Download The habitat of Salpa fusiformis in the California current a

The habitat of Salpa fusiformis in the California
as defined by indicator asscmblagesl
Mary Wilcox
Coastal
Marine
Current
Silver
Laboratory,
University
of California,
Santa Cruz
95064
Abstract
Indicator
assemblages can define the habitat of a planktonic
species. Diatom assemblages that co-occur with Salpa fusiformis,
a common planktonic
herbivore,
suggest a
constancy in the environment
of this tunicate:
S. *siformis
is found with a suite of diatoms whose relative abundances are prcdictablc
to a degree. Preliminary
results indicate
that swarm formation
by S. fusiformis
occurs in waters different
from that of other salp
species commonly found in the California
Current.
IIutchinson
( 1961) defined the “paradox
of the plankton” as the problem of coexistence for potentially competing species in
the rcla tivcly unstructured pelagic environment. Plankton ecologists have suggested
that species potentially competing Ear food
might coexist because of differences in food
preference, predation on one of the competitors, the occurrence of periods when
food was nonlimiting,
differences in seasonal distribution,
and differences in horizontal or vertical distribution,
Alternatively, ecologically
similar species may
occur togcthcr in planktonic communities
because of the instability of such environments, with no, one species favored long
enough to cause the extinction of its competitors. My purpose here is to define the
species habitat of a common pelagic tunicate, Salpa fusiformis, by planktonic
assemblages with which it occurs.
Isoplcths of temperature, or of tcmperature and salinity, delineate habitats for single species and for planktonic assemblages
or “recurrent groups” (Fager and McGowan
1963; Venrick 1971). McGowan and Williams (1973) have pointed out that other
biological
differences
(e.g. competitors,
food organisms, prcda tors, parasites ) as
well as “water history” exist between the
water masses deBncd by tcmpcrature and
salinity.
Certain organisms also identify
the origin of water ( Raymont
1963 ) ;
-_-
changes in community composition have
been predicted by changes in indicator spcties ( Russell 1939).
Yount (1958) felt that all salps seem to
occupy similar niches simultaneously
and
are ecological equivalents for all practical
purposes. Certainly the oceanic distributions of most salp species are widely overlapping, and salps also appear to be nonselective in their diet. Thus, definition of
species niches among salps has been extremely difficult.
Berner (1957, 1967)
found 21 of the 24 species of salps in the
California Current and was unable to associate the occurrence of S. fusiformis with
any set of physical parameters; IIubb,ard
and Pearcy ( 1971) also noted the occurrence of S. fusiformis at all times of year in
the California Current off Oregon. Unlike
the distribution of some other common epiplanktonic
zooplankters, that of salps is
Many tows frequently
rather irregular.
lack specimens within the species range; at
other times populations are so dense as to
be considered swarms. Within the California Current, Berner (1967) found S. fusiformis only at 22% of the stations, while populations exceeding 1 mm3,called swarms here,
occurred only at 3% of the stations sampled
over a lo-year period. Berner ( 1957) found
the appearance of the other dominant salp
species, Thalia democratica, to be related
only to higher temperatures and noted that
values of tempcraturc, salinity, oxygen, and
phosphate did not delineate exclusive spc1 Based on a thesis submitted in partial fulfilltics ranges, However, temperature-salinity
ment for the Ph.D. degree From the University
of
relationships have been useful in showing
California,
San Diego.
LIMNOLOGY
AND
OCEANOGRAPHY
MARCH
1975,
V. 20( 2).
230
Habitat
defined
by indicator
water mass affinities of S. fusiformis in Indian waters (Bhavanarayana and Ganapati
1971) and off New Zealand (Bary 1960).
Due to the complexity of water origin and
mixing in the California Current, species
ranges for salps may best be defined by biological indicators.
The ubiquitous S. fusiformis has recently
been subdivided into four new species (Foxton 1961)) but a wide range of overlapping
forms exists in the California Current (Berner 1957). Berner (personal communication)
has expressed doubts about the validity of
the division, and therefore the S. fusiformis
varieties are considered one species here.
