374
Notes
PATALAS, K. 1971. Crustacean plankton communities in forty-five
lakes in the Experimental
Lakes Area, northwestern
Ontario. J. Fish. Res.
Bd. Can. 28: 231-244.
REED, E. B. 1958. Two new species of Diaptomus
from arctic and subarctic Canada (Calanoida:
Copepoda). Can. J. Zool. 36: 663-670.
RIGLER, F. H., AND R. R. LANGFORD. 1967. Congeneric occurrences
of species of Diaptomus
in southern Ontario lakes. Can. J. Zool. 45: 8190.
ROBERTSON, A. 1975. A new species of Diaptomus
(Copepoda:
Calanoida)
from Oklahoma
and
Texas. Am. Midl. Nat. 93: 206-214.
SANDERCOCK, G. A. 1967. A study of selected
mechanisms for the coexistence of Diaptomus
spp. in Clarke Lake, Ontario. Limnol. Oceanogr. 12: 97-112.
WILSON, M. S. 1941. New species and distribution
of diaptomid copepods from the Marsh collection in the United States National Museum. J.
Wash. Acad. Sci. 31: 509-515.
-.
1951. A new subgenus of Diaptomus
(Copepoda: Calanoida), including an asiatic species
and a new species from Alaska. J. Wash. Acad.
Sci. 41: 168-179.
Limnol.
Oceclnogr.,
Mating
26(Z),
-.
-.
-.
-.
-.
-3
-2
1953. New and inadequately
known North
American species of the copepod genus Diaptomus. Smithson. Misc. Collect. 122(2): l-30.
1954. A new species of Diaptomus
from
Louisiana and Texas with notes on the subgenus Leptodiaptomus.
Tulane Stud. Zool. 2:
49-60.
1955. A new Louisiana copepod related to
Diaptomus (Aglaodiaptomus)
clavipes Schacht
(Copepoda:
Calanoida). Tulane Stud. Zool. 3:
3547.
1958. New records and species of Calanoid
copepods from Saskatchewan
and Louisiana.
Can. J. Zool. 36: 489497.
1959. Calanoida, p. 738-794. Zn W. T. Edmondson [ed.], Fresh-water
biology, 2nd ed.
Wiley.
AND S. F. LIGHT. 1951. Description
of a
new species of diaptomid
copepod from Oregon. Trans. Am. Microsc. Sot. 70: 25-30.
AND W. G. MOORE. 1953. Diagnosis of a
new species of diaptomid copepod from Louisiana. Trans. Am. Microsc. Sot. 73: 292-295.
Submitted:
18 December
Accepted: 1 August
1979
1980
1981.374-377
and the depth distribution
Abstruct-Males
of the Calanoid copepod
Pleuromammu
piseki frequently
have a conspicuous spermatophore
within
their body
cavity. This species is a strong vertical migrator and males with spermatophores
migrate to
slightly
shallower
nighttime
depths than
males lacking them. It is suggested that those
copepods with spermatophores
are ready to
mate and that they aggregate near the surface
as an adaptation to increase their chances of
finding a mate. During mating, this behavior
may be superimposed
on the other environmental cues which affect depth distributions
and may complicate
studies designed to determine the causes and benefits of migration
patterns.
The depth distributions
and vertical
migration patterns of oceanic zooplankton potentially
confer an adaptive advan-
’ This research was supported by the Marine Life
Research Program of the Scripps Institution
of
Oceanography
and the Office of Naval Research.
of an oceanic
copepod’
tage to them in such forms as a reduction
of competition
or predation, or more efficient feeding or metabolism. These potential advantages are, generally, not mutually exclusive
and a species’ depth
distribution
may thus be advantageous in
several different
ways. I present evidence here which supports the hypothesis that, on some occasions, mating behavior may also influence
zooplankton
depth distributions
(Russell 1927; David
1961). This also illustrates the potential
difficulties
of interpreting
depth distributions in terms of a single causal factor.
I thank J. McGowan and P. Walker for
providing data on the copepod depth distributions
and G. Wilson for the photomicrography.
The depth distributions
of a group of
oceanic copepods were determined from
vertically
stratified zooplankton
samples
collected on the CLIMAX
I expedition
(19-27 September 1968) in the eastern
Notes
376
NIGHTTIME
ABUNDANCES
1000
#/
Pleuromumma
piseki
1000 m3
2000
3000
0
0
0
0
0
of Pleuromammu
Fig. 2. Depth distribution
sample and is plotted at mean depth
dividual
1979.)
piseki males and females. Each point represents an inof sampling interval. (Data from McGowan and Walker
phore are ready to mate and that they
migrate to slightly
shallower
nighttime
levels than the rest of the population
so
that by aggregating near the surface they
increase their chances of finding a mate.
