INTRODUCTION
Idotea resecata is a marine isopod found in fairly quiet
bay waters along the California coast. Though much work has
been done on the appendages for the purpose of taxonomy
(e.g. Menzies, 1950), search of the literature reveals little
on the functions of the appendages in Idotea. The more ob-
vious functions such as the use of the peraeopods for loco¬
motion have been assigned to the appendages (Schultz, 1969).
but details of the activities have not been described in
Idotea with the exception of feeding activity (Green, 1957
and Cruz, 1963).
In the kelp beds adjacent to Mussel Point, Pacific
Grove, numerous individuals were found which were missing
and regenerating appendages. Regeneration of the appendages
in Idotea has been described by several investigators
(Boussat, 1958; Varèse, 1960; Glaizal, 1962). However, nothing
has been mentioned about compensatory behavior following
amputation of an appendage.
I decided to examine function of selected appendages
in intact animals and in animals with selected appendages
removed. Observations on regeneration were recorded as
opportunity arose during the study.
I. resecata is a very hardy animal, survives well in the
lab and is very suitable for studies on appendage utilization
and regeneration. The appendages studied were the first
and second antennae, the peracopods, the uropods and the
pleopods. For this investigation, the peraeopods were
considered to fall into groups on the basis of orientation
and function: group one includes only the first peraeopods;
group two includes the second, third and fourth peraeopods
group three includes the last three pairs of peraeopods.
MATERIALS AND METHOD
Animals to be used for laboratory studies were collected
on Macrocystis and maintained on plants held in running sea
water aquaria at the Hopkins Marine Station of Stanford Uni-
versity.
Studies of swimming and of activity on kelp were made
in large aquaria. Ventrally placed structures were viewed
from below in a glass tube of 1.5 cm in diameter and 1 m
in length. Activity of animals on kelp blades was studied
under a dissecting microscope as well as in large tanks.
Removal of the appendages was accomplished by methods
described by Schneiderman (1967), using fine forceps and very
sharp manicuring scissors. Animals were first lightly anes-
thetized using a solution of MgCl, isotonic with sea water.
Their bodies were wrapped in damp cheesecloth to limit
movement, leaving only the area to be operated on exposed.
All operations were performed under a dissecting microscope.
For most appendages studied other than peraeopods (e.g.,
the first antennae, the second antennae, etc.) the appendage
was removed unilaterally in some specimens, bilaterally in
others. Three animals were operated on for each condition.
For peraeopod groups two and three, either one, two, or all
three pairs of appendages were removed. Here, too, operations
were performed on three animals for each condition.
After surgery, animals were kept in 5 inch finger bowls.
three animals to a bowl. Bowls were provided with running
sea water at 13-14° C. and covered with cheesecloth to pre-
vent the animals from escaping.
Macrocystis was added to the
bowls for food. Bowls were cleaned out every two or three
days to remove wastes and replenish the food supply.
UTILIZATION OF APPENDAGES
The First Antennae
The first antennae are difficult to study because of
their small size and rather rapid movements in the normal
animal. They are used in chemoreoption and contain aesthetascs
on the outer segment which are the sensory structures (Schultz,
1969). Aesthetascs face forward amd the two appendages are
moved alternately.
In those animals with only one first antenna removed,
no change in behavior was noted. Removing both first anten-
nae resulted in a decrease in locomotary activity; the animals
remained in a stationary position and were non reactive to
prodding by the experimenter.
The. Second Antennae
The second antennae have five peduncular segments and
a number of segments in the flagellum which increases in di-
rect proportion to the size of the animal (Menzies, 1948).
In adult animals 20-25 mm long the number of flagellar seg-
ments ranged from 14 to 16. In animals 9-15 mm long there
were 8-10 segments.
According to Schultz (1969) the second antennae are mainly
tactile sensory structures. My own observations affirm they
are primarily used for probing the environment. The segments
of the peduncle are bent easily; notches in the margins of
segments allow for increased movement between adjoining seg-
ments (fig. 17). The flagellum, being composed of many seg-
ments, has great flexibility which enables the antennae to
explore the environment very efficiently. Physical inter-
faces, e.g. the air-water interface, are detected by the
second antennae. When the interface is contacted, the animal
alters its behavior, i.e. in the case of a swimming isopod,
the animal changes direction (fig. 1).