In the California Current, S. fusiformis, as
well as T. democratica, Weelia cylindrica,
and Cyclosalpa bakeri, have occurred in
densities exceeding 1 m-3 (Berner 1967).
The conditions that promote formation of
such swarms may differ from those supporting smaller populations and thus the species
habitats for swarm and nonswarm conditions arc considered separately in the following analysis.
J. McGowan and M. Mullin supported
and guided this work. J. Isaacs, J. Enright,
and N. Holland reviewed my thesis, which
generated this paper. E. Silver made helpful suggestions and L. Madin reviewed the
paper.
Methods
Indicator organisms were used to identify
the waters in which salp species occurred,
The ideal indicator group would be an assemblage of organisms readily identified
and numerically abundant enough to give
reliable statistical parameters. The diatoms
arc such a group, and they seem particularly appropriate since they are a principal
food for salps in the California Current
( Silver 1971) . Unfortunately
diatoms have
not been sampled routinely there, However, others (e.g. Fedelc 1933; Van Zyl1959)
report that salps are nonselective feeders;
if so, diatoms from salp stomachs may bc
appropriate samples of phytoplankton.
Tests for selectivity of salps were made
by comparing diatom assemblages from salp
stomachs and from water samples: surface
assemblages
231
water samples were taken by bucket near
Catalina Island (32”31.0’N,
117’29.2’W)
and spccimcns of S. fusiformis and T. democratica captured with a dip net or brief surfact net haul within 200 m of the water sample, Diatoms from stomach and water
samples wcrc acid cleaned and permanently
mounted on slides. Similarity
of assemblagcs from the various sources was measured quantitatively
by the percent similarity index (Whittaker 1952), which gives
the percentage of individuals from two assemblages or samples that belong to the
same species or other category. A typical
calculation is shown in Table 1. The value
of the index may range from 0 to lOO%, with
values increasing as the similarity in species
composition and in relative abundance of
species increases between two assemblages.
The indices wcrc calculated using the 56
categories for classification shown in Table 1.
The constancy of the S. fusiformis environment was cxamincd by studying the
similarity of diatom assemblages from stomachs of salps taken at various times and
locations in the California Current. Salp
spccimcns were obtained from the CalCOFI samples at Scripps Institution
of
Oceanography ( Fleminger 1964). Pairs of
samples taken no more than 100 km apart
were compared with ones from stations at
least 180 km apart and sample pairs taken
within the same month with those taken a
minimum of 4 months apart. Pairs from
swarms (at least 1 specimen m-3) were compared with sample pairs from a swarm station and a nonswarm condition (maximum
densities of 0.1 specimen mb3). Six replicates of each condition were used and the
similarity indices from the contrasts listed
above wcrc entered into a three-way analysis of variance. Salps from the CalCOFI
cruises examined by Berner (1967) were
used in the analysis of variance, with the
cruises and stations determined by a random number table and individual
specimens in the counting tray also selected randomly.
In addition to the salps examined for the
analysis of variance, specimens of S. f&-
232
Silver
Table 1. Sample calculation
of the percent
similarity
index, with siliceous assemblages from
the stomach of a salp specimen collected in February 1950 at CalCOFl
station 70.55 (a) and a
specimen taken in May 1950 at CaZCOFI 90.45 (b).
Category
(i)
a.
2
bi
Min
a;bi
Diatoms
Nitzschia
sicula
Nitzschia
bicapitata-interrupta
_Cyclotella
spp.
Actinocyclus
curvatulus
Pseudoeunotia
doliolus
Coscinodiscus
excentricus+c
Thalassiosira
antiqua septata
&tinoptychus
sp.
heptactis
Asteromphalus
Coscinodiscus
radiatus
nitzschoidcs
Thalassicnema
Hemidiscus
spp.
Chaetoceros
concavicornis
Coscinodiscus
oculus iridis
-7
Rhizosolenia
styliformls
Coscinodiscus
cf. polychorda
Rnizosolenia
hebetata
Coscinodiscus
centralis
Roperia tesselata
Actinoptychusendens
Actinoptychus
undulatus
naviculoid
sp."
