AGGRESSIVE BEHAVIOR IN, ADULT LONGEVITY OF
AND ANATOMY OF MOUTH PARTS OF TIGRIOPUS
CALIFORNICUS BAKER (HARPACTICOID COPEPOD)
James Edward Tearse
Dr. Isabella Abbott advisor
Problems in Marine Bioloby
Biology 175H
Hopkins Marine Station
Stanford University
Spring 1977
SUMMARY
Simple behavior responses in the presence of the same or a
different sex appear to take place in Tigriopus californicus Baker
among males and appears to be inversely related to surface area
and not volume. Among males, encounters including touching for longer
than two seconds and copulating pairs with another animal whether male
or female were common. Females alone showed no antagonistic behavior.
In longevity studies, gravid females outlived non-gravid females and
outlived males, the latter dying in half to greater than half the
period shown by gravid females.
Finally, an anatomy of mouthparts is shown in illustrations
drawn to scale, and indicates distinct differences in a number of
structures that may be of taxonomic significance.
INTRODUCTION
Tigriopus californicus Baker, a harpacticoid copepod, occurs in
high tidepools along the California coast and is noted for its ability to
withstand inhospitable environmental conditions (1). Due to evaporation
and tidal wash, densities vary greatly. During observation in the
laboratory, males displayed aggressive behavior when in very dense
conditions with other males. Few behavioral studies have been conducted
on Tigriopus californicus and aggressive behavior has not been investigated.
The present study was to determine whether this aggression is density
dependent, as a function of volume or surface area, and if it is a
male and/or female trait.
Under starvation conditions there is a differential sex survival.
In earlier longevity studies, Egloff (1966) showed that the mean life
expectancy for males was significantly less than for females. He did
not compare the differential longevity of females to gravid females.
There would be an adaptive survival significance to the species if the
gravid females were hardier since they carry the future generations. This
possibility was investigated in the present study.
The anatomy and mechanism of the mouth parts was also studied as
this knowledge is necessary for the complete understanding of feeding in
Tigriopus californicus. Also, with an accurate study of the anatomy, one
can compare these appendages to those of other Tigriopus species. In
arthropod taxonomy such fine details of anatomy are part of determining
whether these species are different.
This paper investigates aggressive behavior, longevity and illustrates
mouth parts.
Tearse, Tigriopus californicus, Aggressive Behavior
Aggressive Behavior as a Function of
Density in Tigriopus californicus Baker
MATERIALS AND METHODS
Tigriopus californicus were collected from a high tide pool just south
of Mussel Point in Pacific Grove, California. Sexed under a dissecting
scope, males with claspers on their first antennae and females with
egg sacs were used. Animals kept in the laboratory longer than 24 hours
were fed Monochrysis lutherii, a flagellate. Tigriopus japonicus can live
longer than eight generations on Monochrysis lutherii (2) Attempts were
made to simulate the natural crowded conditions in evaporating high tide
pools. To check on the effect of serial dilutions, some animals were
run in sequences of decreasing density, as would occur when the tide
pool fills with splash. Since Tigriopus swims, changes in volume density
were tested. Narrow vials (with 3.1 cm bottom surface area) containing
2ml sea water to which were added 100 males, 100 females and 50/50 males
and females were observed at 20°C with side lighting during a 2 hr period.
They were diluted sequentially to 4ml,8ml, and 16m1. The control was a vial
(with 3.1 cm bottom surface area) of 100 males at each density, this
would check for any volume density dependence. The bottom surface area
was held constant to specifically focus on volume.
Tests for surface area effect duplicated the above procedure except
the volume was held constant at 16ml while the bottom surface area was
increased from 3.1 cm to 10.8 cm and then 14.5 cm'.
Scoring was done during 2 min- periods at the beginning, middle and
end of each 2 hr test period. Scoring was on the basis of 1)encounters,
copepods longer than 2 seconds
defined as interactions of 2or
involving movement, and excluding copulating pairs, 2) tail clasping,
Tearse, Tigriopus californicus, Aggressive Behavior
