-
Pacific Science (1980), vol. 34, no. 2© 1981 by The University
Press of Hawaii. All rights reserved
The Annual Cycle of Oogenesis, Spawning, and Larval Settlement
of theEchiuran Listriolobus pelodes off Southern California!
JOHN F. PILGER2
ABSTRACT: Listriolobus pelodes lives in aggregated populations
in fine-grained sediments along the mainland shelf from northern
California to BajaCalifornia, Mexico. This echiuran forms U-shaped
burrows and uses its pro-boscis to feed on the uppermost layer of
sediment deposited around the burrowapertures. The bacterial flora
of the sediment may be an important food source.Commensals in the
burrow include a polychaete, a pinnixid crab, and a
smallbivalve.
The annual reproductive cycle of a population off Palos Verdes,
California,is defined in terms of coelomic oocyte dynamics,
spawning, and larval settle-ment. Small oocytes are released from
the gonad through most of the year butfully grown oocytes are
present only from mid-fall through spring. It is estimatedthat the
coelomic phase of oogenesis lasts about 5 months. Fully grown
oocytesare removed from the coelomic fluid in the germinal vesicle
stage and accumu-late in the storage organs until spawning.
Spawning takes place in winter andspring a(ld individuals are
spawned out by summer. An annual influx of smalljuveniles into the
population occurs in late winter and spring. The newly
settledjuveniles reach sexual maturity when they are 6 months to 1
year old.
echiurans in all aspects except size. In 1970,during an
investigation of a large oil spillwhich occurred at Santa Barbara,
it wasfound that the number of L. pelodes haddecreased dramatically
(Fauchald 1971).Populations of L. pelodes are found in otherareas
off the southern California coast(Fauchald and Jones 1976; Green
and Smith1975). All of these populations share
threecharacteristics: (I) L. pelodes is a
dominantmacroinvertebrate, (2) they are highly ag-gregated, and (3)
they are located in areascomposed of fine-grained sediments with
ahigh organic content.
In spite of its prominence in localizedbenthic communities,
surprisingly little isknown about the biology of
Listriolobuspelodes. The purpose of this paper is toprovide a
compendium of information aboutthe natural history and behavior of
Listriolo-bus pelodes and, further, to describe itsreproductive
biology in terms of coelomicoocyte dynamics, spawning, and larval
set-tlement. Reproductive cycles have beenstudied in related phyla
such as the annelids
129
Listriolobus pelodes (Fisher 1946) is a deposit-feeding echiuran
found in 18-155 m depthsalong the west coast of North America
fromnorthern California to Baja California,Mexico. Barnard and
Hartman (1959) de-scribed a unique area of fine-grained sedi-ments
dominated by this animal near SantaBarbara, California. The more
peripherallylocated members of this population wereconsiderably
smaller than the centrallylocated ones. These "small phase"
echiuranswere considered to be similar to the larger
1 This research was supported in part by BiomedicalResearch
Support Grants RR07012-09, RR07012-10,and 5S07 RR07012-10 from the
Division of ResearchResources, Bureau of Health Professions,
Educationand Manpower Training, National Institutes of
Health.Scientific Contribution No. 42. Catalina Marine
ScienceCenter, University of Southern California, P. O. Box 398,A
valon. California 90704. Scientific Contribution No.55. Smithsonian
Institution, Fort Pierce Bureau-HarborBranch Foundation, Inc.
Consortium, Fort Pierce,Florida 33450. Manuscript accepted 4
September 1979.
2 Smithsonian Institution, Fort Pierce Bureau, RouteI, Box
194-C, Fort Pierce, Florida 33450.
-
130 PACIFIC SCIENCE, Volume 34, April 1980
White'sPoint
/
.................,\ I I
\ Nautical Mile\,"",
\,... -.... /""---',
" I I
'60 ~//"M--- :"
FIGURE I. The collecting site, station 6C, at White's Point,
California. Inset of California showing the locationof the White's
Point area.
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-
Echiuran Reproduction off Southern California-PILGER
6
131
5
-4E-Q)E::J 3o>>."0o 2
(1)
1
y=1.05x-0.14
r 2 =0.91n:50
..