Coscinodiscus
tabularis
Actinocyclus
ehrenbergii
Asteromphalus
robustus
Nitzschia
kolaczekii
Bacteriastrum
spp.
Thalassiosira
decipiens
pseudonitzschia
group
other Chaetoceros
spp.
Podosira
sp.
Synedra sp.
Skeletonema
costatum
__.
I.
Nitzschia
sp. A."
Nitzschia
prolongatoides
Nitzschia
closterium-Longissima
Navicula
sp.+<
cf. Denticula
Rhizosolenia
alata
Cymbclla sp."
other Synedra
centric
SD.
Acnanthes spp.
Thalassiothrix
spp.
cf. Fragillaria
spp.
cE. Amphiprora
spp.
Coscinodiscus
stellaris
Thalassionema
bacillaris
Pleurosigna/Gyrosigma
spp.
Triceratium
spp.
sp. A
other naviculoid
spp.
other pinnate
spp.
other centric
spp.
Other
siliceous
forms
Distephanus
sp. (silicoflagellate)
radiolaria
and other silicoflagellates
Percent
Similarity
11.3 33.9 11.3
32.3 21.0 21.0
0.2
0.5 0.2
1.5 9.5
1.5
11.1
0.8 0.8
3.7
0.3 3.7
4.1 0.8
0.8
3.2
2.0
2.0
0.0
1.2 0.0
0.9
0.3
0.3
6.0 0.6
0.6
0.4
0.0
0.0
0.9
0.3 0.3
0.0
0.0
0.0
0.1
0.2 0.1
0.5 0.6
0.5
0.5
0.9
0.5
0.0
0.0
0.0
0.2
0.0
0.1
0.1 0.5
0.1
0.1 0.0
0.1
0.1 0.3
0.1
0.6
0.2 0.2
0.1
0.0
0.0
0.0
0.0
0.0
0.1 0.2
0.1
0.0
0.0
0.0
2.1
1.5
1.5
0.0
0.0
0.0
0.1 0.2
0.1
0.2
0.0
0.0
9.7
1.4
1.4
0.0
0.0 0.0
0.1
0.3 0.1
0.0
0.0
0.0
0.0
0.0 0.0
0.6
0.6
0.6
0.0
0.0
0.0
0.0
0.2
0.0
0.0
0.2 0.0
0.0
0.0
0.0
0.0
0.6
0.0
0.0
0.0 0.0
0.0
0.0
0.0
4.0
3.0
3.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.2
0.0
0.0
0.0
0.0
0.0
0.0 0.0
0.4
0.3
0.3
0.1
2.1 0.1
0.2
1.4 0.2
1.9 2.1
1.9
Index
0.7
2.1
0.7
1.9
1.7
1.7
Exi
56
= & minimum
(aibi)
=
54.1%
fc Species
variants
illustrated
in Venrick
(1969).
formis, T. democratica, and C. bakeri were
collected from a number of additional stations throughout
the California
Current.
Individuals
from swarms and from small
populations had been examined for other
purposes and now similarity indices were
calculated between assemblages from stomachs of these specimens,
Distribution
patterns of diatom asscmblages in the California Current are poorly
known. Two quantitative surveys provide
some information about the magnitude of
variation in species composition, although
they do not report on most of the diatom
species commonly found in stomachs of S.
fusiformis. Allen ( 1921) published counts
from an cast-west transect off Long Beach
that can be converted into form suitable for
variability studies. Unless they form larger
colonies, small phytoplankton
arc not included in Allen’s net tows; however, the
dominants in his samples actually were
chain-forming diatoms. Counts from Allen’s
data were converted into frequency distributions and similarity
indices computed
between distributions
at all stations more
than 8 km from land. Reid et al. (1970)
published phytoplankton counts from pump
samples taken over a year during the coastal
plankton survey at a station 12 km offshore
from La Jolla. The size distribution of phytoplankton should be directly comparable
with that taken by salps. Similarity indices
were computed between all pairs of samples.
Results
Similarity indices bctwecn diatom asscmblages from pairs of salps were compared
with those between individual
salps and
water samples, taken within 200 m of each
other, from the Catalina collections.
