Featherstone
Introduction
Polycheria osborni is an amphipod found quite abundantly in
a variety of compound ascidian colonies in the lower intertidal
and subtidal areas from Puget Sound, Washington, to Southern
California. It was found in Aplidium (Amaroucium) by Colman (1898),
Skogsberg and Vansell (1928), and in Clavelina sp. by Alderman (1936).
Abbott and Newberry (1980) found Polycheria osborni in a variety
of other ascidians. Skogsberg and Vansell (1928) described its
structure and some of its behavior. In the present study the dis¬
tribution of Polycheria osborni in ascidians of the lower intertidal
area of Mussel Point, Pacific Grove, California was observed.
including the arrangement of the amphipods in the colony, the
sizes of the amphipods in the various ascidians, and a sex ratio
based on a subset of the population. Behavior of the Polycheria
was also observed; their behavior in the burrow and the movement
of them between burrows.
Featherstone
Distribution and location of Polycheria osborni
Ascidians were collected at low tide in 3 areas at Mussel
Point. The location termed West Point was exposed to considerable
wave action at all but the lowest tides. In the second and third
locations, the Bird Rock area and the Seal Island area, the force
of the waves was broken by offshore rocks. Ascidians were also
collected at Pescadero Point, on the open coast at the north tip
of Carmel Bay. Ascidian samples were taken from the rocks with
a putty knife. These were then brought into the lab where the
ascidian samples were measured, the Polycheria osborni burrow
positions were noted, counted and measured, and additional general
observations were made. The ascidians with Polycheria osborni
still in their burrows were maintained in a shallow trough provided
with a continuous flow of fresh seawater at 10 to 15 7C.
To find the distribution of Polycheria osborni in ascidians,
many samples of the ascidians common on Mussel Point were collected
haphazardly at various places and intertidal elevations, and studied
under a dissecting microscope in the laboratory. Burrows appear
as either zigzag seams or as oval openings on the free exposed
surface of the colony in which Polycheria osborni could be seen.
Surface area of each ascidian colony was calculated roughly from
the length and width of the surfaces bearing zooid apertures. The
number of Polycheria osborni burrows was counted, and the anterior-
posterior length of each occupied burrow was measured to the nearest
O.5 mm. The position of the Polycheria osborni in relation to the
zooids of the colony were also recorded.
Featherstone
Eleven species of colonial ascidians from the Bird Rock and
Seal Rock areas were examined for the presence of Polycheria osborni.
Four of these (Archidistoma psammion, Synoicum parfustus, Aplidium
solidum and Perophora annectens) contained no amphipods. Two more
(Distaplia occidentalis and Clavelina huntsmani) contained only
occasional amphipods. The five species in which Polycheria osborni
were most frequently found were: Aplidium californicum, Polyclinum
planum, Archidistoma diaphones, Archidistoma molle and Archidistoma
ritteri.
The average number of Polycheria osborni for these five
ascidians ranged from O to 21.9/cm for most samples. There wasn't
any trend of Polycheria osborni occurence between the West Point
area and Bird Rock area. In each ascidian species there is great
variance in the average number of Polycheria osborni/cm“. The
population densities of amphipods found in particularly heavily
infected regions of individual colonies ranged as high as 45/cm"
for Archidistoma diaphanes, and 67/cm" for Archidistoma ritteri. The
highest densities on Archidistoma ritteri occured on small colony
lobes where the area of the lobe was less than 1 cm"; therefore, the
actual number of Polycheria osborni counted was less than that
recorded/cm.
Extremely high populations, such as that of 45/cm" for Archi¬
distoma diaphanes, consisted very largely of juvenile Polycheria
osborni in burrows about 0.5 mm long. The density of Polycheria
osborni did vary with size. Fig. 2 shows diagrammatically the
difference in numbers of Polycheria osborni/cm" by size in colonies
of Archidistoma molle and Polyclinum planum. Although it is seen
Featherstone
here that the smaller amphipods occur in greater numbers/cm’.
