Abstract
The mysid shrimp A. sculpta was observed to
associate closely with the kelp canopy of Macrocystis
beds during the day. At night but almost never during
the day, it was found in plankton tows through open
water adjacent to the kelp forest, suggesting nocturnal
movement away from kelp.
In constant light A.sculpta remained closely
associated with kelp in aquaria and virtually never
moved into adjacent open water. When subjected to a
light-dark 14:10 regime which coincided with day and
night, more mysids moved away from kelp at night than
during the day. In constant darkness A. sculpta showed
an endogenous circadian behavioral pattern of close
association with kelp during the subjective day and
movement away from kelp during the subjective night.
This behavior is discussed in terms of possible
selective advantages, such as avoidance of predation
and feeding on zooplankters which are present in the
water column at night.
Introduction
A. sculpta is a mysid shrimp, about lem long,
which inhabits the kelp canopy of Macrocystis beds in
California. Limbaugh (1955) and Clarke (1971) observed
this crustacean in the southern part of its range.
They found it associates closely with kelp blades in
the canopy where its behavior and cryptic coloration
render it inconspicuous. Concealment is essential for
its survival, since many species of fish prey upon it
(Clarke, 1971).
Its complex forward appendages enable it to
filter feed on phytoplankton and other particulates
(Banner, 1948; Tattersall, 1951). It is also known to
consume pieces of kelp and to prey on zooplankters such
as copepods (Hobson, 1976).
There is evidence that A. sculpta does not always
maintain close association with kelp since it was found
in plankton tows through open water adjacent to kelp
forests (Hobson, 1976; Miller, 1976). During
preliminary studies I became interested in the suggested
horizontal migration of A. sculpa, and I undertook this
research to investigate a possible diel pattern of
close daytime association with the kelp canopy
followed by movement away from kelp at night. As well
as field studies, I carried out laboratory experiments
in aquaria where the habitat was simulated in miniature
and stocked with mysids.
Materials and Methods
Studies were conducted at Stanford University's
Hopkins Marine Station in April and May of 1978.
Collections and field studies were conducted near a
fixed buoy 50M from the edge of the Station's south
kelp forest and in an adjacent sand channel. Surface
tows with a 333um mesh zooplankton net, O.5M in
diameter, were made through the canopy and in open
water 10-20M from the kelp bed. A rowboat was used to
tow the net in one study and skin divers towed it in
another. The mysids collected were placed in aquaria
with running sea water. Half of each aquarium
contained suspended blades of Macrocystis pyrifera
while half consisted of an open water section. Three
light regimes were used: 1) a light-dark cycle of
14:10 (LD 14:10), 2) constant light (LL), and 3) constant
dark (DD). Light intensity near the surface of the
open water was 10-20 footcandles in all lighted aquaria.
On a sunny day light intensity near the surface of open
water in the field was slightly greater than that used
in the laboratory. Aquaria contained either 100 or
200 mysids and the number of animals in the open water
was counted at two-hour intervals. A dim flashlight
shaded with red cellophane was used to take readings in
darkness as preliminary studies showed its effect on
the mysids to be minimal.
Results
In all field studies and preliminary field work,
virtually no mysids were found in open water during the
day but some were found in open water at night (Figure
1). The Pairwise Chie Test in conjuction with the
Formula for Combining Probabilities for Independent
Tests of Significance (X* Combined) was used to analyze
field data (Sokal and Rohlf, 1969). In the rowboat
study the nighttime peak number of mysids detected in
open water is significantly greater than the daytime
values (X Combined, p£05). In the diver study the
nighttime value for the number of mysids counted in
open water samples was slightly greater than the
daytime value, but a statistical test failed to show
significance (X* Combined, p£.20). Also in the diver
study, the number of mysids detected in the kelp canopy
at night was slightly less than that during the day but was
again insignificant (X Combined, p£.20).
In the LD 14:10 experiment, peak numbers of
mysids were detected in the open water during hours of
darkness (Figure 2). A mean was calculated for three
consecutive nighttime readings starting with 2200h and
for three consecutive daytime readings starting with
1000h. The Student's t-test for the Comparison of Two
Percentages (PTT) was used to collate the two means
(Sokal and Rohlf, 1969). The nighttime mean was
significantly greater than the daytime mean (PTT, p£.05
).
In the LL experiment, the mysids remained closely
associated with kelp during all periods of observation
(Figure 2). At no time was more than 2% of the total
population detected in open water.
In the DD experiments, peak numbers of mysids in
the open water were detected during the subjective
night (Figures 3 and 4). In the first DD experiment
this pattern persisted for about one week, though the
peak values diminished. Since 92% of the mysids
survived the first DD experiment, the decline was
probably not a function of loss of animals. The mean
night value was significantly greater than the mean day
value for diel periods in all DD experiments (PTT, po.05
).
Discussion
Both laboratory and field results suggest that A.
sculpta remains closely associated with blades of
Macrocystis during the day but does not maintain this
close relationship with kelp at night. Daytime
observations made while snorkeling in the kelp canopy
