Spirorbid Larval Behavior
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S. Jensen
INTRODUCTION
Spirorbids are among the most abundant polychaetes found in the
rocky intertidal zones of Monterey Peninsula, where their coiled cal¬
careous shells encrust rocks and algae. They are filter feeders whose
bodies and tubes are marked by bilateral asymmetries. Recent work on
their systematics (Knight-Jones, et. al., 1975) proposes that they
comprise a distinct family, Spirorbidae, though they are usually
classified with the Serpulidae.
Most of the literature on the group consists of descriptive and
taxonomic studies. However, until recently there has been much dis¬
agreement about classification schemes, so that identifications of
California species have been incomplete and naming inconsistent.
These problems partly account for the fact that very little is known
about most aspects of spirorbid biology,
With the exception of the developmental studies by Potswald
(1965) in Washington, nearly all work on their general biology has
been done in Europe. Several investigators have examined the breeding
cycles, growth, and larval settling behavior of a few species asso¬
ciated with algae in Britain (Knight-Jones, 1951; de Silva, 1962;
Williams, 1964; Gee, 1965). This paper presents observations on the
distributions and settling behavior of intertidal spirorbids on
Monterey Peninsula.
DISTRIBUTION OF SPECIES
All observations were made during April and May, 1976. Spirorbids
were collected from rocky intertidal areas around Hopkins Marine
Station and at Pescadero Point. Species identifications were attempted
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S. Jensen
using a key to well-known species, kindly provided by Dr. E. W.
Knight-Jones, University College of Swansea, Wales.
One species having six tentacles and incubating embryos within
its smooth, dextrally-coiled shell (genus tentatively identified as
Spirorbis) was found exclusively on Cystoceira blades from Pescadero
Point. This was the only specific association with an alga found;
all other species occurred primarily on rocks and shells and some¬
times on encrusting algae, especially Hildenbrandia. These species
were abundant in permanent, frequently awash tidepools and in shel-
tered areas exposed during low spring tides, notably in crevices or
beneath rocks.
Among spirorbids attatched to rocks and shells, at least four
species could be distinguished by obvious variations in tube appear-
ance, operculum structure, and the number, shape, and color of the
tentacles. Frequently, individuals of two or more species occurred
in close association.
The most numerous of these species, especially on the walls of
tidepools, was identified as Pileolaria (Simplicaria) sensu. It had
a large-diameter, chalky, opaque, and fairly smooth tube that coiled
sinistrally. There were seven brightly colored tentacles, and the
eggs were brooded in a modified operculum. This species was occa¬
sionally seen on Bossiella.
Two other common species may have been Paradexiospira (Spiror-
bides) vitrea and Spirorbis marioni, though these identifications
are much more doubtful. The former had six tentacles and a dextral,
translucent, glassy tube with prominent ridges running longitudinally
that gave the mouth a scalloped edge. The latter was also dextral
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S. Jensen
but had seven pale tentacles and a thick, white tube with faint longi-
tudinal ridges. Both brooded larvae in the tubes. At laest one other
species, a sinistral tube-brooder, remains unidentified.
Most adult tubes are between 1.5 and 2.0 mm in diameter, but some
P. sensu tubes may be wider than 3.0 mm.
OBSERVATIONS OF SETTLING BEHAVIOR
All spirorbids reproduce sexually and are hermaphroditic, They
brood their eggs until fully developed swimming larvae are ready to
hatch. For all of the species found here, a majority of the indi¬
viduals examined had broods. Larvae were collected from rocks densely
covered with spirorbid tubes by placing each rock in a small con-
tainer of cold (6° C) seawater and exposing it to bright light from
a microscope lamp. Five or six rocks at a time were so treated.
Usually, within two hours, swimming larvae appeared near the surface
in one or more of the containers.
A newly released larva [Fig. 1] is about 0.3 mm long. It has a
large thoracic region and a much smaller unsegmented abdomen. The
thorax has three distinct segments with extensible notosetae that do
not resemble the adult thoracic setae. The anterior part of the
thorax is partly covered by the collar. There is a distinct head area
with two ventral red eyespots. The gut can usually be seen as some¬
what more opaque than the thorax. The larva swims mainly with a
prototroch of long cilia anterior to the collar. It also has anterior
and posterior ciliary tufts and other less salient ciliary fields. A
prominent feature of the larvae observed settling was the white,
opaque primary shell gland covering much of the dorsal thorax.