the large Polycheria osborni burrows may be as close together as
the smaller. It has been observed that one large burrow may ac¬
tually run into another forming a 'T' or they may be end to end
with a very narrow band of tunic between the two animals,
Polyclinum planum was especially abundant at Mussel Point,
so counts were made of numerous whole colonies of different sizes
to see if a relationship exists between size of the ascidian
colony, which is possibly proportionate to age, and the number of
Polycheria osborni. The results, shown in Fig. 3, indicate that
there is no correlation between the size of the colony and the
number of amphipods/cm present. The Polyclinum planum varied in
firmness and diatom cover within each size class,
One feature of special interest from the standpoint of host/
commensal relations was the placement of Polycheria osborni
burrows in relation to the zooids of the ascidian colony, The
apertures of zooids, like the burrows of amphipods, generally
occur on the distal free surface of the colony or its lobes,
To determine the extent to which amphipods might be interfering
with the activities of the host zooids, numerous colonies of
6 species of ascidians were examined and the positions of amphipods
recorded. There were several possible locations for the burrows
(Fig. 4). Three of the ascidian species, Aplidium californicum,
Polyclinum planum, and Distaplia occidentalis have their zooids
arranged in systems, with a central common cloacal aperture for
the atrial outflow of several zoids, though each zooid has its
own separate oral aperture. In these colonies there are three
Featherstone
possible burrow locations: in the center of the system, on or in
the common cloacal aperture; amongst or on the zooids of the system,
or in the tunic between the systems. The only Polycheria osborni
found within the common cloacal aperture was a newly hatched one,
less than 0.5 mm long which filled little of the aperture. The great
majority of the amphipods were situated in the clear areas of tunic
between adjacent systems, where interference with zooids was mini¬
mal. The ascidians Archidistoma molle, Archidistoma diaphanes and
Archidistoma ritteri have both their atrial and oral apertures
opening independently to the exterior of the colony, and they are
not ordered in systems within the colony. The Polycheria osborni
burrows can be either on the zooids or between the zooids in these
ascidians, and most were found between zoids (Fig. 4). In either
position Polycheria osborni often appear to have pushed the zooid
aside to some extent at the top, as the zooid body descends into
the tunic at an abnormal slant under the body of the amphipod.
Seeing that Polycheria osborni occurs largely in areas betwen
zooids it could be assumed that in such a position it is not hurting
the zooids, whereas, if the Polycheria osborni is on top of the
zooid there might be some damage to them. In addition, if the
Polycheria osborni is competing for tunic with the zooids it
might not be a comfortable burrow. This was observed in Distaplia
occidentalis. In this ascidian the zooids and systems are close
together. As a Polycheria osborni burrowed in, pulling on the tunic,
the zooids were seen to contract, pulling the tunic out of the
grasp of the Polycheria osborni. The closeness of the zooids in
Distaplia occidentalis, a very soft ascidian, is possibly the
Featherstone
reason the incidence is so low.
Size class structure and sexuality of the Polycheria
osborni population.
The body length of Polycheria osborni cannot be measured
accurately without removing the animals from their burrows, How¬
ever, burrow length is easily measured without disturbing the am¬
phipods, and provides a good index to the distribution of size
classes in the population.
A total of 1866 burrows were measured to the nearest 0.5 mm
in 5 species of colonial ascidians in the course of examining the
distribution of Polycheria osborni in ascidians. Results show at
the top of Fig. 5 that 64% of the population is in size classes
0-0.75 and 0.76-1.0 mm. The larger size class populations decrease
sharply to 1.5% in the largest size class, 3.0 mm.
When separate size class distinctions were calculated for
different ascidian species, some interesting differences appeared.
(Fig. 5). The Archidistoma diaphanes histogram is similar to that
of the whole population. Aplidium californicum and Archidistoma
molle show smaller percentages of the smallest animals, 0.5 mm,
and still fewer appear in Archidistoma ritteri while Polyclinum
planum shows a relatively even distribution of sizes. These differences
are statistically significant indicating some relationship between
amphipod size and the ascidian they burrow into (R X C contingency
test P .005).
