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This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute.
Notice: ©1985 Elsevier. This manuscript is an author version with the final publication available and
may be cited as: Wilson, W. H., Jr. (1985). Food limitation of asexual reproduction in a spionid
polychaete. International Journal of Invertebrate Reproduction and Development, 8(1), 61-65.
lnt~rnational Journal of Invertebrate Reproduction and Developnjent, 8 (1985) 61-65
61
Els.:,·ic:r
JIR 00256
Short Communication
Food limitation of asexual reproduction in a spionid
polychaete
W. Herbert Wilson, Jr.
Harbor Branch Foundation, R.R. I, Box 196, Fort Pierce, FL 33450, U.S.A.
Received 22 May 1984
Summary
The rate of asexual fragmentation in the spionid polychaete Pygospio elegans is
'hown to increase in the presence of augmented food levels and is a density-depen·Jc:nt response. Asexual reproduction does not occur when the animals are reproductng sexually.
1sexual reproduction; density dependence; food level; fragmentation; Pygospio
The selective advantages of asexual reproduction are poorly understood. Asexual
reproduction is posited to be advantageous in sequestering available resources to the
nclusion of other genotypes, increasing the likelihood of survivorship of the
~rnotype by spreading the risk of mortality in space and bypassing the propagule
\tages (seeds, larvae) which generally suffer higher mortality rates than juvenile or
.adult organisms [1-5]. For organisms which incorporate both asexual and sexual
:nodes of reproduction into their life histories, clonal proliferation may confer
tncreased fitness by increasing the fecundity of a genotype. Despite the widespread
L"tCUrrence of asexual reproduction, few workers have examined the actual factors
•hich favor or inhibit such reproduction [3,6-11). Here I describe asexual reproduction in a marine polychaete and show that the rate of such reproduction is a
'aource-dependent phenomenon.
P)•gospio e/egans Claparede is a spionid polychaete (12 mm long) with a cosmopolitan distribution in the northern hemisphere [12]. In False Bay, Washington, P.
~~ is an abundant member of the upper intertidal soft-sediment community,
;Qching densities of 50000jm2 [13]. P. elegans reproduces asexually by transversely
· ''l~enting into 4-8 segments. These fragments then proceed to grow their missing
:taior andj or posterior parts while remaining in the tube of the progenitor [14).
'hen regeneration is complete, the clones leave the tube and establish tubes of their
"f.4-&lJ0; 8S j $03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)
62
own in the adjacent sediment. Sexual reproduction also occurs in this species;
females brood encapsulated young which may be released as either free-swimming
larvae or benthic juveniles (15].
· Following the results of experiments on vascular plant asexual reproduction [3]
and preliminary studies on polychaetes (13], I predicted that fragmentation in P.
elegans should be increased at low densities, facilitating the rapid occupation of
space by a genotype. I further predicted that food for this deposit-feeding species
would be a resource whose abundance would be positively correlated with the extent
of asexual reproduction.
To test these predictions, I performed a laboratory experiment testing the effect
of density (high vs low) and food level (ambient vs ad libitum) in a 2 x 2 design. I
established experimental populations of adult P. elegans in azoic sediment in 42-mm
diameter glass jars. The depth of the sediment in each jar was 50 mm. The jars were
placed in running seawater at the Friday Harbor Laboratories. Each replicate of the
low density treatments received 17 worms, corresponding to a density of 12000jm2•
Each replicate in the high density treatments had 68 worms, corresponding to
ambient field density. Food level was increased by the addition of Gerber's mixed
cereal, successfully used by Tenore [16] as a source of detritus for deposit-feeders in
laboratory experiments. The cereal was ground to a fine powder, made into a slurry
in seawater and spread evenly over the surface of the sediment with a syringe. The
food was administered ad libitum and was replenished every 3-4 days; food was
therefore always visible on the sediment surface. Worms in the ambient (no food
added) treatment fed only on the organic matter present in the natural sediment.
Each density-food combination was replicated two times in the high density case and
eight times in the low density case. Thus, each combination of food and density
involved 136 worms distributed across either two (high density) or eight (low
density) replicates. Because I was interested in individual responses, I chose to
equalize the number of worms used per treatment rather than the number of
treatments. After 10 weeks, each replicate was sieved through a 500 mu sieve and the
number of worms counted. The experiment was performed in June-August, 1982.
