out by Purcell (1981a) was
based on similar tail fragments that apparently drifted to surface waters
once separated from their nectophores.
The study of digestion patterns in A. uvaria sets it apart from other
species previously examined. The combination of large gastrozooids in great
numbers implies a novel feeding strategy among siphonophores studied. In
general, Purcell (198 1a) classified siphonophores into one of two categories:
those having small gastrozooids («1 mm) in large number ( 20) and those
having large gastrozooids O 1 mm) in smaller numbers («20). Phynosects
were generally found to have large gastrozooids in smaller numbers.
Further, those species having large numbers of small gastrozooids had a
lower percentage of gastrozooids with prey («202) and consumed smaller
prey items. A. uvaria had a highly variable percentage of gastrozooids with
prey (11-502) and consumed prey of variable size (1.5-14.5 mm). The
variation in the percentage of gastrozooids containing prey may simply be a
result of the fact that the ROV samples digested and egested some of their
gut contents in the time between capture and preservation. The sediment
trap samples had a variation from 412 to 502 while the ROV collected
samples had only from 112 to 172 of gastrozooids containing prey. Biggs
(1977) found that egestion rates in siphonophores could be as rapid as 3-4
hours. However, in the two fragments analyzed by Purcell (1981a), the
percentages of gastrozooids containing prey were 152 and 82.62. This
variation existed even though both samples were fixed immediately after
capture, halting all digestive processes. Unfortunately, the
representativeness of those samples is suspect because they were fragments
found well above the normal depth range of intact individuals. Another
possibility is that the sediment trap samples were preserved at night, while
the ROV samples were collected during the day. Several siphonophores have
been shown to show diel differences in feeding (Purcell, 1981a).
In any case, the physiological characteristics of A. uvaria imply à
synthesis of the two feeding strategies described by Purcell. Apolemia
combines large gastrozooids with great numbers, allowing more flexibility in
its feeding pattern. In particular, large gastrozooids of variable size allow a
wider variety of prey to be consumed and large numbers allow many more
prey to be grazed. A variability in percentages of gastrozooids containing
prey would also imply the ability to be flexible to variations in prey
availability. It would again be interesting to take plankton samples at
depths where Apolemia is found and determine if there is a high degree of
variability in prey abundance. In addition, it would be useful to analyze gut
contents from more samples collected at depth and fixed immediately upon
capture. If variation in percentages of gastrozooids containing prey was
found, this would further support the presence of a flexible feeding strategy.
A study of prey size in relation to gastrozooid size would also be helpful in
deter mining the role of gastrozooid size in allowing consumption of variable
prey.
In summary, the resources of MBARI have allowed a more quantitative
estimate of the depth distribution of a phynosect than previously possible.
Further, it was possible to collect and conduct a gut content analysis of
gastrozooids collected from intact organisms at depth instead of fragments
that have drifted to the surface. This provides a better estimate of the
feeding behavior of the intact colony in its deep-water habitat.
8
—

6:
Z



28

.
.
E



2
6 —
X
6o
oo
11
—
Figure 2. Diagram of Sediment Trap




Am


3





9.
.
ation of Depth Distribution
Figure 3. Graphical Rep
Colonies Observed per Unit Time at Each Depth Range
0.083
401-450
0.037
351-400
0.0365
301-350
0.0033
251-300
201-250
101-200
26-100
0-25
0.02 004 - 006 0.08 0.10
000
Colonies/Min.
13
Figure 4. Comparison of depth distribution near canyon wall
versus one mile off the wall.
Density Near Canyon Wall vs. One Mile Off the Wall
05
Tota
401-450
0.217
301-400
03
9.0005
201-300
00
Canyon Wall Dives
101-200
Water Column Dives (1 mile off wall)
26-100
0-25
0.1
92
0.3
0.0
Colonies/Hinute
Table 1. Depth distribution data - 3/20/89-3/20/90.
Depth Range (m) Depth Reports Time (minutes) Colonies Colonies/Min.
65688
0-25 m
2190 min.
26-100 m
52381
1746 min.
87384
101-200 m
2913 min.
201-250 m
90302
3010 min.
251-300 m
3856
1795 min.
0033
9—-
301-350 m
2// min.
0365
351-400 m
123086
4103 min.
152
037
401-450 m
43893
1463 min.
083
122
Table 2. Canyon wall depth distribution data - 12 dives total.
Depth Range (m) Depth Reports Time (minutes) Colonies Colonies/Min.
0-25 m
7387
246 min.
26-100 m
2928
94 min.
101-200 m
239 min.
7161
5799
201-300 m
193 min.
01
301-400 m
42076
1436 min.
038
54
12739
401-450 m
425 min.
217
92
78090
Totals:
2603 min.
148
057
Table 3. Depth distribution data one mile off canyon wall - 24 dives total.
Depth Range (m) Depth Reports Time (minutes) Colonies Colonies/Min.
0-25 m
8845
295 min.
6285
26-100 m
210 min.
34479
101-200 m
1149 min.
201-300 m
54443
00055
1815 min.
301-400 m
39263
1309 min.
0084
401-450 m
6092
0049
203 min.
Totals:
149407
4980 min.
00261
13
15
Table 4. Dietary information for siphonophore Apole mia sp. Percentages of
gastrozooids with prey calculated by (No. of prey/No. of gastrozooids) X 100.
ROV-samples collected by suction sampler; Trap- samples collected with
sediment trap; n- data unknown.
of prey
consumed
a

.
.
Trap 9/06/89 n 78 6.3 47 1.5-14.5 27 3 70
Trap 9/20/90 n 16 8.0 50 3.0-13.0 25 0 100
Trap 10/4/90 n 34 6.0 41 2.0-10.0 14 0 86
ROV 5/01/90 2 29 6.2 17 1.5-3.9 0 0 100
ROV 5/07/90 1 28 7.9 11 4.6-9.1 0 0 100
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