At no time were larvae actually watched in the process of leaving
Spirorbid Larval Behavior
S. Jensen
a brooding parent. Therefore, since the larvae were obtained from
rocks with mixed populations, they could not be positively assigned
to species. However, all for which the entire settling sequence was
timed developed into sinistral adults, and it is very likely that all
were Pileolaria sensu. Dextral individuals were observed in various
stages of settlement, and no deviations from the events described
below were noted.
Newly released larvae first appeared swimming upward and gathering
at the surface. For at least the first few minutes after release, they
exhibited a strong attraction toward light. The attraction reversed a
short time later, suggesting that these spirorbids are adapted for
brief, short-range dispersal of offspring. When removed by pipette
into collecting dishes, most larvae immediately reversed their photo-
taxis and began swimming along the bottom. It is possible that the
turbulence of pipetting hastened this change; there may be a selective
advantage in limiting dispersal time under turbulent conditions.
The larvae swam along surfaces during a searching period of from
20 minutes to many hours. A larva that was ready to settle would
change its behavior, crawling very slowly over the surface and
changing direction more and more often. During this time, which
usually lasted only a few minutes, larvae sometimes raised up as
though to swim away but appeared to be held in place by a posterior
connection to the substrate. This anchoring could be by the mucous
thread that Potswald (1965) found to be secreted by some searching
larvae.
In the minutes before settling, the larva would repeatedly
reverse direction over a distance of 1 to 2 mm, turning by laterally
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S. Jensen
bending the abdomen. It is not clear whether this activity involves
some preparation of the substrate. It could serve to ensure that
there is adequate free space for tube growth, although tubes often
overlap in the field.
Next, crawling stopped, and the thorax appeared to adhere to the
substrate. The milky, somewhat irridescent, fluid in the primary shell
gland was slowly secreted posteriorly, forced out by vigorous con¬
tractions of the body. During the first five minutes after settling,
the larva would roll from side to side and spread the slowly hardening
fluid around itself. This primary tube seemed to attatch securely to
the substrate at its anterior end and spread posteriorly until it
enclosed the tail. The larva then began to back into the sacklike
tube, stretching it posteriorly and withdrawing completely inside it.
The animal could be seen inside the tube, still rolling from side to
side and stretching out the walls by extending its thoracic setae,
Stretching continued until the tube was about 0.5 mm long and visibly
hardening.
About 10 minutes after settling, the body movements slowed, and
the worm extended its head out of the tube, assuming the adult posi¬
tion with the ventral eyespots away from the substrate. At this time,
the collar was first extended and folded down over the tube opening.
Periodic flexing movements ceased after another 5 minutes.
By this time, rudimentary tentacles were already large enough
to be seen with a dissecting microscope. Often, within minutes after
settling, the operculum had become clearly visible, indicating the
eventual coiling direction. After one or two days, the larval head
and eyespots could no longer be seen.
Spirorbid Larval Behavior
S. Jensen
Following formation of the primary tube, the much slower growth
of the adult tube would begin. Swan (1950) has shown that it is sec¬
reted by paired glands under the collar, and the collar probably
shapes the growing tube. In the first 24 hours, tubes grew anteriorly
about 0.25 mm then began to bend in the coiling direction.
Settling was watched both on rocks and on ceramic tiles, and no
differences in the sequence were apparent.
EXPERIMENTS ON LARVAL SETTLING
a) Detection of a film of microorganisms
Methods and Materials
To test substrate descrimination of settling spirorbid larvae,
groups of newly released larvae were placed with fresh seawater in
clean 12 cm crystallizing dishes and given choices between paired
identical surfaces (granite rocks or brick fragments) of about the
same size, one of which had been kept dry in the lab, while the other
had been kept for two days or more in running seawater and thus allowed
to develop a film of algae and bacteria. The larvae used in each dish
were obtained from a single rock. The dishes were incubated in the
dark either in a 6° C coldroom or in a 12° C running seawater bath,
and the surfaces were later examined under a dissecting microscope.