1 2 3 4Body Weight (g.)
5 6
FIGURE 2. Regression of body volume against wet body weight of
L. pelodes collected from station 6C.
(see Schroeder and Hermans 1975 for areview; Olive 1977, 1978)
and sipunculans(see Rice 1975 for a review; Gibbs 1976),but they
have never been investigated in theEchiura. It is hoped that this
informationwill provide a basis from which furtherstudies of this
interesting echiuran willemerge.
METHODS
Specimens of L. pelodes were collectedfrom a depth of 60 m at
station 6C of theLos Angeles County Sanitation District,
near White's Point, California (33°42.5' N,118°20' W; Figure I).
Supplementary in-formation for certain aspects of this studywas
obtained by using large specimens col-lected from the Santa
Barbara, California,L. pelodes population which was describedby
Barnard and Hartman (1959; 34°23' N,119°38' W). Live animals and
sediment wereobtained with a Shipek grab or box corerand maintained
in flow-through seawateraquariums in a laboratory at the
CatalinaMarine Science Center, University of South-ern
California.
Adults of L. pe/odes are poor burrowersand will die if a burrow
is not constructed.
.,
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132 PACIFIC SCIENCE, Volume 34, April 1980
.......
. . .. .. ~ :
ing of aspects of the animal's behavior whichtake place within
the burrow.
Diameter of coelomic oocytes was used asthe primary index of
gametogenic activity.On a monthly basis from January 1975through
December 1976, a sample of coel-omic fluid was withdrawn with a
syringe from5 females; 40 oocytes were measured in eachspecimen.
The frequency distribution of thesediameters was plotted on a time
scale.
Differentiated gametes accumulate in 2pairs of storage organs
(modified nephridia)until spawning. These organs become
greatlyenlarged by the accumulation of gameteswithin them. A
relative decrease in the volumeof these organs can indicate the
time ofspawning. For each individual, the 4 organswere measured and
their total volume calcu-lated. This volume was divided by the
ani-mal's wet body weight to correct for differ-ences in individual
size. It was not possibleto make these measurements on the
speci-mens from White's Point because they werecollected as part of
a County Sanitation Dis-trict study which precluded their
dissection .To get some idea of the spawning period,however, data
were obtained from specimenscollected from Santa Barbara,
California, asreported by Barnard and Hartman (1959) andothers
(Pilger 1977).
The size-frequency distribution of echiu-rans in the samples
taken at station 6Cindicated the period of larval settlement.Wet
body weight was used as an index ofsize. This measurement was found
to bedirectly related to body volume (Figure 2)and could be
determined accurately andrapidly.
....... ~. ~ :.:', ,", ...
vrjc
nl
J.-Im.
::
:;
.....
......
FIGURE 3. Ventral view of Lislriolobus pelodes. 1m,longitudinal
muscle band; m, mouth; n, nephridiopores;nl, anterior nerve loop;
p, proboscis; S, setae; vnc,ventral nerve cord on mid-ventral
muscle band. Scaleequals 5 mm.
The number of casualties from this problemwas minimized by
artificially constructingU-shaped burrows for some, and by
main-taining others in glass or plastic tubing andproviding
sediment for feeding. Burrows innarrow glass-sided boxes filled
with mudfacilitated the observation and understand-
RESULTS
Natural History Observations
The body of Listriolobus pelodes (Figure 3)is subspherical to
ovoid and measures up to4 em long and 2 to 3 em wide. A green
pig-ment usually is present on the anterior andposterior ends of
the body and 8 regularly-spaced longitudinal muscle bands are
visiblein the body wall. The mouth is located
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-
Echiuran Reproduction off Southern California-PILGER 133
ventrally at the base of the dorsoventrallyflattened, highly
extensible proboscis. Twohooked setae straddle the ventral
midlinebehind the mouth. A pair of nephridioporesopen on each side
of the ventral nerve cordposterior to the setae.
Using a box corer, 10 intact Listriolobuspelodes burrows were
collected from SantaBarbara, California. Careful examination
ofthese burrows showed all of them to beU-shaped with exactly 2
openings to thesurface. Peristaltic contractions of the bodypump
respiratory water through the burrow.The anal vesicles (paired
hind-gut diver-ticulae) intermittently "inhale" and
"exhale"portions of this water.