Since there are often many small burrows around a large burrow
it is assumed that those are newly hatched. The peak in the popu¬
lation of 0.5 mm Polycheria osborni in Archidistoma diaphanes
icates
Featherstone
indicates that there may have been several newly hatched broods in
the sample. Aplidium californicum and Archidistoma molle may
have broods that were hatched several weeks early. Another possible
determining factor is the toughness of the ascidians. It was seen
that the young burrowed very quickly into Archidistoma diaphanes,
a soft ascidian, agreeing with the peak in Fig. 5 for that ascidian,
However, other soft ascidians, Archidistoma molle and Archidistoma
ritteri, do not support the idea of soft ascidians being good
hosts for the O.5 mm animals.
Amphipods are slightly longer than the burrows they occupy,
In order to be able to interpret burrow length in terms of amphipod
length, 15 burrows were measured, then the amphipods were removed
and measured. Results shown in Appendix A indicate a conversion
factor of 1.5:2 for burrow length to amphipod length.
Not only size classes but also the relation of size to sex
and sexual maturity was investigated. Of special interest was the
fact that in previous studies only female Polycheria osborni
had been found among the larger sized animals (Skogsberg and Vansell,
1928; Colman, 1898). In order to look further for male Polycheria
osborni, 107 amphipods of various sizes were removed from ascidians
of different species and from different colonies, and preserved
in 70% ethanol. The samples were haphazard with regard to ascidian
source, but while the larger animals were all taken, the smaller
juveniles were not collected in proportion to the numbers present
in the colonies. The animals were all measured with an occular
micrometer to the nearest 0.2 mm. On close examination, the popula¬
tion could be separated into juveniles, males and females. The
Featherstone
males had two distinct copulatory appendages ventrally on the pos¬
terior edge of the eighth thoracic segment (Fig. 6). These appen¬
dages were clear to white and varied somewhat in size with the size
of the Polycheria osborni. Females could be identified by brood
plates (oostegites) in the thoracic region. Juveniles were those
Polycheria osborni which lacked both copulatory appendages and
oostegites. Fig. 7 shows the number of juveniles, males and females
at sizes from 0.5-3.6 mm. Juveniles were seen from the smallest
sizes through 2.2 mm. Males and females began to appear at 1.8 and
1.6 mmrespectively. Interestingly, in this haphazard sample there
were twice as many females as males. Several of the females
observed were gravid. The eggs in 12 broods were counted; egg
number ranged from 37 to 129 with a mean of 85.3 There was a
clear correlation between the size of females and the number of
eggs being carried (Fig. 8) but not between the age of the eggs
and the number of eggs. The eggs in a single brood were at different
stages of development but there were not distinct separations
in development as would appear if 2 batches were brooding.
Behavior of Polycheria osborni
Burrowing
Polycheria osborni removed from their burrows and placed on
or near ascidians, are capable of forming new burrows; the process
was observed in some detail by Skogsberg and Vansell (1928). In
the present work, experiments were performed to compare the burrowing
activities of Polycheria osborni of different sizes and of Polycheria
osborni on different ascidian substrates. In these experiments,
Polycheria osborni was removed from its burrow and placed on
the same colony or a new ascidian colony or species where it was
Featherstone
observed at short intervals for up to 280 minutes. For the first
30 minutes the transplanted Polycheria osborni was watched con-
tinuously so as not to miss the critical times for Polycheria
osborni that burrowed quickly. The time for burrowing was divided
up into the time that Polycheria osborni walked around on the
ascidian or swam off, the time from beginning the burrow to being
below the surface of the ascidian host, and the time until the
burrow could be closed when Polycheria osborni was disturbed.
Some amphipods when placed on any of the ascidians tested
for burrowing behavior quickly swam away; others were immediately
quiet, sitting on their backs on the ascidian whether they began
to burrow or not. It was observed that if, when Polycheria osborni
was placed on the ascidian, the antennae did not touch the host,
the amphipods often swam away. In contrast, whenever the antennae
contacted the ascidian the amphipods sat quietly on the ascidians
or would climb on the colony if not yet already on it.