If
salps feed selectively, diatom assemblages
from salps should resemble those from other
salps more closely than those from the water. The median value for comparisons
among 6 S. fusiformis specimens was 55%,
between assemblages from 6 S. fusiformis
and 4 T. democratica 53%, and between
the 10 assemblages from salps and the watcr samples 63%. A Mann-Whitney
U-test
Habitat
Table
2.
defined
Results of the analysis
SS
Salp density
effect
Temporal effect
Distance
effect
207
41
294
8,735
9,277
df
MS
1
1
1
35
3ii
207
41
294
249
by indicator
of variance.
P
t35
0.91
0.41
1.09
-
p> 0.10
p ‘ro.10
P>O.lO
-
Grouped Means from the Analysis
Density:
Swarm vs swarm
Swarm VS nonswarm
Time
Same month
More than 3 months
Distance:
43.1
47.3
apart
Within
100 km
More than 180 km apart
44.3
46.1
47.7
42.7
for diffcrenccs
between water-salp and
salp-salp pairs was nonsignificant
(x =
0.67, p > O.lO), as was the same test for differences between S. fusiformis-S. fusiformis
and T. democratica-S. fusiformis comparisons (x = 0.84, p > 0.10). These results
suggest that salps are feeding nonsclectively on diatoms and that the two species
of salps take the same food items when they
occur together.
The possibility was also considered that
salps of different sizes or ages might select
different food items. A number of similarity
indices between individuals from the same
CalCO~FI station were computed to dctcrmine whether comparisons between stomach contents of similar sized salps gave
higher indices than comparisons between
very different sized salps. A regression for
similarity index showed no significant rclationship (23 specimens, slope = -0.33, SE
= 1.69, p > 0.10).
The analysis of variance on similarity indices of diatom assemblages indicates no
significant effect of time, distance, or salp
density (Table 2). Thus the similarity of
assemblages taken in the same month within
100 km (the stations selected ranged from
40-100 km apart) was not significantly
higher than that of assemblages from
swarms sampled over 3 months apart and
over IS0 km apart (the stations selected
ranged from 500-1,000 km from each other).
assemblages
233
Moreover, swarm stations were no more
alike than swarm and nonswarm ones.
However, the variability
of the similarity
indices within each category is high and
the means of the various conditions differ
by as much as 5%. Although no large-scale
diffcrcnces in the diatom assemblages found
with S. fusiformis can be attributed to the
factors tested, it seemed possible that the
use of more samples would indicate significant but smaller differences than could be
detected by the factor analysis. Such an
analysis follows.
Data from 57 stations (those used in the
analysis of variance, together with others
indicated above) over a period of 9 years
were compared (Table 3). The results of
these more extensive comparisons confirm
the conclusions of the analysis of variance:
the diatom assemblages found in waters occupied by S. fusiformis are relatively constant, with about 40% of the individual
diatoms in the assemblages common to all
samples independent of the origin of the
salp.
When the stomachs of T. democratica
and C. bakeri from swarms were contrasted
with those from S. fusiformis, however, important differences were noted (Table 3).
The average similarity index for contrasts
bctwcen individuals
of these two species
and individuals
from S. fusiformis swarm
assemblages, 29%, was significantly
lower
than contrasts between two S. fusiformis
individuals.
When assemblages from these
species were compared with those from S.
fusiformis at low density stations, however,
the average index was significantly higher
(37%) than from comparisons with S. ftiformis taken from swarm waters. If S. fusiformis is feeding nonselectively, then stomach contents indicate that swarm waters of
S. fusiformis differ from swarm environmcnts of other species, but that small populations of S. fusiformis can survive in water
of mixed origin, Whenever S. fusiformis
occurs, however, the waters contain rccurrent groupings of microplankton associated
with S. fusiformis conditions : that is, all S.
fusif ormis comparisons result in similarity
234
Silver
Table
3.
Comparison
of diatom assemblages from stomachs of salps.
Assembjages
from S. Eusiformis
A.
than 0.1/m
) were contrasted:
Mean SI = 40.7,
B.