Listriolobus feeds on deposited sedimentthat surrounds the
burrow apertures. Whilefeeding, the body remains within the
burrowand the proboscis extends onto the substratewith its ventral
side up. The top layer ofsediment is picked up by the proboscis
andtransported by ventral cilia to the mouth.Laboratory
observations showed that certainareas of sediment are grazed while
others aredisregarded. This may indicate that somedegree of
qualitative discrimination takesplace in food selection.
Fecal pellets (0.5 x 1.0 mm ellipsoids) re-main in the burrow
for a while after theirrelease. The combined effect of the
watercurrents from the body's peristaltic contrac-tions and the
"exhalations" from the analvesicles intermittently expels the
pellets fromthe burrow. The feces accumulate on thesediment around
the burrow apertures andgive rise to the hummocky topography of
anL. pelodes bed (R. Given, personal com-munication). Since these
pellets typically arereingested after a short exposure on
thesediment, a rapidly regenerating food re-source is implied
(e.g., bacteria and associatedmicroorganisms) .
The proboscis is highly extensi ble and largespecimens of L.
pelodes have been observedto feed on the sediment as far as 20 cm
fromthe burrow aperture. L. pelodes intermit-tently turns around in
its burrow, thusfeeding on the sediment surrounding bothburrow
apertures. These observations in-dicate that up to 0.25 m2 of
substrate surface
is available for feeding by individual echiu-rans and imply that
in dense L. pelodespopulations (approximately 100 individuals'm- 2)
all of the sediment surface is regularlygrazed. Clearly, the
potential impact of thepresence of L. pelodes on sediment
reworkingand the trophic structure of the benthiccommunity is
great.
At least 3 commensal organisms werefound to be living in the
burrow of Listrio-lobus pelodes: the polychaete Hesperonoelaevis
(Hartman 1961), the pinnotherid crabPinnixa schmitti, Rathbun 1918,
and thebivalve Mysella tumida (Carpenter 1864).Although there does
not appear to be anydirect interaction between these commensalsand
the echiuran, the burrow may affordthem some protection, and
nutritional benefitmay be derived from the water and asso-ciated
organisms which are pumped throughit (Fisher and MacGinitie 1928).
A peritri-chous ciliate was discovered in the coelomicfluid of a
few specimens and a helminthparasite was found embedded in the
pro-boscis musculature of another. Neither ofthese was identified
further.
Oogenesis
Echiuran oogenesis may be divided into 3distinct phases. The
oocytes begin theirdifferentiation in the gonadal phase. Theyenter
the second, or coelomic, phase whenthey detach from the gonad and
continuetheir growth in the coelomic fluid. Finally,as germinal
vesicles, they enter the storagephase when they are removed from
thecoelomic fluid and accumulated in the modi-fied nephridia.
In Listriolobus pelodes the gonad is situatedon the ventral
blood vessel in the posteriorregion of the body (Figure 4). The
bloodvessel is attached to and follows the ventralnerve cord
posteriorally. Near the rectum thevessel separates from the nerve
and attachesto the intestinal caecum. The germ cells arisefrom the
tissue surrounding the blood vesselin this region.
Small primary oocytes (5-7 f.lm) are re-leased from the gonad in
clusters of 20 to 30cells (Figure Sa). One to 3 of these cells
begin
-
134
av
PACIFIC SCIENCE, Volume 34, April 1980
FIGURE 4. Dorsal view of the posterior internal anatomy of L.
pelodes showing the location of the gonad. a, anus;aI', anal
vesicles; bl', blood vessel; cg, ciliated groove; g, gonad; i,
intestine; ie, intestinal caecum; vnc, ventral nervecord. Scale
equals I mm. Adapted by permission of the Smithsonian Institution
Press from United States NationalMuseum Proceedings, Volume 96,
"Echiuroid Worms of the North Pacific Ocean," by W. K. Fisher,
April II, 1946,pages 215-292, Figure 12. Washington, D.C.: U.S.