Only permanently attatched larvae that had begun metamorphosis were
counted as settled.
In another experiment, 20-30 larvae were pipetted into each of
six 50 ml beakers containing about 20 ml seawater. Half of the
beakers had been "filmed" for two days in running seawater, and half
were clean. All were incubated together at 12° C in the dark.
Spirorbid Larval Behavior
S. Jensen
Two of the filmed and two of the unfilmed beakers were examined
8, 21, and 120 hours later, and the other two beakers were left undis-
turbed for 24 hours.
Results
The results of the discrimination experiment are summarized in
Table 1. Levels of statistical significance were determined using
the Chi-square test. It is clear, with both the natural (granite) and
artificial (brick) substrates, that the larvae preferentially select
surfaces that have been submerged.
For the second experiment, as indicated in Table 2, the results
obtained in the first four beakers are confused somewhat by settling
of many of the larvae in the unfilmed beakers onto two or three small
pieces of detrital material accidently introduced with the seawater.
The fact that so many larvae in the unfilmed beakers settled on tiny
loose objects indicates that the much larger unfilmed glass surface
was a relatively unsatisfactory substrate.
In the two undisturbed beakers, there were no interfering debris
in the water, and these results demonstrate that contact with an algal
or bacterial film stimulates earlier settling.
b) Response to a chemical cue
Methods and Materials
In an experiment based on one done by Gee (1965) to test the
specificity of Lithothamnion as a substrate for Spirorbis rupestris,
an extract was made by scraping rocks covered with filamentous green
algae and diatoms and immediately grinding the algae with a mortar
and pestle. To each ml of the ground algae were added about 12 ml of
seawater that had been filtered through an HA Millipore filter of
0.45 Am pore size (hereafter "HAM filtered"). This mixture was
Spirorbid Larval Behavior
S. Jensen
allowed to stand in a 6° C coldroom for one hour with occasional
shaking. It was then centrifuged at 2000 rpm for 15 minutes and the
supernatant HAM filtered to remove algal cells and most bacteria. The
resulting clear solution was poured into Petri dishes containing
pieces of unglazed white ceramic tile with areas of approximately
4 cm". These tiles and control tiles in HAM filtered seawater were
soaked overnight in covered dishes at 3° C.
Into each of three clean glass bowls containing fresh HAM filtered
seawater was placed one tile from each set. Larvae were pipetted into
a collecting dish of HAM filtered seawater and from there into the
centers of the bowls. All were incubated in the dark at 12° C.
In another experiment, larvae introduced to three bowls of HAM
filtered seawater were given a choice between a ceramic tile that had
been soaked for 24 hours in HAM filtered seawater and one soaked for
the same time in a 0.5% solution of algin, a sodium salt of alginic
acid, in HAM filtered seawater. The tiles in two of the bowls had been
soaked at 3° C and those in the third at room temperature.
Results
The results of these experiments are presented in Table 3. All
three replicates of the extract experiment indicated significantly
nonrandom (p £.05) preferences, but the third contradicts the first
two, in which selection was toward the extract-soaked tiles,
In the alginic acid experiment, the sample sizes were smaller.
However, trials 1 and 2 show significant (p £.025) settling in
favor of the control tile. Trial 3, in which the tiles were soaked
at room temperature, gave the opposite result but does not meet the
95% significance level.
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Spirorbid Larval Behavior
S. Jensen
DISCUSSION
The description of settling behavior presented here agrees with
that given by Potswald (1965) for a species he identified as Spirorbis
morchi, and his descriptions of that species support the possibility
that S. mõrchi is a synonym for the species here called Pileolaria
sensu. The observations of Knight-Jones (1951) on settling in Spirorbis
borealis differ only slightly from the present account in that he
reported more rapid secondary tube growth (coiling through 90° within
5 hours after settling) and described a primary tube large enough to
cover only the trunk and abdomen of the animal. The primary tubes seen
here were stretched before hardening so that, after only a few minutes,
the larva could retract its head completely inside.