When the animal is stationary on a kelp leaf, the anten-
nae are held in a forward position but angled approximately
40° from the anterior-posterior axis. At rest on the stipe
of Macrocystis the second antennae straddle the tubular stipe
for balance or for gripping the stalk (fig. 2).
When the animal is walking on kelp, the antennae swing
about irregularly in an arc (fig. 34). There is no set pattern
relating the behavior of the antennae to one another in the
probing, but they never contact each other. An animal about
to go around the edge of a kelp blade from the upper to lower
surface, first bends the antennae to probe the under surface
(fig. 4); following this it may move. In swimming, the anten-
nae are extended straight forward, allowing for smoother
fluid flow about the body. Sometimes the extreme outer
segments of the flagellum are pointed outward. The animal
may be sonsing the presence of a solid surface and is poised
for contact (fig. 5).
The second antennae are fastidiously groomed by the first
peraeopods (fig. 6). Material on the antennae of I. resecata
living on Macrocystis consists of mucus from the kelp and
adhering debris. These appear to be acceptable food sub-
stances, and the second antennae thus play a role in feeding
by serving as collecting devices. Both antennae are regularly
cleaned and the material is brought to the mouth parts. Fur-
ther details are discussed in relation to the first peraeopods.
A specific aspect of behavior was seen in a laboratory
aquarium which indicates the second antennae may participate
actively in another way in bringing detritus to the mouth
parts. Much detritus, mainly Macrocystis debris, had accum-
ulated on the bottom of the tank. On several occasions when
an I. resecata came to rest on the bottom in the detritus,
the second antennae which weré initially extended were pulled
in toward the cephalon as in grooming. A mound of debris was
accumulated in one stroke, and the first peraeopods grabbed
portions of this and transported it to the mouth parts.
The second antennae serve as a pivot point for the body
in certain situations as in righting. An isopod lying on its
back arches its body until only the telson and the tips of
the second antennae remain in contact with the substratum.
The arched position attained is unstable and usually the
animal is able to flip over to one side and right itself
(fig. 7).
The second antennae also act as a pivot point when an
animal alights on a stipe of kelp. As the animal swims the
second antennae are extended anteriorly. Upon contacting
a solid surface the antennae are spread apart and provide
a pivot point as the anterior part of the body swings toward
the surface. The first three peraeopods are used to grab
the substratum and establish a hold (fig. 8).
Removal of one antenna affects the behavior of the ani-
mal but not drastically. Interfaces are still detected.
In walking, the animal compensates by increasing the arc
through which the remaining antenna is swung (fig. 3B).
In swimming, animals with one antenna missing swim in a
curving course toward the side of the remaining antenna in
80% of the cases observed. The intact antenna and the stump
are groomed. In righting themselves the animals arched their
bodies and flipped over to the side lacking the antenna.
Removal of both antennae had more noticeable effects.
In swimming, the air-water interface was not perceived until
the cephalon of the isopod was almost entirely out of the
vater. There is less variation of swimming patterns in these
animals. They tended to swim in straight lines, exhibitihg
little maneuverability. Righting was difficult to achieve.
The Peraeopods
Each peraeopod is composed of six segments. All per-
aeopods are subchelate and biunguilate. Except for the first
peraeopods, they are mainly locomotary and clinging appendages
and the biunguilate condition is a modification for walking.
The first peraeopods are used more like hands, and are closely
associated with the mouth parts.
The number of setae decreases as one goes from the first
to the seventh peraeopods. The first peraeopod has three
main clusters of setae, each centered around a large file
seta. The second, third, and fourth peraeopods each have two
clusters of setae and the last three pairs of legs have only
one main cluster.
As mentioned previously; the first peraeopods are used
in grooming the second antennae (fig. 6). The animal places
one of its antennae betwéen the dactylus and the propodus
of the two first peraeopods and then pulls the antenna through
from base to tip. The three large file setae and their clusters
may be acting as food strainers. Mucus and debris are scraped
off and brought to the mouth parts where the material is mas-
ticated. This behavior may be regular, with the animal repeatedly
cleaning off first one antenna and then the other. If the
food is rejected by the animal the material is removed from
the mouth parts by the first peraeopods. The first peraeopods
also actually hold chunks of food for the mouth parts to
10
chew on. If one of the first peraeopods is removed, the
second peraeopod on the same side takes over the job of groom-
ing the antennae, though this is done somewhat inefficiently,
often leaving the outer segments of the flagellum untouched.
