Behavioral Ecology VoL 8 No. 1: 83-86
Egg trading in the simultaneously
hermaphroditic polychaete worm
Ophryotrocha gracilis (Huth)
GabrieHa Sells, M. Qotilde Premoli, and Fiammetta Turri
Department of Animal Biology, University of Turin, Via A. Albertina 17, 10123, Turin, Italy
Ophryotrocha gracilis (Huth 1934) is a simultaneously hermaphroditic polychaete worm with external fertilization and a brief
protandrous phase. The mating system of this species seems to meet conditions leading to the establishment of egg-trading
behavior. Experiments showed that mating occurs in pairs composed of two simultaneous hermaphrodites; sex roles are sequentially alternated and self-fertilization is avoided. Egg reciprocation is kept evolutionarily stable by laying eggs in multiple,
small-sized egg clutches and reducing the reproductive success of pairs in which one of the two partners does not reciprocate
egg exchange. The frequency of ovigerous hermaphrodites in mass cultures of O. gracilis is about 50%. Such a high mate
availability preadapts hermaphrodites of O. gracilis to change partners very frequently and to reduce investment in parental
care, contrary to what is observed in another egg-trading, simultaneously hermaphroditic species, O. diadtma. Laboratory
populations of O. diadtma have a frequency of only 17% ovigerous hermaphrodites. Key words: egg trading, Ophryotrocha gracilis,
Ophryotrocha diadtma, Poh/chaeta, protandry, simultaneous hermaphroditism. [Bthav Ecol 8:83-86 (1997)]
E
gg trading is a mating behavior in which a simultaneous
hermaphrodite gives its own eggs to be fertilized in exchange for the opportunity to fertilize those of a partner. A
hermaphrodite is likely to fertilize more eggs in this manner
than if it does not reciprocate egg exchange. Up to now, egg
trading has been reported in only two distant taxa of simultaneous hermaphrodites with external fertilization: in some
coral reef fish, HypopUctrus nigricans, Serranus tortugarum
(Fischer and Petersen, 1987), and S. tabacarius (Petersen,
1991) and in a polychaete worm, Ophryotrocha diadtma (Sella,
1985).
Because sperm are assumed to be less costly than eggs, in
the mating system of an egg trader, an individual fertilizing
the partner's eggs without giving any of its eggs would apparently gain an advantage. But the potential advantages of behaving only as a male are severely restricted by the egg reciprocation mechanism: an individual cannot fertilize more eggs
than it releases. Both the egg-trading hermaphroditic species
of coral reef fish and the polychaete O. diadtma have evolved
two basic mechanisms that reduce the reproductive success of
nonreciprocating individuals: (1) Ripe eggs are released gradually; either the clutch of ripe eggs is divided in small parcels,
as in serranid fish (Fischer, 4980), or a few oocytes reach maturation contemporaneously and spawning involves small-sized,
multiple dutches, as in the polychaete O. diadema (Sella,
1988). In this way losses are minimized if the partner deserts
or does not reciprocate. (2) Egg laying is delayed if the partner is not ready to reciprocate; such a delay sharply rfiminiihes reproductive success of both partners (Fischer, 1980; Premoli and Sella, 1995b).
If reproductive success of male-behaving individuals is lower
than that of egg-trading individuals, a female-biased allocation
of sexual resources is expected. When there is no male-male
competition, only the minimal quantity of sperm necessary to
fertilize the partner's eggs should be produced. A female-biased reproductive effort will allow a hermaphrodite to pro-
Received 30 October 1995; first revision 30 October 1995; Kcond
revision 13 March 1996; accepted 21 March 1996.
1045-2S49/97/$5.00 O 1997 International Society for Behavioral Ecology
duce more than half as many eggs as a pure female and fertilize more than half as many eggs as a pure male (Chamov
et al., 1976). In H. nigricans (Fischer, 1981) and in O. diadtma
(Sella, 1990), testicular tissue is about one-ninth and one-fifth
of ovarian tissue, respectively.
We tested the basic assumptions of the egg-trading model
by analyzing the mating system of a simultaneously hermaphroditic polychaete worm, O. gracilis, in which the hermaphroditic phase is preceded by a protandrous phase, as in O.
diadema. We investigated (1) whether, in a mating pair between two hermaphrodites, allocation of spawning roles follows a regular alternation and to what extent males (i.e.,
young individuals still in the protandrous phase) are ignored
as mates since they are unable to reciprocate egg exchange;
(2) what safeguards have evolved against adults that fail to
reciprocate egg exchange; and 3) whether simultaneous hermaphrodites self-fertilize when no mates are available. To stabilize an egg-trading system, self-fertilization should be impossible or highly detrimental in terms of offspring fitness.