Government Printing Office, 1948.
to differentiate and are distinguishable bytheir increased
diameter (Figure 5b). Theremainder do not differentiate but
remainattached to the definitive oocytes. When anoocyte reaches a
diameter of 40 11m itseparates from the cluster to continue
differ-entiating as a single cell (Figure 5c). Uponattaining a
diameter of about 100 11m theoocytes are selectively removed from
thecoelomic fluid by the long coiled nephrosto-
mal lips (collecting threads). These threadslead to the
nephridia (storage organs) whereoocytes are stored until spawning.
Theoocytes are spawned in the germinal vesiclestage.
Gametogenic Cycle
The data presented in Figure 6 show theannual cycle of coelomic
oocyte growth in
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Echiuran Reproduction off Southern California-PILGER 135
cFIGURE 5. Coelomic oocytes from L. pe/odes. a, a cluster of
small primary oocytes recently released from the
gonad. Scale equals 10 Ilm. h, two differentiating oocytes still
attached to the cluster of small cells. Scale equals 10Ilm. c, a
coelomic oocyte that has dissociated from the cell cluster and is
undergoing the remainder of oogenesis as asingle cell. Scale equals
20 Ilm.
100
80
1 6097 E 405
-3 20
~25%
~b~555M J J
5S
5o
5N
5D
~ 80
~
-
136 PACIFIC SCIENCE, Volume 34, April 1980
-CJ)........
C')
E 20E .,- ,X " " 0.82 g), but lackedcoelomic gametes altogether,
were sometimespresent in the samples but could not be sexed.The
samples contained a greater percentageof individuals lacking
coelomic gametes inwinter (26 percent) and spring (29 percent)than
in summer (2 percent) and fall (11 per-cent). This indicates that
in many individualsthe coelomic gamete population became de-pleted
in winter and spring. This was pro-bably due to the accumulation of
fullydifferentiated gametes in the storage organsprior to spawning
(see Gamete Accumulationand Spawning below). By summer, the
coelo-mic gamete population in most of the indi-viduals had been
replenished with smallgametes released from the gonad. These
datasuggest that the gonad of L. pelodes may haveundergone a period
of quiescence which wasobscured by slight asynchrony in the
popula-tion and the sampling bias described above.To test such a
hypothesis would requireexamination of the gonad and careful
repeti-
-ea 44 4X gg
-
Echiuran Reproduction off Southern California-PILGER 137
tive monitoring of the coelomic gametepopulation in single
individuals over time.
Duration 01' OogenesisThe entry of oocytes into the coelomic
fluid during the summer defines periods ofoocyte growth which
can be used to estimatethe duration of oogenesis (Figure 6). In
May,June, and possibly July, 1975, small oocyteswere released into
the coelom. The first ofthese became fully differentiated by
October,indicating that the duration of oogenesis wasroughly 5
months (May to October). In asimilar manner, the oocytes that
reachedmaximum diameter in October 1976 areassumed to have been
released into thecoelomic fluid in April or May of that year.Thus,
the term of oocyte growth was estimat-ed as having been 5-6 months.
Estimatesfrom oocytes released at other times duringthe year are
not practical since the growthof discrete size classes could not be
followedwith certainty.
Oocytes that have reached maximum dia-meter are removed from the
coelomic fluidand accumulated in the storage organs fromOctober
through May.
Gamete Accumulation and Spawning
Annual fluctuations of the storage organvolume in specimens
collected in differentyears from Santa Barbara, California,
werepooled and are shown in Figure 7. The dataindicate that the
storage organs are smalland empty in summer. The volume
increasessharply in fall and continues through mid-winter as more
and more gametes becomefully differentiated and are removed from
thecoelomic fluid. The steady decline in storageorgan volume noted
throughout the springis assumed to be the result of spawning.
Bysummer the animals are spawned out and thestorage organs are
empty again.
Juvenile Settlement
During late winter and early spring 1975,a large influx of very
small juveniles «0.25 g)appeared in the population at White's
Point
(Figure 8). These were assumed to be recentlysettled
individuals. By April and May theyaccounted for as much as 75
percent of thepopulation (by numbers). With time theseindividuals
grew and appeared in progressive-ly larger size classes or died.