Removing both first peraeopods is compensated by the second
peraeopod pair taking over the role of the first.
The second peraeopods are well endowed with setae and
are used in grooming the first peraeopods when the latter are
burdened with debris. In an animal missing one of the second
peraeopods, e.g. the left, the animal compensates by bringing
the left first peraeopod to the right second peraeopod to be
cleaned.
The second, third, and fourth peraeopods are grouped to-
ion. The
gether in part because of their anter
other
basis faces forward in the restin
segments are bent at an angle towards the surface of the kelp.
The fifth, sixth, and seventh peraeopods, on the other hand,
are oriented posteriorly. The basis faces toward the rear
and the other segments are usually aligned with the basis
in a resting animal.
On a stipe of Macrocystis the fifth through the seventh
peraeopods are used mainly for gripping the stipe. Often
the fourth pair joins the other three pairs. This leaves
the first three peraeopods free to groom and carry out
feeding activity. This separation of function brings to mind
the more extreme case of the arcturid isopods.
11
The second through the fifth peraeopods are the major
ambulatory legs. Locomotion is ditaxic with the body remaining
fairly rigid (fig. 9). The unguis is used to establish
anchorage in ambulatory movement. If the unguii are removed
from all of the peraeopods the animal cannot carry out nor-
mal walking, and only inefficient slow gliding movement re-
sults. While the animal is walking, the first peraeopods are
extended in quick, alternating movements in front of the body,
acting like probes. The sixth and seventh peraeopods are
normally inactive in slow walking. Their activity increases
with the speed of movement. Monotaxic movement is sometimes
exhibited by the appendages, with the frequency of this in-
creasing as one goes from the seventh to second peraeopods.
The first legs have never been observed moving monotaxically.
In backward motion the sixth and seventh peraeopods play
more of a role, moving ditaxically; their posterior orientation
The animal also employs
allows this to be done efficiently.
these two pairs of legs extensively in turning around.
In fast swimming the peraeopods are all pulled in close
to the body giving a streamlining effect (fig. 10). The first
three pairs are oriented forward, dactylus out, poised for
grasping. The fourth through the seventh legs are oriented
posteriorly with the fourth pair bent at the basal-ischium
joint and the last three pairs aligned straight back. To
slow forward movement in swimming the last three peraeopods
are extended laterally. Similarly in slow swimming the last
three pairs of peraeopods are extended laterally perpendicular
to the body where they act as water brakes, disrupting the
flow of water about the body (fig. 11).
The braking phenomenon is also observed when an isopod
falls freely through the water. This was seen in isopods placed
in the glass observation tube and also in aquaria. The animals
are not long term swimmers and after a few swimming excursions
they appear to tire. When a tired animal is placed in the water
it falls, making no effort to swim, but the fall is retarded,
and balance is maintained, by the lateral perpendicular exten-
sion of all the peraeopods except for the first pair (fig. 12).
Once the animal has rested, swimming my be resumed again.
The first, second, and third peraeopods are used in a
swimming animal to grab an algal stipe when the animal initially
contacts it. As soon as the hold is established, resulting
in a stable position with the last four peraeopods firmly
gripping the stipe, the first peraeopods are pulled up to
assume the role of aiding the mouth parts once more.
Removal of the first peraeopods does not affect ambulatory
motion. When the second peraeopods are removed, the animal
compensates by using the first peraeopods as locomotary ap-
pendages. The first peraeopods become much more important
for locomotion if peraeopods two and three, or two to four
inclusive, are removed. The animal also compensates for lack
13
of support in the peraeon segments by arching its body. In
w-alking there is increased pleopod action to give a forward
thrust. The animals have much difficulty in moving around
the edge of a kelp blade from the upper surface to the under-
side; the anterior appendages are clearly needed to establish
the initial hold.