The general results of the filming experiments are entirely con¬
sistent with those of Gee (1965) and Knight-Jones (1951). They demon¬
strated that larvae of two species of Spirorbis in Britain settle
more readily on surfaces that have been filmed.
The contradictory, yet nonrandom, results of the extract experi¬
ment are difficult to interpret. An unknown factor other than chemical
substances associated with the algae must be involved. This may have
been some accidental difference in the handling of the tiles before
soaking. It is also possible that larvae selectively settle near other
spirorbids, so that once a larva has settled, it increases the prob¬
ability of further settlement nearby. Such gregarious behavior has
been demonstrated for S. borealis in Britain and for other benthic
invertebrates (Knight-Jones and Stephenson, 1950; Knight-Jones, 1951).
The alginic acid experiment yielded no evidence that this poly¬
saccharide, a structural element of many algal cell walls, promotes
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S. Jensen
settling, and the two replicates that showed significant avoidance
of the algin solution may have been influenced by the same unknown
factor operating in the extract experiment.
Usually, in the present work, the larvae of more than one species
were tested simultaneously, yet no differences in response to filming,
position on the substrate, or settling behavior were noticed inter¬
specifically, with the exception that dextral and sinistral animals
settling together sometimes seemed to have different rates of secondary
tube growth. On intertidal rocks and shells at Hopkins and at
Pescadero Point, it appears that a number of species settle according
to identical or nearly identical criteria. The overlap of distributiions
is striking, especially in the low intertidal. In many areas, any
surface with more than a few spirorbids nearly always has a mixed
population, and it is not uncommon for a relatively small rock to
bear four intermingled species. Further settling experiments are
needed to help determine whether the apparent common preferences are
real.
SUMMARY
The settling behavior of spirorbid polychaete larvae from rocky
intertidal habitats on Monterey Peninsula was described, and settling
of larvae from mixed populations was shown to be promoted by the
presence of a suitable substrate bearing a film of microorganisms.
At least four species occupy rocks and shells in these areas,
including Pileolaria sensu and possibly Paradexiospira vitrea and
Spirorbis marioni.
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ACKNOWLEDGEMENTS
The author gratefully acknowledges the
invaluable help and advise of Drs. Isabella
Abbott and Robin Burnett and wishes to
express thanks to all the faculty and staff
of the Hopkins Marine Station of Stanford
University.
S. Jensen
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S. Jensen
REFERENCES
de Silva, P. H. D. H. 1962. Experiments on choice of substrata by
Spirorbis larvae. Brit. J. Exp. Biol., 39: 483-490.
Gee, J. M. 1965. Chemical stimulation of settlement in larvae of
Spirorbis rupestris (Serpulidae). Animal Behavior, 13 (1):
181-186.
Knight-Jones, E. W. 1951. Gregariousness and some other aspects of
the settling behaviour of Spirorbis. J. Mar. Biol. Assoc. U. K.,
30: 201-222.
Knight-Jones, E. W. and J. P. Stephenson. 1950. Gregariousness during
settlement of the barnacle Elminius modestus Darwin. J. Mar. Biol.
Assoc. U. K., 29: 281-297
Knight-Jones, P., E. W. Knight-Jones, and T. Kawahara. 1975. A review
of the genus Janua, including Dexiospira (Polychaeta: Spirorbinae).
Z0ol. J. Linn. Soc., 56: 91-129.
Potswald, H. E. 1965. Reproductive biology and development of Spirorbids
(Serpulidae, Polychaeta). Doctoral dissertation, University of
Washington Department of Zoology. 330pp.
Williams, G. B. 1964. The effect of extracts of Fucus serratus in
promoting the settlement of larvae of Spirorbis borealis (Poly-
chaeta). J. Mar. Biol. Assoc. U. K., 44: 397-414.
Spirorbid Larval Behavior
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Figure 1
Schematic ventral view of a spirorbid larva: a-head,
b-red eyespot, c-prototroch, d-collar, e-position of
dorsal primary shell gland, f-metatroch, g-abdomen.
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