METHODS
Study jmii&ftl
Ophryotrocha gracilis is a member of the interstitial fauna of
unstable, coarse sands and is commonly found in the North
Sea and in the English Channel. Populations of this species
are known to be present in sands around the isle of Syit, Helgoland, and at Plymouth and Roscoff. Nothing is known about
the density in natural populations of O. gracilis, but it is assumed to be low, as for most species living in interstices between grains of sand (Svedmark, 1964; Westheide, 1984). This
species' behavior can be studied only under laboratory conditions, since the maximum body length of adults is only
about 5 mm, which corresponds to a body length of 35 setigerous segments.
The adolescent male phase usually starts when the body
reaches a length of seven segments and sperm are produced.
in four body segments, from the third to the sixth (Akesson
B, personal communication). The male phase lasts for 6
weeks; this lapse of time approximately corresponds to onefifth of the reproductive life of O. gracilis. A simultaneous
Behavioral Ecology VoL 8 No. 1
84
production of sperm and oocytes begins at a body length of
15-16 segments. On the average, ovaries are active in 14 segments (from the 7th to the 21st). In laboratory populations
this potychaete is a continuous breeder. Worms breed only in
pairs. Mating is achieved by pseudocopulation; i.e., a process
of external fertilization, which maintain* partners of a pair in
close physical association before synchronous release of gametes. Eggs are laid in transparent mucous cocoons. Under
our rearing conditions, during the first 8 weeks of the hermaphroditic phase, the number of eggs per cocoon reached
a mean value (± SD) of 11.2 ± 0.5 per week (n = 20), remained on this value for the next 13 weeks, and then slowly
decreased. This long plateau phase makes fertility relatively
age independent compared to O. diadema (Akesson, 1976).
The mean number of eggs per cocoon has a very small standard error; such a small dutch-size variation makes measuring
reproductive success as the number of cocoons or measuring
it as the number of eggs per cocoon nearly equal (Sella, 1988,
1990).
Eggs are laid on average once a week. Fertilized eggs can
be easily recognized because cleavage can be observed by
means of a stereomicroscope (120X), whereas unfertilized
eggs degenerate within 24 h after spawning. Although in some
Opkryotrocha species (e.g., O. labrvnica and O. diadema ) eggs
are continuously guarded at least by one of the parents, in O.
gradlis only 15% of pairs (n = 48) stay near their cocoon on
the first day after a cocoon is laid. This frequency drops to
8% on the second and third day (Sella G, personal observation).
Animals for the present study were reared in filtered sea
water at a density of 0.022 g/1, at a constant temperature of
12°C, fed parboiled spinach, and kept under ambient light
conditions. Separated pairs or triplets of worms were maintained in 10-ml glass bowls or petri dishes. Mass cultures of
about 100 individuals were maintained in 1500-ml bowls. Water and food were changed once a week. In mass culture, the
breeding sex ratio (i.e., the ratio of protandrous males and
hermaphrodites without oocytes to ovigerous hermaphrodites) is generally 1:1.
When it was necessary to recognize members of a pair, one
of the two partners was marked either by placing a worm in
a 0.04% of a 1:300,000 Nile sulfate blue solution in sea water
for 24 h. (Nile sulfate blue stains the mucous glands of the
head and chetigerous segments for about 3 weeks) or by cutting off the last two body segments with a razor blade after
the worm had been lightly anesthetized with a solution of 7%
MgO ,. The cut heals very rapidly and the last two body segments regenerate within 15-20 days. In other species of this
genus, cutting of! the last two segments does not modify sexual behavior (Rolando, 1981).
Allocation of spawning roles
We determined the pattern of allocation and alternation of
spawning roles by observing egg laying in 10 pairs of hermaphrodites, each kept in a separate bowl. Observations were
made every second day until each pair had spawned six times.
One of the two partners of each pair had been marked with
Nile sulfate blue. Since the body walls are transparent, the
egg spawner can be recognized because after spawning it appears void of eggs, while the fertilizer still ha* its oocytes. The
experiment lasted for 30 days.