and the adjacent open water corroborate these results
with the added information that the mysids are so
cryptic in both behavior and coloration that they are
seldom seen. The mysids which were observed, however,
were intimately associated with kelp.
In the rowboat study, the fact that the nighttime
peak value for the number of mysids detected in open
water is significantly greater than the daytime values
(x* Combined, p.05) suggests movement away from kelp
at night. Taken in the context of Hobson's (1976) and
Miller's (1976) work together with preliminary studies,
all of which showed an increase in the number of mysids
detected in open water at night over daytime values,
these results suggest a behavioral pattern which
lates to laboratory findings.
These results do not necessarily show mass
movement from the kelp bed across long distances to
open water. It is possible A. sculpta comes out of
hiding among kelp blades and swims in water immediately
adjacent to them. Perhaps only some of these mysids
move far enough away from kelp to be taken in plankton
tows 10-20M off the canopy. A better method of
demonstrating this behavior may be to dip net in open
spaces in the canopy during the diel cycle.
Possible advantages for a behavioral pattern of
nocturnal movement away from kelp were not examined in
this research, but previous work gives grounds for
speculation. Hobson (1976) shows that A. scupta's
consumption of copepods increases at night over the
amount consumed during the day. In addition, the
number of copepods and other zooplankters present in
the water column is known to be greater at night than
during the day (Hobson, 1976). This abundant food
source and the protection against predation which the
darkness provides probably combine to make A. scupta's
behavioral pattern highly adaptive.
An interesting and unexpected result obtained
from the DD experiments was the existence of an
endogenous rhythm of association with kelp during the
subjective day and movement to the adjacent open water
during the subjective night. For a small animal, which
may have to move some distance to reach the protection
of kelp before daylight subjects it to diurnal or
crepuscular predatory fishes, the ability to anticipate
dawn provided by an endogenous clock would be essential.
If any compass orientation is required to home back to
the kelp canopy a clock is again necessary to account
for movement of celestial objects used as navigational
cues.
Many intriguing aspects of this patterned
behavior remain to be examined. Characterization of the
behavior in the field is only fragmentary and
suggestive. Properties of the biological clock such as
its free running period and Zeitgeber also provide a
fertile area for further investigations.
Summary
1) Field observations showed that A. sculpta remains
close to kelp blades in the Macrocystis canopy
during the day.
2) A. sculpta was detected in open water adjacent to
the kelp forest at night but virtually never during
the day.
3) In laboratory experiments in constant light, mysids
remained closely associated with kelp and almost
none moved away from kelp into adjacent open water.
4) In LD 14:10 in aquaria, significantly greater
numbers of mysids were observed in open water during
darkness than during light (PTT, p.05).
5) In DD A. sculpta exhibited an endogenous circadian
behavioral pattern of close association with kelp
during the subjective day and movement away from
kelp during the subjective night.
The nocturnal abundance of zooplankters and the
protection against predation which darkness affords
probably combine to make A. sculpta's nighttime
movement away from kelp highly adaptive behavior.
Works Cited
Banner, Albert H. 1948. A taxonomic study of Mysidacea
and Euphausiacea (crustacea) of the northeastern
Pacific. Transactions of the Royal Canadian Institute,
20: 97-102.
Clarke, William D. 1971. Mysids of the southern kelp
region. pp. 369-380 in Wheeler J. North ed. The
biology of giant kelp beds (Macrocystis) in California.
Verlag Von Cramer. Germany.
Hobson, Edmund S. and Chess, James. 1976. Trophic
interactions among fishes and zooplankters near
shore at Santa Barbara, California. Fish. Bull. U.S.,
74: 567-598.
Limbaugh, Conrad. 1955. Fish life in the kelp beds and
the effect of kelp harvesting. University of
California Institute of Marine Resources, 55-59:
134.
Miller, W. Allen. 1976. The effects of giant kelp on the
distribution of plankton and nutrients. Unpubilshed
Manuscript. Carleton College, McAllistir, Minnisota.
Sokal, Robert R. and Rohlf, F. James. 1969. Biometry
and practice of statistics in biological research.
W. H. Freeman and Company, San Francisco. 608-620.
Tattersall, W.M. and Tattersall, Olive. 1951. The british
Mysidacea. Ray Society. London. 408.
10
Acknowledgments
I wish to thank my advisors Charles Baxter and
Lawrence Harding for their guidance, fellow students
for helping with plankton tows during all hours of the
night, and Carl Johnson and Richard Roux for extricating
me from Monterey Bay after one such ill-fated voyage.
Figure 1
Figure 2
Figure 3
Figure 4
11
Illustrations
Mysids Observed/Zooplankton Sample
Versus Time
% Total Population in Open Water
(LD and LL)
Versus Time
% Total Population in Open Water
(DD EXP. 1)
Versus Time
% Total Population in Open Water
(DD EXPS. 2 and 3)
Versus Time
Figure 1
Mysids Observed/Zooplankton Sample versus Time
The mean number of mysids detected in three
replicate surface tows of a 333um mesh zooplankton net,
0.5M in diameter is graphed versus time for two
different studies. In the rowboat study each tow lasted
three minutes and tows were made only in open water 10-
20M from the kelp forest (boxed points). In the diver
study, 50M tows were made through the kelp canopy (points
which are not boxed) as well as through the open water.
Vertical bars indicate standard error.
v