The number ofindividuals in the smallest size class remainedvery
low from fall 1975 through early spring1976. Another influx of
juveniles occurred inMay and June 1976. This settlement
involvedfewer individuals than the one in 1975, wasrestricted to a
narrower time-frame, and wasrecognizable as a distinct size class
for ashorter time.
Growth to Sexual Maturity
The growth of newly settled juvenilesduring their first year can
be predicted fromthe data presented in Figure 8. Since
thepopulation size frequency distributions arenoticeably skewed by
the influx of newlysettled juveniles, changes in the arithmeticmean
weight for the entire population are apoor indication of growth
during the firstyear. In this case, fluctuation of the mostfrequent
size class (i.e., the mode) moreadequately represents the weight
changes ofindividuals in these small size classes.
Growth of newly settled juveniles in thepopulation is seen
graphically as a progres-sion of the modal size class toward
heavierweight classes. The difference in weight of themodal class
in the 12 months followingsettlement gives an indication of
juvenilegrowth during the first year. In July 1975, themodal class
was 0-0.25 g and comprisedindividuals which had settled in the
precedingfew months. By January 1976, this class hadincreased to
0.75-1.0 g and was still quitedistinct. In June-July 1976, the mode
for theindividuals which had settled in 1975 wasabout 1.50 g. Thus,
in the 12 months follow-ing their settlement in 1975, the
juvenileL. pelodes had attained a weight of approx-imately 1.5
g.
Based on the examination of several hun-dred individuals of
various sizes, the smallestindividual in this study having
differentiatinggametes in the coelomic fluid weighed 0.82 gand was
assumed to have been about 6
-
138 PACIFIC SCIENCE, Volume 34, April 1980
801975 Jan.
n=44Feb.
n=50Mar.
n= 71Apr.
n=119
40
May June July Sept.80 n=118 n=108 n =107 n=128
» 400cQ)
::J0"Q)~
LL Oct. Nov. Dec. Jan.n=135 n=140 n=130 n=103
Q)
>-roQ)
a:Feb. Mar. Apr. May
n=143 n=95 n= 128 n=127
12345 12345
Weight (gJ
Sept.n=94
Julyn=121
Junen=140
1 2 3 4 5
FIGURE 8. Relative size frequency distributions of L. pelodes
collected from January 1975 through September1976 at station 6C.
The size classes are in 0.25 g increments of wet body weight.
months old. Thus, it is hypothesized thatL. pelodes juveniles
from White's Point reachsexual maturity when they are 6 months to
Iyear old.
DISCUSSION
Listriolobus pelodes was first describedfrom the stomach of
flounders (Fisher 1946)
and at White's Point it is known to be themain food item of the
dover sole, Microsto-mus pacificus (Lockington 1879; J.
Allen,personal communciation). The bat ray,Myliobatis californicus,
Gill 1865, is a sus-pected predator also since it is present withL.
pelodes at White's Point and it is knownto eat other echiurans
elsewhere (Fisher andMacGinitie 1928).
Listriolobus pelodes uses its proboscis
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:Belke _ MiNigy;
-
Echiuran Reproduction off Southern California-PILGER 139
to feed on sediment deposits in a manner simi-lar to that
described for Echiurus echiurus(Gish~n 1940), Ochaetostoma
erythrogrammon(Chuang 1962), and Bonellia viridis (Jaccariniand
Schembri 1977a, 1977b). Since L. pe/odesfeeds on sediment in some
areas while leavingother areas undisturbed, it is hypothesizedthat
the proboscis is able to discriminatequalitatively between
different sedimenttypes. Jaccarini and Schembri (1977b) re-reported
that Bonellia viridis has a feedingpreference for nutrient-rich
fine sediments.The sensory structures that are involved inthis
discrimination have not been described.Recent evidence indicates
that this type ofselectivity is not restricted to adults
sinceechiuran larvae show similar substrate pref-erences at the
time of settlement (L. Suer,personal communication; see also
Pilger1978).
Risk (1973) stated that the burrow ofListri%bus pe/odes is a
gallery consisting ofas many as 4 openings to the surface.