Removal of the fourth or the fifth peraeopod results in
difficulty as far as turning around on a stipe is concerned.
These center peraeopods are used as pivot points in an intact
animal, and the animal must compensate for their absence by
moving the pivot point forward or backward to a less favorable
location on the body.
Removal of two of the last three pairs of legs impairs
gripping of the substratum, but the animal can still hold on
reasonably well with just the seventh pair. If the last
three pairs are removed the animal has difficulty in holding
onto a stipe or blade of kelp.
In swimming, the animal compensates for the loss of the
last three pairs of peraeopods by having the fourth peraeopods
assume the role of a water brake (fig. 13).
One peculiar use of the fifth, sixth, and seventh per-
aeopods is their role in the removal of the anterior molt.
When an isopod molts, the posterior half of the exoskeleton
is molted first; then after a period of approximately 24
hours the anterior molt is discarded. In removing the pos-
14
terior shell the animal merely attaches to the kelp with
the last three pairs of legs and then pulls itself out of the
old exoskeleton, segment by segment, in a series of undulating,
worm-like movements. Removal of the anterior shell is quite
different. The animal does not attach its anterior legs and
then walk backward out of its shell using the last three
pairs of legs. Instead, it bends its body sharply ventrally
and then the last three peraeopods grasp the anterior exo-
skeleton and are used to pull it off. This may demonstrate
the animal's tendencies always to move forward in locomotion
and also the importance of the second through the fourth per-
aeopods in accomplishing any ambulatory locomotion. Surgical
removal of the last three peraeopods causes the animal great
difficulty in removing the anterior molt. One such animal
took 6 hours to remove the anterior shell, whereas normally
it only takes 15 minutes.
The Uropods
Each uropod consists of two recognizable segments, a
proximal protopodite and a distal endopodite. The uropods
form valves covering the ventral side of the branchial chamber.
They serve to protect the pleopods, which are very delicate
structures and, in addition, they direct water flow through
the pleotelson.
Proper regulation of water flow in the branchial chamber
increases the efficiency of the pleopods in respiration and
15
swimming (fig. 14). Water currents are drawn into the chamber
when the pleopods are raised and water is forced out when
they are lowered. The uropods can regulate the aperture
through which the stream of water flows, thus controlling
the speed of locomotion by the Venturi principle.
The endopodite of the uropod is free to move separately
from the protopodite and thus is able to narrow or widen
the posterior aperture of the pleopod chamber. It is pos-
sible that the endopodites help steer the body in swimming
by acting as rudders. However, the right and left endopodites
have never been observed to move independently of each other
in a way suggesting steering control.
The animals with one uropod removed always swam on a course
curving toward the side with the remaining uropod. The water
stream is deflected off the uropod and toward the opposite
side, creating a force pushing the animal to the side with
the uropod remaining (fig. 15).
In animals lacking both uropods, the two rami of each
pleopod were more widely separated than usual, so the exopodites
extended laterally out of the normal chamber. This resulted
in less water being jetted out posteriorly, and the animals
without uropods swim more slowly.
16
The Pleopods
The pleopods are the five pairs of appendages of the
anterior pleonal segments. They are biramous, each pleopod
having a free endopodite and exopodite coming off the basis.
The second pleopod in the male is sexually modified, having
a style. This modification is common to all members of
Valvifera (Schultz, 1969). In addition, the first and second
pleopods and the exopodite of the third pleopod have hairs
around the periphery. This is the case in Sphaeromatids,
also (Schultz, 1969). Size of the pleopod increases going
from the first to the fifth pair and the length of the hairs
decreases in going from the first to the third pair.
The presence of the hairs suggests that the first and
second pleopods are natatory. The hairs themselves are na-
tatory in structure. In an individual hair, there is one
main root with small bristles branching off from it (fig. 16).
The fourth and fifth pleopods lack hairs and have very thin
membranes, suggesting that they play more of a role in respir-
ation than locomotion.
Animals lacking the first and second pleopods are not
able to swim. The remaining pleopods beat but no motion is
generated. Animals missing the fourth and fifth pleopods are
able to swim but the incidence of death is high (50%). Thus
it seems that there are specialized functional modifications
in the pleopods of I. resecata.