The process of burrowing was described by Skogsberg and
Vansell (1928), but their studies involved only one species of
host ascidian. Present work shows that speed of burrowing may
vary, not only with the individual and with its size but also
with the host species (Fig. 9). Four Polycheria osborni placed
on Archidistoma psammion did not burrow, but sat or walked on the
ascidian, occassionally swimming off. Polycheria osborni burrowed
most quickly into Archidistoma molle, Archidistoma diaphanes,
and Aplidium californicum. Interestingly, all the experimental
amphipods burrowed into Clavelina huntsmani although they are
found in that host infrequently in the field. The actual mechanism
of burrowing was closely observed on Archidistoma molle supporting
the behavior described by Skogsberg and Vansell (1928).
Featherstone
Polycheria osborni has been found to occur in a much greater
variety of ascidian hosts than previously recorded. Observations
on distribution of Polycheria osborni in different ascidian species,
in relation to the zooids, and the length of time for burrowing
in different ascidians suggests some reasons for the distribution
to be as it is. No Polycheria osborni burrowed into Archidistoma
psammion and none were found around China Point burrowed into
this ascidian. However, two were found burrowed into Archidistoma
psammion at Pescadero Point. This ascidian is very tough when
pressed on. It is possible that this hardness creates a major
barrier for this animal which is going to simply pull with its
pereopods and push with its back to make a burrow. Several of the
other ascidians looked at but found lacking in Polycheria osborni
are encrusted with sand which would make it virtually impossible
for Polycheria osborni to catch hold of the surface membrane to
pull itself into a burrow. Polyclinum planum which had a medium
density of Polycheria osborni in the field and into which only
half the experimental animals burrowed into may also be firm enough
to present some barrier to Polycheria osborni.
Behavior of Polycheria osborni in its burrow.
Studies were made to determine how much of the time the Poly-
cheria osborni spend holding their burrows closed, holding their
burrows open, or holding their burrows open while kicking their
pleopods. Ten minute observations were carried out on 13 Polycheria
osborni in 3 different ascidian species. An event recorder coupled
with a strip chart recorder was used to keep records of when the
animals closed their burrows, when they opened them, and when
they kicked their pleopods. From these records, total duration
for each activity in the 10 minute period could be obtained,
11
Featherstone
In addition, further observations of the animal in its burrow were
made to elucidate the direction of current flow, caused by the kicking
pleopods, the part this current plays in feeding, and a little
on how the animal feeds. The addition of a mixture of graphite
particles and diatoms added to the water in which the ascidian
sample was sitting allowed for easy observation of the current.
Behavior in the burrow of the 13 amphipods studied for 10
minutes each showed variation; each individual Polycheria is
represented by a separate vertical bar in Fig. 10. The Polycheria
osborni in the 3 different ascidians showed no significant differences
in behavior, therefore observations made in all 3 hosts have been
combined. The amount of time the 0.5 mm long Polycheria osborni were
closed, 0 minutes, is statistically significant in relation to the
other sizes of Polycheria osborni (RX C contingency test P.005).
Larger amphipods don't show behavior differences correlated with
size. Polycheria osborni may kick its pleopods for 15 seconds
without stopping or it may kick once, stop, and kick again. The
number of starts in Fig. 10 refers to this and not to the total time
spent kicking. Most of the Polycheria held their burrows open for
at least 2/3 of the time observed, and were kicking for less than
1/3 of the time open.
As Skogsberg and Vansell (1928) described, the current that
the pleopods make brings in water predominantly from the sides of
the burrow although water is also brought in from both ends. The
water then shoots almost straight up from the central part of
the body. This current brings fresh water by the gills, and also
by any eggs that may be in the brood pouchs of females. In addition,
some water does pass through the antennae bringing with it food
particles that get caught in the antennae. One or two antennae
12
Featherstone
are then bent quickly onto the burrow where the gnathopods scrape
the food off from the base to the tip of the antennae. Graphite
particles were observed in the antennae spread out and extended
perpendicular to the surface of the colony. After having been
scraped off by the gnathopods, the antennae again extended with
no graphite on them.
Another 'in burrow' behavior sometimes noted is that of
Polycheria turning around in its burrow. An amphipod does this
by rocking on its dorsal side up onto its head. It then rotates.
head down in the burrow while using its pereopods to pull it
around. Turning around takes about 3 seconds, and may be accom¬
panied by several false starts in which the amphipod rocks but
not all the way to its head.
Movement of Polycheria from one burrow to another.