Assemblages
were contrasted:
EromS.
captured
S2 = 245,
Eusiformis
at
df
captured
14 nonswarm
= 13,
at
number
34 swarm
stations
of
(salp
comparisons
stations
densities
no higher
= 91
(salp
densities
ol: comparisons
= 578
at
least
1/m3)
n
Mean SI = 42.9,
from S.
C. Assemblages
blages
from salps
taken
SL = 196,
df
fusiformis
captured
at 34 swarm stations:
= 33,
at
number
14 nonswarm
stations
were
contrasted
with
assem-
n
Mean SI = 38.5,
SL = 236,
D.
Assemblages
Erom S. Eusiformis
from other
species
o1-salps
(6 1.
species
was swarming:
Mean SI = 29.3,
Mean SI = 36.9,
L-
the
tests
were performed
following
comparisons:
to
test
captured
democratica
SL = 235,
B.
Assemblages
from S. fusiformis
blages
from other
species
of salps
these latter
species
were swarming:
dE = 47,
df
at
number
of
comparisons
34 swarm stations
and 3 C. backeri)
= 42,
number
of
were compared with
assemblages
at stations
where the given
comparisons
captured
at 14 nonswarm
stations
(6 x. democratica
and 3 C. backeri)
S2 = 224,
whether
df
= 22,
number
A vs. B, tLb6 = 0.47,
were contrasted
from stations
existed
with
assemat which
= 126
between
the
means
of
p > 0.10
B vs.C,
t46
= 1.32,
p > 0.05
B vs.D,
t41
= 4.00,
p < 0.01
D vs.E,
t55
= 1.93,
p < 0.05
indices of about 40% (A, B, and C in Table 3).
The similarity indicts derived from comparisons of S. fusiformis and T. democratica
may be better understood by studying Table 4, which presents numerically dominant
categories of diatoms found in the two species of salps. The values in the table are
derived from average values for 48 S. fusif ormis specimens and for 6 T. democratica
and indicate that assemblages of diatoms
from the two salp species differ not in the
presence or absence of diatom categories,
but in their ratios of abundance. The data
also demonstrate the intermediate position
of the assemblages from stations with low
densities of S. fusiformis, as compared with
assemblages from stations at which either
salp is swarming.
Data from Allen ( 1921) and Reid et al.
= 306
oE comparisons
differences
significant
= 476
(1970) provide an indication of phytoplankton variability
in the California Current.
For stations on a MO-km transect line surveyed by Allen, similarity indices averaged
35% between stations in June 1918 and 38%
between stations in July 1917. Indices computed between stations taken in the 2 years,
however, averaged 16% (median 14.5%))
indicating large differences in phytoplankton species composition, at least between
those dates. Reid et al. took their samples
at different times over a year at one geographic position, and the average similarity
index between the samples was 28% (median 21.4%). This average is probably unrealistically high, since several of the most
abundant
categories probably
included
forms that could be considered different
species.
Assemblages from S. fusiformk stomachs
Uabitat
defined
by indicator
Table 4. Percentage composition
of dominants
in salp stomachs. Percentages were calculated from
average values for all salps used in this St&y.
--- _-~
----.. ---. ---... __.-___ _______ ___
Z
Salpa
iusitcrmis
----__
swarm
nonswarm
(Z)
(Xl
Nitzscllin
sicula
~~
Nitzschia
bicnpitata
Cyc10te1.1.a
spp.
-Pscudoeunotia
doliolus
Thalassionema
nitzschoicles
naviculoid
other
sp.
pinnatcs
ThaLia
-.-democratica
swarm
(2)
--
10.0
22.0
0.3
11.3
6.6
26.8
0 .4
2.2
30.4
9.0
3.9
0.4
6.1
5.2
3.4
16.2
1.9
3.1
0.3
24.3
17.2
could not be contrasted directly with those
from the phytoplankton
surveys discussed
above.