F«U«w4ag Fischor (1980), the probability that in each pair
the observed alternation in spawning roles by both partners
could occur by chance was determined by means of a runs
test (Sokal and Rohlf, 1981) and by reference to tables compiled by Swed and Eisenhart (1943). The probability values
for each pair were then transformed to their natural loga-
rithms and combined to give an overall probability, which is
distributed as a chi-square, with X a number of degrees of
freedom corresponding to twice the number of separate probabilities.
Duration of the pair bond
To test whether O. gracilis shows a tendency to spawn with
die same partner more than once, we placed six ovigerous
hermaphrodites with a similar body length in each of four
petri dishes. Population density was 1 individual/5 ml of sea
water. To facilitate identification of partners, two hermaphrodites were colored with Nile sulfate blue, two had the last
three segments cut off, and two had a markedly different body
length (e.g., 17 and 24 segments). To control whether a pair
persisted after more than one spawning, the position of each
observed pair and its cocoon was marked on the outer side
of each Petri dish with a waterproof pen. The experiment
lasted 3 weeks.
Mate
If O. gradtis is an egg trader, die presence of ripe oocytes in
both partners is critical for mating success. Therefore, to evaluate mating success of males, 24 bowls each containing two
20-segment long .hermaphrodites and one 7-segment long
male were set up. Each hermaphrodite could mate either with
the male or with the other hermaphrodite. Under random
mating, expected frequencies of the two kinds of pairs were
8 and 16, respectively. Animals were observed until a pair was
formed in each bowl and one of the partners had laid eggs.
Reproductive success and fallure to
reciprocate egg exchange
If reciprocal spawning is critical to mating success, the reproductive success of a hermaphrodite mated to a male is expected to be lower than that of an ovigerous hermaphrodite
paired with another ovigerous hermaphrodite. In the former
case, the simultaneous hermaphrodite can act only as a female, since alternation of sex roles and reciprocation of fertilization is precluded.
Twenty-two 20-25 segment hermaphrodites were kept with
a 3-4 segment juvenile, to verify when juveniles reached the
adolescent protandrous male phase and began to fertilize
eggs. Each pair was kept in a separate 10-ml bowl. We recorded time intervals (expressed in days) between die hermaphrodite's successive spawnings. After the male attained the
16 segment stage, it became hermaphroditic and began to
spawn. From this moment onward, die pair was considered as
consisting of two simultaneous hermaphrodites. Time intervals between successive spawnings were recorded again. Following work on O. diadema (Sella, 1988, 1990), we estimated
the reproductive success of both kinds of pairs as die mean
number of cocoons per individual per day.
The distribution of the number of cocoons and time intervals between successive spawnings did not fit a normal distribution. Therefore, both groups of data obtained from die hermaphrodite-male and hermaphrodite-hermaphrodite pairs
were compared using the nonparametric Wilcoxon two-sample test (Sokal and Rohlf, 1981).
Self-fertiBzatkm ability
We kept 24 ovigerous hermaphrodites isolated in 15-ml bowls
for 3 monuis and checked every 2 days for spawning and fertilization.
Sella et al. * Egg trading in O. graciHs
86
Table 1
rataaneota hennaphrodlte. of
SMMllllg 1
apawmngi nqnenices uj
O. graciHs
Pair
1
2
3
4
5
6
7
8
9
10
Spawning sequence
A
A
B
B
A
A
B
B
B
B
B
A
A
B
B
B
B
A
A
A
A
B
A
B
A
A
A
B
A
B
Tablet
Sex allocation pattern, and characteristica of the mating systems of
the two simultaneous hermaphrodite. O. gradlu and O. diadtma'
Characteristic
B
A
B
A
B
B
A
A
A
A
A
B
B
A
A
B
B
B
A
A
A
B
B
B
B
B
A
B
A
A
A, egg released by the unmarked partner, B, egg released by the
marked partner.
RESULTS
All pain showed a tendency to reverse sex role* regularly at
each spawning (Table 1). In 3 out of 10 pairs, spawning roles
were regularly alternated in 6 subsequent observed spawning
bouts, in 5 out of the remaining 7 pairs, spawning roles were
alternated 5 out of 6 times. The spawning sequence was not
random (run test x 1 " 54.84, df = 20; p<.001), indicating
that partners actively alternated sex roles during the experiment
All 24 animals were involved in copulation, and therefore
12 pairs were formed in the 4 groups of 6 hermaphrodites.
In two of these pairs (17%), egg reciprocation continued for
four successive spawnings, in two pairs (17%) for three spawnings. In four pairs (33%) partners deserted after two spawnings, and in four pairs (33%) partners immediately after a
single spawning.