Published accounts of Polycheria leave the impression that the
amphipods, once burrowed in, remain permanently in place. Preliminary
observations showed that this is not the case, and that Polycheria
osborni does change burrows. To study amphipod movements, a colony
of Archidistoma diaphanes (4.8 X 2.8 cm) with 110 Polycheria osborni
burrowed into it (28 large and 82 newborn) was placed in a fingerbowl
with a very gentle flow of fresh sea water. At numerous times
during 9 days the colony was observed and the location of the
Polycheria osborni burrows plotted, the orientation of the larger
burrows noted and the general condition of the colony recorded,
The plottings of consecutive observations were compared to see
where old burrows and Polycheria osborni disappeared, where new
burrows appeared and where large burrow orientations changed,
13
Featherstone
The results of these observations show that the Polycheria popula¬
tion is quite dynamic. In Fig. 11, in all but 2 of the time inter¬
vals between successive observations, both large and small Poly-
cheria moved, changing burrows or disappearing completely. By
the end of 9 days all of the large amphipods had either changed
burrow or orientation within their burrow. All but 9 of the small
Polycheria had changed burrow location. During the time the colony
was kept in lab the Hopkins Marine Station seawater system ex-
perienced "bubble disease" and the Archidistoma diaphanes colony
got many air bubbles just below the surface tunic. Two small
Polycheria osborni were seen to be on top of these air bubbles
but were not in the same locations at the next observation, Con¬
siderable debris also settled on the ascidian during the 9 days
and no Polycheria osborni were observed in areas where the debris
was especially heavy.
To see if laboratory results reflected events actually occuring
in the field, 2 colonies of Archidistoma diaphanes were observed
on three successive days without disturbing them in any way. The
first colony observed consisted of several lobes in a 3.5 cm X 4 cm
area. In this sample the number of large (2 mm) and small ( 1 mm)
Polycheria osborni/lobe were counted, then counted again after 23
hours and compared. In the second sample a colony of Archidistoma
diaphanes (1.5 cm X 2 cm) was carefully diagrammed and the location
and orientations of the burrows recorded, then observed again
after 24 and 48 hours.
The results show that Polycheria do indeed move about.
Observations on the numbers of amphipods/lobe in the Archidistoma
diaphanes colony (Fig. 12) indicate that there was only one lobe
Featherstone
which had the original number of Polycheria at the end of the second
observations. In the colony where amphipod positions were mapped,
(Fig. 13) the number of Polycheria osborni/lobe did not change
much, but movement was still clearly evident.
Studies of Polycheria osborni movement indicate that when
amphipods leave burrows, these soon disappear, and few vacated burrows
are normally seen on infested colonies. To investigate how quickly
the ascidians were able to reshape the tunic after the burrow
was vacated, Polycheria osborni (1.5 mm) were carefully removed
from colonies of Polyclinum planum and Archidistoma diaphanes and
the burrows observed intermittently for 3 days.
Vacated burrows on Polyclinum planum took a mean of 20 hours to
disappear, one shallow burrow took only 5 hours to disappear,
while another deeper burrow took 30 hours. Burrows in Archidistoma
diaphanes took on the average 24 hours to disappear. On Polycheria
burrow extended all the way across a small lobe, and when it was
removed the ascidian did not fill that burrow but instead remained
as 2 smaller lobes.
Featherstone
Summary
1. In the period April-June, 1980, ascidians were collected with
Polycheria osborni burrowed into them.
2. Polycheria osborni was abundant in Aplidium californicum, Archi¬
distoma diaphanes, A. molle, A. ritteri and Polyclinum planum;
scarce in Distaplia occidentalis and Clavelina huntsmani; and
absent in Archidistoma psammion, Synoicum parfustus, Aplidium
solidum and Perophora annectens.
3. Found predominantly between zooids of the ascidians, the Polycheria
don't appear to injure their hosts.
4. Two-thirds of the population of Polycheria observed were juveniles
less than 1.25 mm long.
5. Different species of ascidian had different size amphipods
predominating.
6. Male Polycheria osborni were reported for the first time and
can be identified by 2 copulatory appendages on the eighth thoracic
segment.