17
The pleopods also may be associated with circulation.
There is a direct correlation between the rate that the
pleopods beat and the rate of heart beat. When the heart con-
tracts the pleopods are extended ventrally and when the heart
expands the pleopods are brought back into the branchial
chamber. This implies that the pleopods are either aiding
in pumping the blood are moving as a consequence of the ex¬
pansion and contraction of the heart.
When the pleopods stop beating, the heart stops momentarily.
The heart starts beating again at a constant rate if the
pleopods remain stationary. This rate is maintained until
the pleopods start beating again. Then the heart beat rate
speeds up or slows down to coincide with the pleopod beat rate.
The pleopods may recommence beating before the heart does.
This suggests that the two structures are not enervated by
the same nerve.
18
REGENERATION
The sequence of events during regeneration of appendages
varies with the type of appendage, the level at which the
original appendage was lost, the extent of damage to the
tissues remaining within the stump and the stage of the an-
imalin the intermolt cycle. Since it was not possible to
monitor these factors in the time period of this study, only
a general descriptionof the process will be made. Regen-
eration was studied up to the third molt for the first antennae
and up to the second molt in the other appendages (see fig.
17-36). A typical sequence is given by a description of
the events in the regeneration of a peraeopod.
The ischium of the peraeopod is the amputation site.
Immediately after the operation bleeding occurs. This stops
shortly thereafter. A dark green scab is sometimes present.
Three to four days later an exoskeletal covering is formed
over the wound. The area beneath the wound site is white.
After the first molt a stump is formed. It is rounded
in appearance, unsegmented and colorless. The second molt
produces a peraeopod with the same number of segments as in
a normal limb. However, the segmentation is not complete,
the joints are not entirely movable and the setae are absent
(fig. 31). The regenerate résembles the juvenile peraeopod
described by Menzies (1948).
Removing a limb may have an effect on the molting cycle
of the animal. It was observed that in large animals 25-30
mm long the period between the first and second molts was
much shorter than the period between the operation and the
first molt. This suggests that amputation alters the molting
rhythm.
Regeneration was abnormal in the uropods and the pleopods.
Both were structurally distorted relative to normal appendages.
The uropod had setae on the posterior part of the protopodite
and were thickened (fig. 36). This thickening has also been
reported in Idotea baltica by Glaizal (1962). The regenerated
pleopods differed from the normal paddle shape in having a
very irregular surface (fig. 33).
20
SUMMARY
1. The marine isopod Idotea resecata is found in the fairly
quiet waters of Monterey Bay living on the brown kelp Macro-
cystis. The utilization of selected appendages of I. resecata
was studied.
The second antennae probe the environment and may be
used in feeding in conjunction with the first peraeopods.
They act as pivot points in righting and in alighting on kelr
while swimming.
The first peraeopods are used in grooming the sécond
3.
antennae in a manner suggesting a feeding mechanism. They are
associated with the mouth parts and are used for locomotion
only in compensation behavior.
4. The second, third, and fourth peraeopods are oriented
forward. Along with the fifth pair, they are the main ambu-
latory legs.
The first three peraeopods are used for grabbing kelp
5.
when the animal is swimming.
6. The last three peraeopods are used for gripping the
substrate and are used as water brakes in swimming behavior.
They are also used for removing the anterior molt.
The uropods direct water flow through the pleotelson.
The pleopods show functional differentiation. The first
two pleopods possess hairs and are natatory. The fourth and
fifth pleopods are probably used mainly for respiration.
9. The correlation between the rate that the pleopods beat
and the rate of heart beat suggests that the pleopods play
a role in circulation.
10. Observations on regeneration were done.
0
ACKNOWLEDGEMENTS
I would like to thank Dr. Donald P. Abbott for his
invaluable guidance on this project.
21
22
LITERATURE CITED
Boussat, M. 1958. Territoire de régénération des antennes
de l'Isopode Idotea baltica (Aud.). Compt. rend. soc.
biol. 246: 2530-2532.
Cruz, A. de la. 1963. Observations on the feeding activity
of the isopod, Idothea baltica (Pallas). Publ. Seto
ma. biol. Lab. 11: 165-170.