The total final abundances of P. e/egans in each of the four treatments are given
in Fig. 1. By inspection, the addition of food andjor low density is associated with
increased abundances, compared with the ambient food/high density (NF-HI)
treatment. When the data are analyzed in a 2 X 2 contingency table, a significant
difference in the number. of worms recovered was found (x 2 = 60.68, P < 0.001).
When the contingency table is subdivided into two-way comparisons between
treatments, one finds that there are significantly fewer worms in the HI-Nf
treatment when compared with the remaining three treatments (x 2 > 20.0 for all
three comparisons, P < 0.001). The three remaining treatments do not differ in the
number of worms recovered (x 2 test; P > 0.07 in all three comparisons). Thus, the
low numbers of worms recovered in the HI-NF treatment is solely responsible for
the significant difference in the overall 2 X 2 contingency table. (Use of parametric
statistics was prevented by uncorrectable heterogeneity of variance (Bartlett's test;
P < 0.05).) The experimental results are as expected; I earlier demonstrated a
significant density effect [13]. The significantly lower rate of asexual proliferation in
63
400
0
"§.
300
0
:
... 200
':!
0
~
100
F- LO
f - HI
Nf-LO
Nf - HI
Fig. 1. Final abundances in the various treatments. Cross-hatched areas represent the number of Py gospio
elegans at the beginning of the experiment. NF. no food added; F. food added; HI, high density; LO, low
density.
·
the food-limited high density treatment contrasts with the higher rate observed in the
high density treatment where there was a surfeit of food. I have therefore succeeded
in dissecting apart the effects of density and food level in this experiment; it is
otherwise difficult to separate the effects of food and density because altering the
density also alters per capita food level.
A possible explanation for the significantly low proliferation rate in the high
density-low food treatment would be that fragmentation occurred at the same rate as
in other treatments but that survivorship was lower. This explanation is discounted
by noting that fewer than 20% of the worms in both replicates of the NF-HI
treatment had fragmented compared with greater than 80% of the worms in all
replicates of the three remaining treatments (Mann-Whitney U-test, P > 0.20 for all
three comparisons). Thus, suppression of fragmentation is induced by conditions of
high density and low food. The alternative explanation that asexual reproduction
occurs only when worms reach a certain size, a size they might not attain in the
food-limited treatment, is disallowed because I used only the largest Pygospio in
setting up the experiment.
These data show that the rate of asexual fragmentation in P. elegans is a response
to food levels, in turn a density-dependent resource. The selective advantage of such
behavior is best interpreted as a means of efficiently sequestering available resources.
I have previously shown that high P. elegans densities cannot exclude the immigration of a co-occurring spionid, Pseudopolydora kempi [22]; the fragmentation response does not confer a means of excluding potential competitors through direct
interactions. The fragmentation response does, however, provide an efficient means
of exploiting high resource levels.
Superimposed on the effect of food and density, a distinct seasonality in fragmentation in the Washington population is seen. I first performed this experiment in
~ovember-December, 1981, and found only three asexually produced individuals
from an original total of 384 worms. (In June-August, 1982, I found over 1100
asexually produced individuals from an original total of 544 worms, Fig. 1.)
~ovember-December corresponds to the time when sexual individuals appear in the
population. Males, normally not distinguishable from females in the population [25],
develop secondary sexual structures and become ripe in November. Gametes in both
64
sexes are present only in November and December. Asexual reproduction occurs in
the. field from March until October. There is therefore a temporal separation of
sexual and asexual reproduction in the Washington population. In Scandinavian
waters, Rasmussen [23) showed that fragmentation in Pygospio elegans could be
induced by lowering seawater temperature. However, he did not describe the
densities at which the P. elegans were held so it is not possible to know if the
Scandinavian populations respond only to temperature or if they elicit a density-dependent fragmentation response subject to a greater constraint of seasonality.
Nonetheless, the data presented herein show that food is a density-dependent
resource that strongly affects the rate of fragmentation in a Washington population
of P. elegans.
Acknowledgements
I would like to thank K.J. Eckelbarger, S. Lidgard, J.D. Orcutt, R.R. Strathmann
and S.A. Woodin for useful comments on the manuscript. A.O.D. Willows made the
facilities of the Friday Harbor Laboratories available to me. This research was
supported in part by a grant from the Theodore Roosevelt Fund of the American
Museum of Natural History. This paper is Contribution 438 of the Harbor Branch
Foundation.
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