defined as a male clasping the caudal rami of another Tigriopus and
being pulled, 3) copulating pairs, in which a male was attached dorsally to
the cephalothorax of another Tigriopus, male or female. Qualitative
observations were made of bumping into or crawling over other animals
and short burstwhere violent swimming shorter than two seconds was
observed and interpreted as getting out from under other Tigriopus.
RESULTS
Encounters were seen as two males touching ventrally and
swimming, usually in tight circles. Also, they were observed as a few male
Tigriopus aggregating on another male Tigriopus and jerking at him.
The encounters were only found in vials with males present and never in
the female vial. They were never observed to last longer than 30
min. Bumping and short burst observed in all containers decreased
qualitatively with increasing surface area, while remaining constant
with increasing volume. Only in male vials was a decrease in the mean
number of encounters per 2 hr period with increasing bottom surface area
found (Graph 1). The only statistically significant drops in the number
of encounters were in the sequentially run males from 3.1 cm“ to 14.5 cm
(p 0.025) and in the fresh males from 3.1 cm to 10.8 cm (p 0.025).
Other data appear in the graph for comparison.
Copulating pairs occurred in vials containing males and males plus
females, but never in the female vials. Copulating pairs were observed
between 2 males (maximum 12 sightings during a two hour test period)
Tearse, Tigriopus californicus, Aggressive Behavior
even with gravid females present. Also males attempted the copulating
pair position with females. But, gravid females sucessfully shook off
males grabbing onto her egg sac or attempting to copulate. Copulating pairs
formed of a male and an egg-bearing female were observed in other
populations of Tigriopus californicus.
Greater than 87% of all encounters involved 2 animals (Graph 2).
Tail clasping, counted as an encounter involving 2 animals, comprised 147
of the total encounters.
When the vials were undisturbed for 1 hour, greater than 70% of the
populations, including females, were resting or crawling on the bottom.
DISCUSSION
Encounters are solely a male aggressive trait and have an inverse
(Table 1)
dependence on surface area but not on volume./ General observations
show that Tigriopus is mainly benthic. Hal Townsend (unpublished) has
found that in a nine cm water column under 110 LUX light conditions,
approximately 50% of the active populations of Tigriopus californicus
is within the bottom quarter of the water column.
Decreasing the probability of bumping, a preliminary to an encounter,
has a direct effect of decreasing the probability of encounters.
Increasing the volume does not significantly decrease the amount of
bumping, whereas increasing bottom surface area does. Therefore, to decrease
the number of encounters it is more effective to increase the bottom
(Graph 1)
surface area than to increase the volume./ Since the probability of two
animals bumping is so much greater than three or more at the same time,
Tearse, Tigriopus californicus, Aggressive Behavior
it follows that the frequency of encounters involving two animals is
greater than three or more. (Graph 2)
Males apparently will form copulating pairs with males, not
limiting themselves to females. This indicates that males will
try to form copulating pairs with anything. To the author tail clasping
could be an unsucessful attempt at the copulating pair position. In this
study the maximum of 12 sightings of male-male copulating pairs could
be the same four pairs counted at each scoring. Encounters, since they
terminated within thirty minutes, did not have this scoring problem.
The egg sac-carrying females were very defensive, protecting
their egg sacs and avoiding the copulating pair position. This would
improve the survival chances of the eggs, as well as reduce the time wasted
by a male trying to copulate with a gravid female. In other populations
of Tigriopus, female egg sacs have been eaten while still on the female.
Also, a male copulating with a gravid female is wasting his efforts since
one copulation can result in insemination of up to 12 broods (3).
Tearse, Tigriopus californicus, Adult Longevity
Longevity in Adult Tigriopus californicus
Baker Under Starvation Conditions
METHODS AND MATERIALS
Ninety-nine adult Tigriopus californicus were collected from a
high tide pool on Mussel Point, Pacific Grove, California. Under a