7. Polycheria sit in their burrows kicking their pleopods to make a
current, feeding and occasionally closing the burrow over them.
8. The population of Polycheria is dynamic, the amphipods change
burrow location and orientation.
9. Ascidian hosts usually reshape within 24 hours after a burrow
has been vacated.
16
Featherstone
Acknowledgements
I would like to thank all the HMS spring students for making
this an enjoyable project, Dr. Robin Burnett for his statistical
help, and especially Dr. Donald P. Abbott for his assistance in
identifying ascidians, and his great encouragement and advice
throughout the project.
Literature cited
Abbott, D.P. and Newberry, A.T. 1980. Urochordata: The tunicates,
Chapter 12, in R.E. Morris, D.P. Abbott, and E.C. Haderlie,
Intertidal Invertebrates of California, Stanford University
Press, Stanford, California. 928 pp.
Alderman, A.L. 1936. Some new and little known amphipods of
California, Univ. Calif. Publ. Zööl. 41 (7): 53-74.
Colman, W.T. 1898. On a collection of crustacea from Puget Sound,
Ann. N.Y. Acad. Sci. XI: 259
Skogsberg, Tage and Vansell, G.H. 1928. Structure and behavior of
the amphipod, Polycheria osborni. Prod. Calif. Acad. Sci.
(4) XVII: 267-295.
17
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Archidistoma diaphanes
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Aplidium californicum
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Archidistoma molle
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Archidistoma ritteri
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Polyclinum planum
333
530
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Figure Captions
Fig. 1: Average and maximum distributions of Polycheria osborni/cm
on different species of ascidians at West Point (left)
and Bird Rock (right). rectangles: 1 mm-1 Polycheria
osborni.
Fig. 2: Number of Polycheria osborni/cm in relation to burrow
lengths: 0.5, 1.0, 1.5 and 2.0. Burrows not to scale.
Fig. 3: Average and maximum numbers of Polycheria osborni/cm'
on Polyclinum planum colonies with surface areas from
1-66 cm". Each column shows mean, range, standard deviation
and 95% confidence limits of the mean,
Fig. 4: Position of Polycheria osborni in relation to the ascidian
zooids.
Fig. 5: Percent Polycheria osborni found in burrows to the nearest
0.5 mm burrow length in 5 ascidian hosts.
Fig. 6: Morphology of the ventral side of the 8th thoracic and
Ist abdominal segments of Polycheria osborni.
Fig. 7: Total numbers of juvenile, male and female Polycheria
osborni at body lengths from 0.6 to 3.6 mm measured to the
nearest 0.2 mm.
Fig. 8: Number of eggs in the female Polycheria osborni brood pouch
for amphipod lengths 2.4-3.8 mm.
Fig. 9: Length of time for Polycheria osborni to make a new burrow.
Non-burrowing time is the time spent walking on the ascidian
or swimming in the dish. Time for burrowing, solid line shows
the time until the Polycheria osborni is below the surface
of the ascidian, dashed line is the time until the burrow
can be closed.
Fig. 10: Behavior in the burrows of Polycheria osborni, 0.5-2.0 mm
in length, for 10 minute trials. Showing the amount of time
the burrow was closed, the amount of time was open and
the pleopods were kicking (cross hatching). The number
in the bar indicates the number of times the kicking starts.
Fig. 11: Movement of Polycheria osborni in an Archidistoma diaphanes
colony in the laboratory. The lines indicate the number of
adults and young Polycheria osborni which have moved
from an old or to a new burrow since the last observation.
Nequals the number of individuals present at each obser-
vation. Days 1-9 were 5/9-5/17.
Fig. 12:
Change in numbers of Polycheria osborni/lobe in an Archi¬
distoma diaphanes colony in the field. First bar of each
pair is the ist day, the 2nd bar is the 2nd day.
Fig. 13: Movement of Polycheria osborni within an Archidistoma
diaphanes colony in the field. Graphical data, except for
the total number of Polycheria osborni present, is since
the previous observation.
amphipod
length
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Appendix A
—

8084

—
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
burrow length
2 Archidistoma diaphanes
Archidistoma ritteri
o Polyclinum planum