Diatom
taxonomy
and species
groupings in the various studies were not
the same. I used species descriptions from
Vcnrick ( 1969) and Cupp ( 1943); categories of Allen (1921) and Reid et al. (1970)
did not include species dominant in salps or
included them under some more general
heading
(e.g. “Nitzschia
spp.-large”).
Thus comparisons could only be made between the rclativc variability
of phytoplankton assemblages in stomachs of salps
and in water samples. A x2 test indicated
that the temporal variability in diatom asscmblages taken in S. fusiformis stomachs
was very significantly lower than that noted
in water samples (x2 against Allen’s data
is 317, with 1 df; against the Reid et al.
data it is 60.3 with 1 df, both with p <
0.005). If the two surveys discussed here
reflect the normal variability
of assemblages in the California Current, the rcsults indicate that the environment of the
salp is significantly more constant than that
of the California Current over time,
Allen’s samples were taken over 3-day
periods in both 1917 and 1918 and thus
provide a nearly synoptic measure of the
spatial variability
expected on a scale of
about 170 km. This measure of the natural
spatial variability
of assemblages in the
water can be contrasted with that of assemblagcs from S. fusiformis waters. In
the analysis of variance presented above,
one set of similarity indices was calculated
assemblages
235
for comparisons of assemblages taken from
stomachs of salps captured within 100 km
in the same month. These salps, however,
were actually captured within a few days of
each other, due to the closeness of the given
stations on the CalCOFI cruise grid. Similarity indices from these assemblages in
salp stomachs were compared with those
obtained directly from water samples on
Allen’s 1917 and 1918 transect lines. The
assemblages from stomachs and from water
samples did not differ significantly
(p >
0.20, t-test ) , suggesting that phytoplankton
is equally heterogeneous, whether sampled
directly in the water or in salp stomachs,
when material is taken within a few days
and within a few hundred kilometers. In
fact, assemblages of diatoms in S. fusiformis stomachs, independent of the time or
place of capture, are approximately
as
homogeneous as assemblages sampled by
Allen from the water over the 170 km, 3day survey: similarity indices from June
1918 samples and July 1917 samples did
not differ significantly from all S. fusiformis
comparisons (A, 13, C in Table 3, t-test, p >
0.10).
Discussion
This study demonstrates a technique for
identifying
the pelagic habitat of S. fusiformis, a member of a planktonic group
whose niches seem particularly
difficult to
define. Salps are nonselective feeders and
thus are potential competitors for food when
they co-occur. Some evidence suggests that
they may normally be food limited and
that swarm formation may occur only when
particularly
high concentrations
of food
arc available ( Silver 1971). The survival
of these nonspecialized feeders may be due
to their high growth rates and short gencration times ( Silver 1971; Heron 1972a,b),
with rapid population expansion possible
during the onset of phytoplankton
blooms.
Salps may be an example of opportunistic
plankton (Silver 1971), a solution to the
paradox of the plankton
predicted
by
Hutchinson
( 1961) ; Heron
( 1972a,b)
reached a similar conclusion, characterizing
these organisms as colonizing species. Such
236
Silver
opportunism may be one mechanism by
which salps can coexist with slower growing herbivores such as copepods, and this
strategy is supported by a very significant
negative correlation in abundance bc twecn
salps and copepods and salps and euphausiids in the California Current (Berner 19.57).
An additional mechanism permitting coexistence of such ecologically similar forms
could be through appropriate behavior of a
prcda tor. Some predators of salps are
known, but in most cases the regularity of
predation is unknown, Pelagic turtles, cod,
sardine, and tuna have all been found to
contain salps. Only in a few cases have
salps appeared to be fairly consistent dietary items for a predator: flying fish off
Barbados (Hall 1955), albatross in the
Antarctic
( Foxton 1966), sablefish off
southern California (Farris and Ford 1967),
and two species of deep-sea smelt (Cailliet
1972). The role of predation on salp populations is not yet clear, although future studies may show it to be an important control.
The evidence presented here shows that
salps are nonselective feeders on diatoms.