dissecting scope they were sexed for 1)males with claspers on first antennae,
2) females, 3) females with eggs visible in ovarian tubes or egg sacs
(labelled gravid females). To prevent cannibalism, they were separately
put in 1-2 ml filtered sea water (.45mWhatmanand GF/C fiber glass
filters). The water and container was changed every second day to prevent
oxygen depletion and algal growth. They were given no food and exposed
to laboratory conditions (15-22°C). Death was determined under a
dissecting scope as no movement, external or internal.
RESULTS
Fifty percent longevity occurred at 6.8 days for males, 7.4 days
for females and 13.3 days for gravid females. Mean male life expectancy
was 7.3 days. By the end of the experiment, 1002 death had not occured
in the females or gravid females, precluding calculation of the mean
life expectancy. By probit analysis and linear regression, the curves were
shown to be statistically different. Males + Females (p«0.002),
Females + Gravid Females (p«0.02), Males + Gravid Females (p«0.000001).
Tearse, Tigriopus californicus, Adult Longevity
DISCUSSION
Gravid females outlived non-gravid females, and both outlived
males, the latter dying in half the period shown by gravid females.
This trend of longer living females agrees with Egloff's study (1966)
where he found the mean life expectancy for starved males to be 9
days (23 C) or 14 days (15°c) and for starved females to be 33 days
(23°C and 15°C); whereas when fed, the mean life expectancy for males
was 66 days (23°C) or 120 days (15°C and for females was 33 days (23°c)
or 48 days (15°C) for females.
The present study's mean life expectancy for males of 7.3 days
compares well with Egloff's mean of 9 days. The seven to eight fold increase
shown by Egloff in mean life expectancy when males are fed, would
reflect an adaptive strategy. Under low food conditions, a short male
life wherein males can perform their primary role of insemination
would minimally deplete nutrient supplies needed by gravid females.
When food is plentiful, the tide-pool can support a larger population
allowing males to live longer. In this study, non-gravid females also
show this adaptive strategy of minimally depleting food supplies.
The longevity of starved gravid females can be postulated to
result from 1) eating the hatched nauplii or retracting nutrients from
the egg sac (i.e., food source) 2) having more food reserves. This
differential survival would result in populations being skewed toward
gravid females in tide pools with too little food.
Tearse, Tigriopus californicus, Anatomy of Mouth Parts
Anatomy of Mouth Parts
MATERIALS AND METHODS
Mouth parts of Tigriopus californicus collected from
Mussel Point, Pacific Grove, California were dissected off, mounted
in polyvinyl alsohol, and immediately drawn. A micrometer in the
compound scope was used for measurements. All parts are drawn to scale.
Drawings were checked with mouth parts of other Tigriopus californicus
to insure accuracy. Also, drawings were compared to limited electron
micrographs of whole Tigriopus californicus. Fecal pellets were examined
for kinds of food in order to estimate the use of different mouth parts.
ANATOMY OF THE MOUTH PARTS
Understanding the anatomy of the mouth parts is a preliminary
step in determining the mechanism and function of them.
A composite drawing shows the orientation of the mouthparts. (Figure 1)
Some have been left out for clarity.
The labrum is a wide flap which terminates in 4 chitinous teeth
and covers the mouth and part of the mandibles. (Figure 2)
The mandible is a long arm ending with 2 rows of teeth. The smaller
teeth are next to the labrum. The mandible has a biramous palp with four
spines and 11 slender setae. (Figure 3)
The first maxilla has 7 basal spines forming a cradle with which to
shove food forward. It has sensory setae on the side palps with a
numbering of 2-3-4-2(?). (Figure 4)
The second maxilla has a large claw with two pectinate setae at the
Tearse, Tigriopus californicus, Anatomy of Mouth Parts
distal end of this stout structure. There are four sets of palps,
two with three hairy spines; each in line proximal to the claw. The last
two are side by side at the base of the appendage.
The maxilliped consists of a dactylus with one spine at the
distal end of the massive second joint. The setae of the second joint are
bipectinate.
MECHANISM AND FUNCTION
Macrocylops albidis Jurine, a cyclopoid copepod is a related