Thisstudy has shown, however, that the burrowsare U-shaped with
exactly 2 openings to thesurface. U-shaped burrows with only 2
open-ings have also been described for the mud-dwelling echiurans
Urechis caupo (Fisher andMacGinitie 1928) and Echiurus
echiurus(Gislen 1940). Bonellia viridis, on the otherhand, lives in
rock burrows having multipleopenings (Schembri and Jaccarini 1978),
butthese dwellings are formed by the action ofUpogebia de/tura; B.
viridis is only a second-ary resident that makes few, if any,
modifica-tions to the burrow system (Schembri andJaccarini
1978).
The location of the gonad is known onlyin a couple ofechiuran
species. Spengel (1879)described the gonad of Bonellia
viridis,Rolando 1821, as being an area of differen-tiating
peritoneal cells surrounding the post-erior part of the ventral
blood vessel. Morespecifically, the gonad covers the blood
vesselwhere it extends from the intestine to theventral nerve cord
and a little farther anter-iorally. In Lissomyema me/lita (Conn
1886)the gonad surrounds a posterior muscle bandwhich "extends from
the intestine to theventral nervous chord." Conn (1886)
clearlydescribed the gonad as being in the same
location as in Listri%bus pe/odes but inter-preted the
underlying structure as being mus-cular rather than vascular. The
associationof L. pe/odes' gonad with the posterior ventralblood
vessel corresponds well with what isknown in these 2 other echiuran
species.However, the location of the gonad inechiurans which lack a
vascular system(e.g., Urechis caupo) remains an enigma.
Scant information is available on thereproductive rhythms
ofechiurans other thanL. pe/odes. From this study and the
parenthe-tical remarks by others on 3 additionalechiurans, it is
known that gamete produc-tion is seasonal in some species, and in
atleast 1 other species it is continuous. Hiraiwaand Kawamura
(1936) mentioned that eggproduction in Urechis unicinctus occurs
dur-ing winter off Japan. The oogonia of Ikedo-soma gogoshimense
first appear in the coelo-mic fluid in May and June and
differentiationcontinues into winter (Sawada and Ochi1962). Urechis
caupo off southern California,on the other hand, produces gametes
con-tinuously (MacGinitie 1935) and contains alloocyte size classes
in the coelom simultane-ously throughout the year (Gould 1967).
Miller and Epel (1973) attempted to deter-mine the time course
of oogenesis in Urechiscaupo by radioactive labeling and
subsequentmonitoring of coelomic oocytes. This ap-proach failed
because it required that theanimals be retained in the laboratory
for anextended period and Urechis caupo is not ableto maintain
oogenesis during such a treat-ment. Das (1976) calculated the rate
ofribosome production for different oocytestages and, considering
the total amount ofribosomal material in a fully differentiatedU.
caupo oocyte, predicted that oogenesiscontinues for 135 days. The
value determinedfor Listri%bus pe/odes in this study (5-6months) is
considerably longer; the methodsused in this study were also cruder
than thoseused by Das. It is not possible to evaluatesuch
differences without more specific infor-mation about the process of
oogenesis inL. pe/odes.
Assuming continuous gamete productionin Urechis caupo,
Gould-Somero (1975) rea-soned that the frequency of each oocyte
size
-
140
class is proportional to the relative amount oftime an oocyte
spends in that particular sizerange during oogenesis. Her data
suggest thatthe rate of oocyte growth varies duringoogenesis.
Specifically, the larger oocytesseem to grow faster than the
smaller ones.Since gamete production by the gonad of L.pe/odes may
not be a continuous event, thereasoning used by Gould-Somero
(1975)cannot be applied here. It can be argued,however, that
differential oocyte growth mayoccur in L. pe/odes, since a
preponderance ofsmall oocytes was present in the coelomicfluid
during several months of the studyperiod though there never was a
month whenlarge ones prevailed.
The accumulation of ripe oocytes in thestorage organs of L.
pe/odes takes place infall, winter, and part of spring. Spawning,
onthe other hand, is limited to the springmonths. Thus, it is
probable that at least thefirst batch of oocytes produced in the
repro-ductive season remain in the nephridia for afew months before
they are spawned. Themetabolic requirements of these oocytes maybe
low since growth is negligible during thisphase. From November
through Marchspecimens can be obtained routinely whichcontain large
quantities of gametes in thestorage organs. This observation is in
con-trast to the situation in the Sipuncula wheregametes are rarely
found stored in thenephridia (Rice 1975).
