Sandy-lobed Ascidians
Mulroy, W.
Abstract
The similarities in external morphology and distribution have made
distinction between Aplidium arenatum, Aplidium propinquum and Synoicum
parfustis difficult in the field and laboratory. Quantitative differences
have been found between the species in: height and area of the lobe top;
the shape, color, and clustering of lobes; the number of systems per lobe;
the number of zooids per lobe; the number of zooids per system; zooid size
the number of rows of stigmata; the number of developing embryos in the
atrium; and habitat.
1
Sandy-lobed Ascidians
Mulroy, W.
Several species of compound ascidians in which the colonies form
sandy lobes occur near the Hopkins Marine Station at Pacific Grove, California.
They are so similar that species identification in the field and even in
the laboratory may be difficult. The original descriptions of these species
bring out more similarities between them then differences. Van Name (1945)
described Aplidium arenatum as forming irregularly club-shaped lobes thickly
encrusted and impregnated with sand, the top of the lobes in a cluster
fitting together to form an even surface. He found that lobe height reached
25 mm but was usually less than 5mm. Aplidium propinquum was described by
Van Name (1945) as irregularly club-shaped lobes similar to A. arenatum,
although reaching a greater height. Ritter and Forsyth (1917) described the
external appearance of Synoicum parfustis as sand-encrusted, club-shaped
masses having a peduncle twice as long as the rounded head. The height of
an individual lobe reaching 6 cm.
In addition to the similarity in appearance, all three species are
distributed in the low intertidal zone. The zooids all have three body
regions and form systems in a transparent, common test. Positive identification
requires studying such details of zooid structure as the contour of the
stomach wall and the number of rows of stigmata in the pharynx.
The objective of this study was to describe the range in variation for
each species, both in external characteristics of the lobes and in zooid
characteristics, to make identification easier in the field and in the
laborator)
Laboratory studies
Methods:
The methodology was simple. All sand encrusted lobes that looked
Mulroy, W.
Sandy-lobed Ascidians
different were collected from the intertidal zone. The organisms were
then placed in bowls of seawater, relaxed overnight with menthol, then
fixed in 10% formalin for 48 hours. Only well-expanded zooids in which all
rows of stigmata were clearly visible were used for detailed study.
Measurements of zooids and lobes were made with a Vernier caliper.
Results:
A quantitative measure of shape of the lobes is provided by plotting the
height of a lobe against the area of the lobe top (greatest width of the
lobe multiplied by the greatest thickness) for many lobes of each species,
The results are shown in Figurel. and original data are tabulated in Appendix
A. Aplidium arenatum has the smallest and thinnest lobes of the three species.
The largest lobe measured 35 mm in length. A. propinquum lobes reach 50
mm in length and are more plump than those of A. arenatum. Synoicum parfustis
lobes reach the greatest size. Note the overlap in size and shape of
the three species in the smaller lobe size range. Clearly, other characteristics
must be studied for identification.
Another value showing some differences between species is the number
of systems per lobe (Figure 2.). A small area at the top of the lobe allows
only a small number of systems. On the graph note that the 7+ category
of the number of systems includes both A. arenatum whose maximum value
was 8 systems in a lobe and Synoicum parfustis which commonly had over
12 systems in a large lobe. In the area range of 50-200 mm’, A. propinquum
and Synoicum parfustis have similar values.
Figure 3. compares area of the lobe top with the total number of
zooids in the lobe. A. arenatum never had more than 27 zooids in the lobes
studied. In Synoicum parfustis large lobes have well over 100 zooids.
Sandy-lobed Ascidians
Mulroy, W.
Although A. propinquum lobes do not have as many zooids as large lobes
of Synoicum parfustis, zooids are more densely packed in a given area.
Figure 4. displays the number of zooids per system. Aplidium arenatum
averaged 5.4 zooids per system, and lobes containing one zooid were unique
to A. arenatum. Most systems of Synoicum parfustis had between 6-8 zooids.
In A. propinquum, with an average of 10.8 zooids per system and as many
as 15 zooids in one system, the individuals are more densely packed than
in the other species.
The variation in size and shape of lobes of each species is compared
in Figure 5. From observations made on each species the external appearance
of the lobes in clusters is as follows:
Aplidium arenatum is a small, brown, slender lobe occurring in tightly
packed clusters. The small size, heavy sand-encrustation and flat lobe
tops give the cluster top the appearance of a continuous mound. Sand
not only encrusts the outside of the common test but is loosely spread
throughout the inside also.
Aplidium propinquum forms more plump lobes and larger clusters than
A. arenatum. The lobes range up to 50 mm in height although 30 mm is more
common. The heavy sand encrustation gives way to the transparent tunic
at the bright orange-red, common cloacal aperture, giving the system opening
a distinctive crater-like appearance.