Additional
data show that phytoplankton
assemblages from stomachs of salps taken
within 100 km of each other and within a
few days resemble each other to the same
extent as phytoplankton
populations taken
directly from the water in a comparable
sampling grid. Moreover, the Catalina surface samples showed that S. fusiformis and
T. democratica use the same diatoms when
they occur together. This nonselective feeding allowed stomach contents to be used as
samples of phytoplankton, the indicator assemblages used to identify the salp habitat.
Stomach contents are used as water samples
for phytoplankton
because phytoplankton
have not been collected during the routine
CalCOFI, or California Current, surveys.
I have shown that different species of
salps may occupy different waters, identified by assemblages of indicator organisms.
Using hydrographic
parameters, Berner
( 1957) and Yount ( 1958) had difficulty in
defining the habitat of individual species in
the North Pacific, although such parameters
have been useful elsewhere (Bary 1960;
Bhavanarayana and Ganapati 1971). My
results suggest that more subtle biological
parameters may be used to define the species habitat, that the habitat has a high degree of consistency (at least in terms of diatom associations), and that the optimal
habitats of the species are probably different.
It has been possible to define the species
habitat of S. fusiformis in the California
Current using indicator assemblages because the temporal variability of diatom associations occurring with this salp is significantly lower than expected from water
samples : i.e. the salp exists within a restricted range of conditions in the current.
(In fact S. fusiformis occurs at only a fraction of the stations sampled in the California Current, as noted above.) The overall
variability in phytoplankton assemblages in
the S. fusiformis environment is equivalent
to the variability of phytoplankton in water
samples on a transect of a few hundred
kilometers. The diatom assemblages characteristic of S. fusiformis waters are dominated by five species (Table 4). The relati.onship of this assemblage with California
Current hydrography
is unknown;
very
little has been published on the groupings
or spccics of diatoms present in the open
California Current.
The diatoms, and presumably other physical and biological properties of the water,
are different in swarm conditions for T.
democratica and for S. fusiformis.
Thus
there may bc a “right” kind of water in
which S. fusiformis occurs, indicated by the
recurrence of particular ratios of five dominant diatom species. In nonswarm situations, however, S. fusiformis stomachs often
have, in addition to the typical swarm species, an admixture of the flora typical of T.
t7emocratica swarm conditions ( Table 4).
These low density populations may represent expatriate populations
or “seeder”
populations, capable of sustaining themselves in waters of mixed origin, but not
capable of swarm formation there. The
habitat of maximum population
growth,
however, is quite different for S. fusiformis
from that for T. clemocratica or C. bakeri,
Habitat
defined
by indicator
as indicated by assemblages from stations
at which these species swarm. Thus the two
dominant species in the California Current,
T. democratica and S. fusiformis, appear to
utilize the same food when they co-occur,
but apparently arc found in biologically different waters when swarming. This tcndency to occupy
different waters is also
suggested by a negative correlation
in
abundance between the two species at CalCOFI stations ( Silver 1971).
References
statistical studALLEN, W. E. 1921. Preliminary
ies of marine phytoplankton
of the San Diego
Spec. Publ. Bernice
I?.
region, California.
Bishop Mus. 7, p. 537-554.
BARY, B. M.
1960. Notes on ecology distribution and systematics of pelagic Tunicata from
New Zealand.
Pac. Sci. 14: 101-121.
BERNER, L. 1957. Studies on the Thaliacca
of
the temperate northeast Pacific Ocean.
Ph.D.
thesis, Univ. Calif., San Diego. 144 p.
-.
1967. Distributional
atlas of Thaliacea
in the California
Current region.
CalCOFI
Atlas 8. 322 p.
BEIAVANARAYANA, P. V., AND P. N. GANAPATI.
1971. Species groups among pelagic tunicates in the western part of the Bay of Bengal.
Mar. Biol. 11: 173-177.
CAILLIET, G. M.
1972. Feeding habits and distribution of two deep-sea fishes off Santa Barbara, California.
Ph.D. thesis, Univ. Calif.,
Santa Barbara. 88 p.
CUPP, E. E. 1943. Marine plankton diatoms of
the west coast of North
America.
Bull.
Scripps Inst. Oceanogr. 5( 1) : 238 p.
FAGER, E. W., AND J. A. MCGOWAN.