Glaizal, M. 1962. Sur la régénération de l'uropode du Crustace
Idotea baltica (Aud.). C. R. Acad. Sci. Paris. 254: 2452-
2453.
Green, J. 1957. The feeding mechanism of Mesidotea entomon
(Linn.) (Crustacea: Isopoda). Proc. Zool. Soc. Lond.
129: 245-254
Menzies, R. J. 1950. The taxonomy, ecology and distribution
of northern California isopods of the genus Idothea
with the description of a new species. Wasmann Jour.
Bio. 8(2): 155-195.
Menzies, R. J. and Waidzunas, R. J. 1948. Postembryonic
growth changes in the isopod, Pentidothea resecata
(Stimpson) with remarks on their taxonomic significance.
Bio. Bull. 95(1): 107-113.
Schneiderman, H. A. 1967. Insect surgery, p. 753 to 765.
In Wilt, F. H. and Wessells, N. K. (ed.) Methods in
developmental biology. T. Y. Crowell Co., New York.
23
Schultz, G. 1969. How to know the marine isopod crustaceans.
Wm. C. Brown Co., Dubuque, Iowa. 359 p.
Varèse, J. 1960. Sur la régénération des antennes de l'lsopode
Idotea baltica (Aud.). C. R. Acad. Sci. Paris. 250: 3399-
3340.
FIGURES
Fig, 1: Detection of the air-water interface by the second
antennae and consequent change in the direction of
swimming.
Fig. 2: Position of the second antennae of an isopod resting
on an algal stipe.
Fig. 3A: Arc of second antennae in walking isopod.
Fig. 3B: Arc of single antenna of walking, unilaterally
amputated isopod.
Fig. 4: Antennae probing the underside of a kelp leaf.
Fig. 5: Antennae position of swimming isopod.
Fig. 6: Grooming of the second antennae by the first peraeopods.
Fig. 7: Righting of isopod lying on its back.
Fig, 8: Sequence of movements involved in alighting on stipe.
Fig. 9-13: Positioning of peraeopods.
Fig. 9: Walking
Fig. 10: Fast swimming.
Fig. 11: Slow swimming, showing water braking by the last three
peraeopods.
Fig. 12: Free fall through water.
Fig, 13: Slow swimming in isopod missing the last three peraeopods.
Fig. 14: Water flow through the pleotelson. Arrows indicate flow.
Fig. 15: Water flow through the pleotelson in isopod with only
one uropod. Arrows indicate water flow.
FIGURES:
Fig. 16: Natatory hair of first pleopod. Scale in mm.
Fig. 17-19: Regeneration in the second antennae. Scale in mm.
Fig. 17: Normal left antenna.
Fig. 18: After one molt, 11 days. Amputated at fifth peduncular segment.
Fig. 19: After two molts, 20 days.
Fig. 20-23: Regeneration in the first antennae. Scale in mm.
Fig. 20: Normal left first antenna.
Fig. 21: After one molt, 5 days. Amputated at third segment.
Fig. 22: After two molts, 26 days.
Fig. 23: After three molts, 37 days.
Fig. 24-26: Regeneration in the first peraeopods. Scale in mm.
Fig. 24: Normal first peraeopod (left).
Fig. 25: After one molt, 6 days. Amputated at merus.
Fig. 26: After two molts, 21 days.
Fig, 27-31: Regeneration of the peraeopod. Scale in mm.
Fig. 27: Normal sixth peraeopod.
Fig. 28: After one molt, 16 days. Amputated at the merus.
Fig. 29: After two molts, 21 days.
Fig, 30: After one molt, 16 days. Seventh peraeopod amputated
at the ischium.
Fig. 31: After two molts, 21 days.
Fig. 32-33: Regeneration of the first pleopod. Scale in mm.
Fig. 32: Normal pair of first pleopods.
Fig. 33: After two molts, 33 days. Amputated at the basis.
First molt, after 25 days produced no change.
FIGURES
Fig. 34-36: Regeneration in the uropods. Scale in mm.
Fig. 34: Normal left uropod.
Fig. 35: After one molt, 15 days. Right uropod. Essentially
unchanged since operation.
Fig. 36: After two molts, 31 days. Note two setae.
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