Judging by the conditions of the storageorgans, Fisher (1946)
noted that a specimenof L. pe/odes that was collected in June
hadrecently spawned. His observation corre-sponds with the period
defined by the presentstudy. The only other information on
echi-urans available for comparison concernsUrechis caupo. This
echiuran is reported toundergo partial or complete spawning in
latesummer (Ricketts and Calvin 1962) despitethe fact that its
gamete production is con-tinuous.
In studying the L. pelodes population atSanta Barbara, Barnard
and Hartman (1959)stated that "mature individuals are
thosemeasuring 25 mm or longer." According totheir data (Table 6,
p. 13), these individualsshould weigh about 6 g. Personal
observa-
PACIFIC SCIENCE, Volume 34, April 1980
tions ofechiurans from the same area (though13 years later)
reveal that the lower weightlimit of sexual maturity is closer to
0.8 g.
Fisher (1946) noted that his smallestsexually mature specimen
was just 7 mm long.Since individuals of that size usually
arecontracted as small spheres (Pilger, personalobservation), the
volume of his specimenprobably was about 0.2 ml and weighedabout
0.2 g (see Figure 2). Barnard andHartman's data support this
conversion(1959: 13, Table 6). Fisher's observation isinconsistent
with the findings of this studythat sexually mature individuals
weigh atleast 0.8 g. A possible reason for this differ-ence is that
Fisher's specimen probably wasone of the "small phase" individuals
whichhad been loaned to him by Hartman (Fisher1946, pp. 238-239).
If this is true, thencaution must be exercised in extending
thereproductive information in the present studybeyond populations
of normal-sized indivi-duals.
SUMMARY
I. LiSlriolobus pelodes lives in U-shapedburrows which it
fashions in soft muddybottoms off southern California. Theproboscis
is a highly extensible structureadapted for feeding on the
uppermostlayer of sediment around each of theburrow apertures. Some
degree of nutri-tional discrimination may take place. Theability of
these echiurans to feed regularlyon large areas of the sediment
surfaceimplies that the impact of their presenceon the trophic
structure of the benthiccommunity is great.
2. The gonad of L. pelodes surrounds themost posterior portion
of the ventralblood vessel. Early in oogenesis clustersof primary
oocytes detach from the ovaryand differentiate in the coelomic
fluid.When an oocyte reaches 40 /lm diameterit separates from the
cell cluster andcompletes oogenesis as a single cell. Fullygrown
oocytes are selectively removedfrom the coelomic fluid and
accumulatedin the modified nephridia (storage organs)
•
-
Echiuran Reproduction off Southern California-PILGER 141
until spawning. The oocytes are spawnedin late prophase of the
first meioticdivision but complete their maturationprior to
syngamy.
3. Gametes are released by the gonad duringmost of the year but
fully grown oocytesare present only in late fall, winter,
andspring. The duration of oogenesis isestimated to be 5-6
months.
4. Spawning takes place over an extendedperiod from late winter
through springand individuals are spawned out bysummer.
5. Juvenile recruitment occurs in late winterand spring. Newly
settled juveniles becomesexually mature when they are 6 monthsto I
year old.
ACKNOWLEDGMENTS
This work was part of a doctoral disserta-tion submitted to the
Department of Biology,University of Southern California,
LosAngeles. The advice, encouragement, andfriendship of Drs.
Kristian Fauchald andRussel Zimmer have been invaluable through-out
this study. The following people contri-buted in many ways: Dr.
Robert Given,Richard Beckwitt, James Coyer, Jack Engle,Greg
Hageman, Fred Piltz, and BruceThompson. I thank the Los Angeles
CountySanitation Districts and their staff for pro-viding ship
time, information, and theirlarge collection of L. pelodes. The
supportof Dr. Mary E. Rice and the facilities of theSmithsonian
Institution, Fort Pierce Bureau,are gratefully acknowledged. Ms.
June Jonesskillfully typed the manuscript. My wife,Patty,
contributed immeasurably and de-serves special thanks for her
efforts.
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