Although this may not be sufficiently
advantageous for a general explanation
of
vertical migration,
it could conceivably
alter the details of an existing migration
pattern.
The difficulty
of finding a mate is suggested by the low abundance
of each
species in this oligotrophic
and very diverse community and by the observation
that most P. piseki males (>80%) still retain a spermatophore
as they begin their
early morning migration to greater depths.
Although the 25-m sampling intervals do
not permit resolution of aggregations on
finer vertical scales, one might suspect
that the surface layer provides a convenient reference point in an otherwise unstructured
environment,
which the copepods use for orientation. This behavior
could also have some type of cost (in the
form of increased predation near the surface, etc.) which
would explain
why
males lacking spermatophores are not aggregated in the O-25-m layer. There is
probably
neither a cost nor benefit in
terms of feeding, since there were no
depth differences
in the degree of gut
Notes
x night sample
Pleuromamma pisekl’ @early morning
‘30 $2 WITH SPERMATOPHORES
0
40
20
60
g
o-25
$
25-50
X
F
50-75
X
% 75- 100
80
100
xx
x
@
X
Fig. 3. Each x is percentage
of males with a
spermatophore
in a sample. Early morning samples
were collected
while copepods
were migrating
downward to deeper depths.
fullness within the nighttime depth distribution of P. piseki (Hayward 1980).
This hypothesis, however, will be difficult to prove since it is not certain that
the presence of a spermatophore
indicates that a male is ready to mate, nor is
there a suitable index of which females
need to mate. The females of this species
do not retain spermatophores
and it is
also not known how long it takes a male
to produce a spermatophore, or when and
where in the water column mating takes
place. Clearly
alternative
hypotheses
could also explain the observed patterns
and the proposed explanation must be regarded as speculative.
In spite of this, the data do show, regardless of the cause, that two different
fractions of the adult population of P. piseki have slightly different depth distributions. If these represent different adaptations
to the environment,
it will
complicate
studies designed
to determine the causal factors of zooplankton
depth distributions.
For example, if during a sampling period a significant fraction of the population
altered the details
of its normal depth distribution
due to
mating activity, this might mask the long
term distribution
and make it appear less
beneficial
or even disadvantageous
in
377
terms of factors which are generally selected for.
Two other copepod species from the
CLIMAX
I samples (Scolecithrix
danae
and Undinula darwini) did not show any
depth differences
in percentages
with
spermatophores,
although ~80% of the
males had them. However, both of these
species are, at best, weak migrators, and
the postulated mechanism may be most
advantageous
for strongly
migrating
species. Mating aggregations
are common in the nekton (e.g. squid and schooling fish), and plankton swarms consistent
with mating have been observed for euphausiids (Komaki 1967). This brief report is offered in the hope of leading others who may have appropriate
sets of
samples to determine whether the phenomenon
described
here is common
among migratory copepods and what significance it may have.
Thomas L. Hayward
Scripps Institution
of Oceanography
University
of California, San Diego
La Jolla 92093
References
DAVID, P. M. 1961. The influence of vertical migration on speciation in the oceanic plankton.
Syst. Zool. 10: 10-16.
HAYWARD, T. L. 1980. Spatial and temporal feeding patterns of copepods from the North Pacific
central gyre. Mar. Biol. 58: 295-309.
KOMAKI, Y. 1967. On the surface swarming of euphausiid crustaceans. Pac. Sci. 2 1: 433448.
MCGOWAN, J. A. 1974. The nature of oceanic ecosystems, p. 9-28. Zn C. B. Miller [ed.], The biology of the oceanic Pacific. Oregon State.
-,
AND P. W. WALKER. 1979. Structure in the
copepod community
of the North Pacific central gyre. Ecol. Monogr. 49: 195-226.
-,
AND P. M. WILLIAMS. 1973. Oceanic habitat differences
in the North Pacific. J. Exp.
Mar. Biol. Ecol. 12: 187-217.
RUSSELL, F. S. 1927. The vertical distribution
of
plankton in the sea. Biol. Rev. 2: 213-262.
Submitted:
13 May 1980
Accepted: 11 September 1980