Synoicum parfustis described by Abbott and Newberry (1980) as clusters of
orange, tear-drop shaped lobes are unmistakable when they attain this shape
in well-developed lobes. Smaller lobes are not as distinctively shaped;
however the heads are still more rounded than flat-topped, and the bright
orange systems can be seen in life.
Sandy-lobed Ascidians
Mulroy, W.
The growth of the lobes of each species is also reflected in Figure
5. A. arenatum, A. propinquum, and Synoicum parfustis all form new lobes
from the bases of existing lobes. All three species can add to the area
of a lobe top by increasing the number of systems in the same lobe head.
A. arenatum and A. propinquum have a third method of growth, branching of
a lobe. A new system may form and split the lobe top forming two heads.
This lobe branching adds to the tight clustering of the colonies,
Figure 6. compares the zooids of each species. The drawings, taken
from Van Name (1945), show the number of rows of stigmata and the contour
of the stomach wall. It is the zooids which give the lobes their characteristic
color. The oral and atrial siphons of A. arenatum are brown. The brown
coloring fades to a transparent thorax. In A. propinquum the thorax is
a bright orange-red color, and in Synoicum parfustis it is bright orange.
Figure 7. displays the variation found in the number of rows of stigmata
for each species. A. arenatum always has 5 rows of stigmata. A. propinquum
has 15-21 rows of stigmata, while Synoicum parfustis has 10-16 rows.
All counts were made on well-expanded zooids in which the number of rows
could be counted accurately.
te zige ahe
In Figure 8., species are compared with respect tozooid. Since the
length of the post-abdomen is too variable, the combined length of the
thorax and abdomen was used as a measure of size in well-expanded zooids.
A. arenatum was the smallest, maximum length 6 mm. The largest zooid of
A. propinquum is 9 mm. Synoicum parfustis has the largest zooids with some
reaching over 12 mm in length of thorax and abdomen combined.
Figure 9. shows the number of developing embryos found in brooding
zooids. A. arenatum and A. propinquum showed comparable numbers of developing
Sandy-lobed Ascidians
Mulroy, W.
embryos in the atrium. Synoicum parfustis has the greatest capacity for
brooding embryos of the three species by carrying up to 13 developing
embryos.
Field studies
The second phase of the project was conducted to see if any distributional
differences could be seen between the three species.
Methods:
From collection of specimens, the general area where lobes occurred
was known. Fourteen areas were then selected for study on the basis of
surf exposure, depth, and the presence of at least one of the three ascidian
species being studied. Abundance and distribution of each species was observed
in each area from the zero tide level to the ocean floor.
Each area was divided into three ranges of depth below the zero tide
level: +0.25 to -0.25 m,-0.25 to -0.75m and -0.75 to -1.0 meters. The
zero tide level was determined using a meter stick and the Monterey Bay
tide charts and measuring the zero tide level from the height of the low
tide.
The abundance of each species in each area and depth was measured
on a scale of 0 (species not present), 1 (species present), 2 (species
present in good amount), or 3 (species present in large amount).
Results:
General patterns of distribution are shown in Figure 10. Synoicum
parfustis occurs lower down in the areas studied, A. arenatum and A. propinquum
higher up. A. arenatum was not as abundant as either Synoicum parfustis
or A. propinquum. Looking at Table 1., the environments for Figure 10.,
further information is revealed about distribution. Synoicum parfustis
Sandy-lobed Ascidians
Mulroy, W.
grew best in protected areas of moderate surf and on horizontal surfaces
in the lower portion of the depth studied. A. propinquum grew best in
exposed areas with horizontal or vertical surfaces, while A. arenatum
grew best in exposed areas on horizontal surfaces.
Discussion:
From the studies on distribution a logical question follows: what
are the advantages of being a sandy-lobed, colonial tunicate? One advantage
is in space competition. These compound ascidians are benthic organisms.
By having only a stalk attached to the substratum most of the tunicate's
mass is out in the free-flowing water. The lobe shape allows more individuals
per given area of substratum, and prevents the organism from being as
easily overgrown by flat colonial tunicates or sponge as an encrusting
colony might be. Since the tunicates are filter-feeders, a lobe sticking
out from the substratum may get better circulation of water and thus more
food. Finally, the hard sand encrustation protects the tunic from abrasion
in the surf as well as discouraging predators. With these advantages in
gare
mind, the distribution of the three species can, be explained.
The distribution of Aplidium arenatum and A. propinquum higher up
and in heavier surf as compared to Synoicum parfustis appears to be related to
the morphological differences between the species. A. arenatum and A.
propinquum form tightly packed clusters; the lobes are smaller than in
Synoicum parfustis and are more heavily sand encrusted. These characteristics
provide more protection from the heavyy surf and from desiccation at low
tide. Synoicum parfustis on the other hand, form loose clustering colonies
which doeé not allow the individual lobes to support each other in the
heavy surf or to retain water in the cluster at low tide. The large size
7.