1963. Zooplankton species groups in the North Pacific.
Science 140 : 453-460.
FARRIS, D. A., AND R. F. FORD. 1967. Food rclationships and general population
ecology of
the sablefish,
Anoplopoma
fimbria
and the
Pa&c
hake, Me&c&us
productus.
San Diego State Coil., Final Rep. Contracts
M-4
and M-8. (Mar. Rcs. Comm., State of Calif.)
FEDELE, M. 1933. Sulla nutrizione
degli animali pelagici.
3. Ricerchc
sui Salpidae.
Boll. Sot. Nat. Napoli 45: 49-118.
FLEMINGER,
A. 1964. Distributional
atlas of
Calanoid copepods in the California
Current
region, part 1. CalCOFI Atlas 2. 313 p.
FOXTON, P. 1961. Salpa fusiformis
Cuvier and
related species.
Discovery Rep. 32: 3-32.
-.
1966. The distribution
and life-history
of Salpa thompsoni Foxton with observations
on a related species, SaZpa gerlachei Foxton.
Discovery Rep. 341: l-116.
assemblages
237
in
HALL, D. N. F. 1955. Recent development
[he Barbadian
flying fish fishery and contributions
to the biology of the flying fish,
Hirunclichthys
affinis.
Colon.
Off.,
Fish.
Publ. 7. 41 p,
1972a.
Population
ecology of a
IIERON, A. c.
colonizing
spccics :
the
pelagic
tunicate
ThaZia democratica.
1. Individual
growth
rate and generation time. Occologia
(Berl. )
10: 269-293.
-.
1972b.
Population
ecology of a colonizing species : the pelagic tunicate
Thalia
democratica.
2. Population
growth
rate.
Occologia ( Berl. ) 10 : 294-312.
HUBBARD, L.T., AND W.G. PEARCY. 1971. Geographic distribution
and relative
abundance
of Salpidae off the Oregon coast. J. Fish.
Res. Bd. Can. 28: 1831-1836.
I~UTCIIINSON, G. E. 1961. The paradox of the
plankton.
Am. Nat. 95: 137-145.
MCGOWAN, J. A., AND P. M. WILLIAMS.
1973.
Oceanic habitat differences
in the north Pacific.
J. Exp. Mar. Biol. Ecol. 12: 187-217.
RAYMONT, J. E. 1963. Plankton and productivity in the oceans. Pergamon.
REID, F. M. I-I., E. FUGLISTER, AND J. B. JORDAN.
1970. The ecology of the plankton
off La
Jolla, California,
in the period April through
September 1967. Part 5. Phytoplankton
taxBull.
Scripps
onomy
and standing
crop.
Inst. Oceanogr. 17: 51-66.
RUSSELL, F. S. 1939. Hydrographical
and biological conditions
in the North Sea as indicated by plankton organisms.
J. Cons., Cons.
Int. Explor. Mer 14: 171-192.
1971. The habitat of Salpa fusiSILVER, M. W.
formis
(Chordata:
Tunicata)
in the California Current
as defined by stomach content studies and the effect of salp swarms on
the food supply of the plankton community.
Ph.D. thesis, Univ. Calif., San Diego. 135 p.
VAN ZYL, R. P. 1959. A preliminary
study of
the salps and doliolids
off the W. and S.
coasts of So. Africa.
S. Africa Dep. Commerce Ind. Invest. Rep. 40.
and ecolVENRICK, E. L. 1969. The distribution
ogy of oceanic diatoms in the North Pacific.
Ph.D. thesis, Univ. Calif., San Diego. 655 p.
1971. Recurrent groups of diatom spc-*
ties in the North Pacific.
Ecology 52: 614625.
WHITTAKER, IX. II.
1952. A study of summer
foliage insect communities in the Great Smoky
Mountains.
Ecol. Monogr. 22: l-44.
YOUNT, J. L. 1958. Distribution
and ecologic
aspects of central Pacific Salpidae (Tunicata).
Pac. Sci. 12: 111-130.
Submitted: 28 January 1974
Accepted: 21 October 1974