Ee


DD
o
r
—

MYSIDS/ SAMPLE
S

r
20
20
e
14
Figure 2
% Total Population Observed in Open Water (LD and LL)
Versus Time
The % of the total mysid population observed in
open water in aquaria (as opposed to a section of each
aquarium which contained kelp) is graphed versus time
for separate studies which used different light regimes.
In the light-dark (LD) experiment a 14:10 cycle was used
and one tank was lit with natural light which diffused
through a laboratory skylight while another tank was lit
artificially. In the graph of constant light (LL)
experimental results vertical bars denote standard error.
All LD and LL aquaria contained 100 mysids and were
exposed to a light intensity of 10-20 footcandles.
POP.

IN OPEN WATER

0


—
C
16
Figure 3
% Total Fopulation in Open Water (DD EXP. 1)
Versus Time
The % of the total mysid population observed in
open water in aquaria (as opposed to a section of each
aquarium which contained kelp) is graphed versus time
for three replicate tanks which each contained 100
mysids and were exposed to constant darkness (DD).
Vertical bars denote standard error.
4
+
%
POP.
IN
OPEN
WATER
f




2
18
Figure 4
% Total Population in Open Water (DD EXPS. 2 and 3)
Versus Time
The % of the total mysid population observed in
open water in aquaria (as opposed to a section of each
aquarium which contained kelp) is graphed versus time for
two constant dark (DD) experiments. In experiment 2 there
were 200 mysids/replicate while in experiment 3 there
were 100 mysids/replicate. Three replicates were used
in both experiments. Vertical bars denote standard error.
2

—



r1
POP.
IN


OPEN
WATER
—