In all 24 trios, hermaphrodites mated with the other hermaphrodite. Males were never involved in roarings.
In 7 out of 22 pairs, the adolescent male fertilized the partner's eggs for die first time at a body length ranging from 7
to 11 segments, while in the other 15 pairs the first fertilized
cocoon was observed when the male partner had reached a
body length from 12 to 17 segments.
Distribution of time intervals in days between successive
spawnings in hermaphrodite-male pairs (median = 7, range
=21, n = 22) and in hermaphrodite-hermaphrodite pairs
(median = 6, range = 12, n *• 22) were significantly different
(Wilcoxon two-sample test, t s = 2.12; p < .02). Similarly, die
distribution of numbers of cocoons per hermaphrodite per
day of hermaphrodite-male pairs (median =• 0.059, range =
0.109, n = 22) and of hermaphrodite-hermaphrodite pairs
(median = 0.125, range « 0.438, n = 22) showed a highly
significant difference (t, = 4.35, p < .001).
During the 5-month isolation period, die 24 ovigerous hermaphrodites never spawned eggs, and all of them resorbed
their oocytes.
DISCUSSION
The mating system of O. gracUis seems to meet three conditions leading to die establishment of an egg trading system:
(1) mating occurs in pairs of ovigerous hermaphrodites,
which sequentially alternate sex roles more than once. (2) No
male competition is present; males are never involved in mating (when given a choice), and pairs are always formed between two simultaneous hermaphrodites. As expected, reproductive allocation to gonads is female biased since ovarian
Ratio between trsrlnilar and
ovarian masses
% Origerous hermaphrodites in
mass cultures
Ratio between male and hermaphroditic
phase duration
Egg reciprocation behavior
Egg parceling behavior
Ability to time egg laying
Male competition
% Stable pair bonds after four
consecutive spawnings
Investment in bipareutal care
O. gradEs
O. diadtma
1:4
1:
47 J%
175
1:4
Ye*
yes
yes
absent
12
yes
yes
yes
Very low
15%
Very low
30%
High
*
• Data for O. diadma from Sella (1985, 1988, 1990. 1991) and
PremoH and SeOa (1995a,b).
mass is 3-4 times as large as testicular mass. (3) Self-fertilization does not occur.
For such a mating system to be stable, two mechanisms reducing thefitness*of nonreciprocating hermaphrodites have
evolved. First, few eggs reach maturity simultaneously and
clutches are released frequently (once a week), even if no
morphological constraints to production of large dutches are
present This form of egg parceling is common to other simultaneously hermaphroditic species of Ophryotrocha (Premoli and Sella, 1995b); in contrast, gonochoric species of
Ophryotrocha (which have a similar body size and morphology
and do not parcel their reproductive effort) release 100-300
eggs every 2 weeks. Second, hermaphrodites that were not
reciprocated by their partner because diey were forcedly mated to males laid eggs for longer time intervals compared to
pairs of simultaneous hermaphrodites. Therefore, their reproductive success was significantly lower than that of reciprocating hermaphrodites. Thus, hermaphrodites seem to be able
to recognize the sexual phenotype of their partner and, by
timing spawning activity according to the sexual condition of
their partner, diey reduce the risk of being cheated by a nonreciprocating partner. H. nigricans (Fischer, 1980) and O. diadema (Sella, 1988) show the same behavior. This means that
both groups of egg parcelers have a partial behavioral control
on egg releasing.
Comparing die results of the present study with those of
previous studies on the mating system of O. diadtma (Sella,
1988, 1990, 1991), diree main differences between the two
mating systems emerge (Table 2). O. gracilis males do not
have any reproductive success, at least under our experimental conditions. Under natural conditions, however, occasionally a highly variable reward in male fitness can be expected,
as indicated by the results of experiment 2. Male competition
(i.e., interference by young males or hermaphrodites playing
the male role) seems to be almost absent in O. graciHs. Therefore, a greater female-biased allocation of reproductive effort
in this species is expected compared to O. diadtma. Although
die bias in female allocation to gonad mass and number is
about the same in both species, O. gracilis further reduces its
male investment by shortening die duration of the protandrous phase compared to die whole reproductive life duration.