crustacean whose feeding mechanism has been described (Fryer 1957)
Applying Fryer's interpretation, a general idea of the feeding mechanism
of Tigriopus californicus can be derived.
In describing the appendages, Fryer states
... the most important movements of the appendagea are
those of adduction and abduction... and consist essentially
of a lateral swing through 90°. (Fryer, pp. 7-8)
The labrum which usually "hangs" away from the mouth can be brought
up to cover the mouth. The teeth at the terminal end can then help to
hold the food material while other appendages work on it.
The mandible has three types of movements. There is a rocking motion
of both appendages in the horizontal plane. The cutting motion is a
vertical movement with the dentitures of the mandible crossing. And
oscillating alternately increases and decreases the horizontal distance
between the mandibles. The movements are intermittent in nature and
are useful for tearing the food into manageable pieces. The Tigriopus,
as compared to the cyclopoid mandible, seems to have more of a masticating
Tearse, Tigriopus californicus, Anatomy of the Mouth Parts
motion with the two rows of teeth. Large sections of exoskelton seen in
fecal pellets give evidence that mastication is not thorough, and pieces
are ingested intact.
The role of the other appendages is best described by Fryer.
As food is forced into the mouth the maxillules (lst maxilla)
momentarilly release their hold, move back and again grip the
food, thus drawing the entire mass forward. In this they are
usually assisted by brisk pushing movements of the maxillae (2nd
maxilla) and maxillipeds. As one part of the body of a
chironomoid larva is eaten , a feeding copepod sometimes
moves along to a new region, using the maxillules, maxillae, and
maxillipeds for this purpose much as human hands move along a
rope. (Fryer p. 11)
... the... role... of the more posterior appendages ...(is)
to push food into the mouth. (Fryer , p. 14)
COMPARISON OF THE MOUTH PARTS
The anatomy of the mouth parts have individually been described
in Tigriopus by Sars
(1911) on T. fulvus of Norway and by
Mitakidis: (1949) of T. lilljeborgii in Whitely Bay, England.
A comparison of these with that of Tigriopus californicus shows the
similarities and differences (Table 2). Enough dissimilarities in mouth
parts are present to indicate that these are three related but distinct
species.
Tearse, Tigriopus californicus
12
ACKNOWLEDGEMENTS
I wish to thank Dr. Isabella Abbott for her advice and guidance in
all areas of this project. Also, thanks go to Dr. Donald P. Abbott for
his help with dissection, Dr. Robin Burnett for his help with statistics,
and Lynn Hodgson for her relentless help and encouragement.
Tearse, Tigriopus californicus
LITERATURE CITED
Baker, (1912) lst Annual Report, Laguna Marine Laboratory, pp. 108-109.
(3) Egloff, D. A. (1967) Biological
aspects of sex ratio in
experimental and field populations of the marine copepod Tigriopus californicus,
Stanford University, Ph. D. Dissertation.
Fryer, G. (1957) The feeding mechanism of some freshwater cyclopoid
copepods, Zoological Society Proceedings, Vol. 129, pp. 1-25.
(1) Kontogiannis, John E. (1973) Acquisition and loss of heat resistance in
adult tide-pool copepod Tigriopus californicus, Physiol. Zoology, V. 46,
p. 50-54 .
Mistakidis, M. (1949) A New Variety of Tigriopus lilljeborgii Norman,
Dove Marine Laboratory Report, 3rd. Ed., No. 10, p. 55.
(2) Provasol, L., K. Shiraishi, and J.R. Lance (1959) Nutritional idiosyncrasies
of Artemia and Tigriopus in monoxenic cultures, Ann. New York Acad. Sci. Vol.
77, pp. 250-261.
Sars, G. O. (1911) Crustacea of Norway, Vol. V, p. 54, Pls. XXI,XXXII.
Townsend, Hal, Unpublished results, Hopkins Marine Station.
Tearse, Tigriopus californicus, Aggressive Behavior
TABLE 1
Occurrence of Encounters
Volume
n= 3 runs
volume males females males/females males fresh
1007
2m1
1002
1007
4m1
1002
1007
1902
1002
1007
1007
8m1
1002
1007
1007
16m1
Surface Area
n= 4 runs
area males females males/females males fresh
1007
3.1 cm 1002
07
507
757
757
10.8 cm 757
757
14.5 cm 757
07
507
Tearse, Tigriopus californicus, Aggressive Behavior
GRAPH 1
Occurence of Encounters
There is a significant dependence of encounters on surface area
in all male tubes. Sequentially run males from 3.1cm2 to 14.8 cmP,
Fresh males from 3:lem2 to 10.8 cm?.
Sequentially run males —
Fresh males
X-X
9....0
Males plus females
4
3
12
E 10
O 8
N7
6
S 4
2