Sandy-lobed Ascidians
Mulroy, W.
of the lobes also would be a disadvantage in heavy surf by putting more
stress on the stalk than a smaller lobe would be subjected to. For these
reasons Synoicum is better suited for moderate surf and greater depth
below the zero tide level.
dgeme
Acknow
I woull like to thank Don Abbott for his enthusiasm and guidance
which made this project so enjoyable. Special thanks goes to Keith Kohatsu,
the best T.A. ever. And finally my gratitude must be expressed to all the
students in the spring class, the faculty and the rest of the staff who
make Hopkins such a special place.
10.
Literature Cited
Abbott, D.P. 1975. Phylum Chordata: Introduction and Urochordata, pp
638-625. In Smith, R.I..and S.F. Carlton., Light's Manual: Intertidal
invertebrates of central California coast. Univ.Calif. Press. 716 p.
Abbott, D.P. and A.T. Newberry.1980. Urochordata: The Tunicates. pp.
177-226, pls. 57-67. In Morris, R.H., D.P. Abbott and E.C. Haderlie,
eds., Intertidal Invertebrates of California. Stanford University
Press. 690 pp. 200 pls.
Jackson, J.B.C. 1979. Morphological strategies of sessile animals, pp
499-503. In G. Larwood and B.R. Rosen, eds., Biology and Systematics
of colonial organisms, Academic Press, 589 pp.
Ritter, W.E. and R.A. Forsyth 1917. Ascidians of the littoral zone of
Southern California. Univ. of Calif. Publ. Zool. 16:439-512, pls. 38-46.
Van Name, W.G. 1945. The North and South American Ascidians. Bull. Amer.
Mus. Nat. Hist. 84:1-476, pls. 1-31.
11.
Figure Captions
Figure 1. Area of the lobe top vs. height of the lobe for three ascidian
species. Numbers above bars show number of lobes involved.
Figure 2. Area of the lobe top vs. the number of systems per lobe for
three ascidian species. Numbers above bars show number of lobes
involved.
Figure 3. Area of lobe top vs. the number of zooids per lobe for three
ascidian species. Numbers above bars show number of lobes involved.
Figure 4. Variation in number of zooids per system for three ascidian
species. Numbers above bars show number of systems involved.
lobes
Figure 5. A comparison of lobe shape for,up to 50 mm in height for
three ascidian species.
Figure 6. A comparison of zooid structure taken from Van Name (1945).
Variation in the number of rows of stigmata per zooid for
Figure 7.
three ascidian species.
Figure 8. Variationg in combined length of thorax and abdomen for three
ascidian species.
Figure 9. Numbers of developing embryos in the atrium in brooding
zooids of three ascidian species.
Figure 10. Distribution in depth below the zero tide level vs. the
environment studied.
0
le Captions
Table 1. Environments (A-N) for Figure 10.
Table 2. Summary: Characteristics useful for separating three species
are summarized in the following table.
12.
e
Table 1
Environments for Figure 10
I. Exposed areas
(heavy surf)
AREA
VERTICAL SURFACE
HORI ZONTAL SURFACE
-0-
-O-
-0-
-O-
-O-
-0-
-0-
-0-
-0-
-0-
-0-
-0-
II, Protected area (moderate surf)
VERTICAL SURFACE
HORI ZONTAL SURFACE
AREA
-O-
-O-
-O-
-O-
-O-
-0-
-0-
-O-
-O-
+-prasa
O - alan
BOTH SURFACES
-0-
-0-
-O-
-0-
-0-
-0-
BOTH SURFACES
-0-
13.
Table 2
Aplidium
arenatum
Brown
1. Lobe color
Heavy
2. Sand encrustation
Flat
3. Lobe top
4. Clustering of colony.
Tight
1. Lobe formation
New lobe growth by:
at base of another
lobe.
2.Splitting of one
lobe to form two.
5. Lobe shape
Flat
35 mm
6. Maximum height of
lobes.
7. Visibility of systems
Difficult to see.
in vivo.
1-8
8. Number of systems/lobe
1-27
9. Number of zooids/lobe
(range)
sastem
5.4
10.Number of zooids/lebe
(means.
11.Number of rows of stigmata 5
12.Combined length of
zooid abdomen and thorax
1-6 mm
1-5
13.Number of developing
embryos in atrium
Heavy surf
14.Distribution
14.
Synoicum
parfustis
propinquum
Orange
Orange-red
Heavy
Moderate
Flat
Rounded
Tight
Separate
1.Lobe formation
1.Lobe formation
at base of
at base of anothe
another lobe.
lobe.
2.Splitting of one
lobe to form two.
Rounded
Flat
60 mm
50 mm
Orange-red, crater-/Orange. Clear
like
1-6
1-15
5-124+
8-62
6.6
10.8
10-16
15-21
2-12 mm
2-9 mm
1-14
1-6
Heavy surf
Moderate surf
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