In O. gracilis pair bonds appeared relatively looser than in
O. diadema (Table 2) when kept under experimental conditions where more dian one partner were available. In O. grac-
Behavioral Ecology Vol. 8 No. 1
Ms fecundity i» not strongly age dependent; the reproductive
rate of ovigerous hermaphrodites is stable for a long time. In
mass cultures the proportion of ovigerous hermaphrodites is
about 50% of the breeding population. In contrast, in laboratory populations of O. diadema the number of eggs per cocoon is strongly age dependent (Akesson, 1982), and the frequency of ovigerous hermaphrodites is only 17%. Consequently, frequent partner changes may increase the risk for
an ovigerous hermaphrodite of encountering a subadult or
an old individual with few eggs. Therefore, in O. diadema the
pair bond is protracted for as long as both partners have eggs
to mutually fertilize, while in the former species the population breeding sex ratio is much more favorable to a partner
tempted to desert.
Finally, in the mating system of O. graatis there is a low
investment in parental care compared with O. diadema (Table
2), in which nearly all mating pain care for eggs (Premoli
and Sella, 1995a). Tune devoted by O. gradhs hermaphrodites
to caring for offspring is probably the minimum required to
bring embryos to complete development In contrast, in O.
diadema, because the population male-biased sex ratio deprives parents of the prospect of mating again soon, biparental care may be considered a by-product of the convenience
of staying close to a good partner (Sella, 1991).
We do not know whether the different frequencies of ovigerous hermaphrodites we observed in captive populations
are also present in natural populations of both spedes. If the
frequencies are the same in nature, a key factor in shaping
one mating system over another could be the different availability of simultaneous hermaphrodites.
The first draft of the manuscript greatly benefited from critical comments by E. Balleoo, D. Peoani, B. Akenon, and two anonymous referees. Financial support was provided by the Italian Mlniitero Unlveraita Ricerca Sdenorica Tecnologica (MURST 60% grant).
REFERENCES
Akesson B, 1976. Morphology and life cycle of OpkryotroAa diadema,
a new polychtete species from California. Ophelia 15:25-35.
Akenon B, 1982. A life table study on three genetic strains of Ophryotrocha Oaiema (Polychaeta, Dorvillddae). Int J Invert Reprod 5:
59-69.
Charncrr EL, Maynard Smith J, Bull JJ, 1976. Why be an hermaphrodite? Nature 263:125-126.
Fischer EA, 1980. The relationship between mating system and simultaneous bermaphroditism in the coral reef fiih Hjpopltdnu mgricans (Serranidae). Anim Behav 28:620-633.
Fischer EA, 1981. Sexual allocation in a simultaneous hermaphroditic
coral reef fish. Am Nat 117:65-82.
Fischer. EA, Petersen CW, 1987. The evolution of social patterns in
the seabanet Bioscience 37:482-489.
Petersen CW, 1991. Sex allocation in hermaphroditic seabasses. Am
Nat 138:650-667.
Premoli MC Sella C, 1995a. AUoparental egg care in the poh/cbaete
worm Opkrfotrodut diadrma (Polychaeta). Ethology 101:177-186.
Premoli MC, Sella G, 1995b. Sex economy in benthic polychaetes.
Ethol Ecol Evol 757-48.
Rolando A, 1981. Early courtship and sexual differentiation in Opkryotrocha labrmUa La Great and Bacci (Polychaeta, Dorvilleidae).
Monlt Zool Ital 155:3-61.
Sella C, 1985. Reciprocal egg trading and brood care in a hermaphroditic polychaete worm. Anim Behav 33:938-944.
Sella C, 1988. Reciprocation, reproductive success, and safeguards
against cheating in a hermaphroditic polychaete worm, Opkryotrocka diadtme AkaiDn, 1976. Biol Bull 175:212-217.
Sella G, 1990. Sex allocation in the simultaneously hermaphroditic
polychaete worm Ophxjotrocha diadema. Ecology 71:27-32.
Sella G, 1991. Evolution of biparental care in the hermaphroditic
polychaete worm OpkryotrocJia diadema. Evolution 45:63-68.
Sokal RJ, Rohlf FJ, 1981. Biometry. San Francisco: W J t Freeman.
Swed FS, Eisenhart C, 1943. Tables for testing randomness of grouping in a sequence of alternatives. Ann Math Scat 14.-66-87.
Svedmark B, 1964. The interstitial fauna of marine sand. Biol Rev 39:
1-42.
Westheide W, 1984. The concept of reproduction in polychaetes with
small body size: adaptation in interstitial spedes. Fortschr Zool 29:
265-287.