L
2 4
16
VOLUME IN ML
15a
N6
C5
U4
k

—
10.8 14.5
AREA IN CM
15b
Tearse, Tigriopus californicus, Aggressive Behavior
GRAPH 2
Frequency of Encounters
Greater than 87% of all encounters involed only 2 Tigriopus.
Sequentially run males
Fresh Males
Males plus Females
L
I
L
ax
u

0

38890
38o
- 2OUO
X


O
Tearse, Tigriopus californicus, Adult Longevity
GRAPH 3
Adult Longevity in Tigriopus californicus
Gravid females outlived non-gravid females and outlived males,
the latter dying in half to greater than half the period shown by
gravid females.
Gravid Females + Females (p«0.02)
Gravid Females + Males
(p40.000001)
Females + Males
(p60.002)
Gravid females
g..
Females
-
Males
D


—
N
O
r
O
O


—
N

—

—
O
o


oo
m-

o



5
Tearse, Tigriopus californicus, Anatomy of Mouth Parts
Figure 1 Composite of Mouth parts- Some appendages omitted for clarity.
Man-Mandible, lst Mx- First Maxilla, 2nd Mx- Second Maxilla
Mp- Maxilliped, Lab- Labrum
Figure 2 Labrum — Ventral View
Figure 3. Right Mandible- Ventral view
Figure 4. Left lst Maxilla- Ventral View
Figure 5. 2nd Malillae- Ventral view, with Anterior View below
Figure 6. Maxillipeds- Ventral view with Anterior View below
MAN-
IST MAX
2ND MAX

l



—







T
.





MP
50um




LAB
20
FI
N
50um
A
15um


A


50um
50um
2
23
A








b-



2

50um
6
Z


/


50um


S





A
26
Tearse, Tigriopus californicus, Anatomy of Mouth Parts
Table 2
MANDIBLE
T. lilljeborgii (Mistakidis, 1949)
a. A slender pectinate seta at the base of the smaller teeth.
b. Biramous palp with 12 slender satae
c. Out of five slender spines, only four with fine hairs. (Pl. I, fig 6)
T. fulvus (Sars, 1911)
a. No pectinate seta at the base of the teeth.
b. Biramous palp with 9 slender setae (Mistakidis says perhaps 10,
Sars' figure not clear)
c. 5 slender spines with hairs (Pl. I, fig. 6)
T. californicus (Baker, 1912)
a. No seta at the base of the smaller teeth.
b. Biramous palp with 11 slender setae.
c. 1 slender spine no hairs. (Fig. 58B, p.108)
T. californicus (This study)
a. A slender pectinate seta at the base of the teeth.
b. Biramous palp with 11 slender setae.
c. 1 slender spine with no hairs.
Tearse, Tigriopus californicus, Anatomy of Mouth Parts
Table 2 continued
lst MAXILLA
T. lillijeborgii (Mistakidis, 1949)
a. The number of setae on the lobes is 4-3-5-3 (4?) (Pl. II, fig. 7)
T. fulvus (Sars, 1911)
a. The number of setae on the lobes is 4-3-3-3 (Pl. XXXI)
T. californicus (This study)
a. The number of setae on the lobes is 2-3-4-2 (? on last 2)
2nd MAXILLA
T. lilljeborgii (Mistakidis, 1949) (lst Maxilliped)
a. Middle lobe with 3 spines. (Pl. II, fig 8)
T. fulvus (Sars, 1911)
a. Middle lobe with 2 spines (Pl. XXXI)
T. californicus (This study)
a. Middle lobe with 3 spines.
27
Tearse, Tigriopus californicus, Anatomy of Mouth Parts
Table 2 continued
MAXILLIPED
T. lilljeborgii (Mistakidis, 1949)
a. Dactylus with 2 inequidistant spines.
b. Seta of 2nd joint bipectinate (Pl. I, fig. 9)
T. fulvus (Sars, 1911)
a. Dactylus with 1 spine.
b. Seta of second joint smooth. (Pl. XXXI)
T. californicus (Baker, 1912)
a. Dactylus with 1 spine.
b. Seta of second joint smooth. (Fig. 59B, p. 109)
T. californicus (This study)
a. Dactylus with 1 spine.
b. Seta of second joint bipectinate.
28