from their stony coral hosts in the Red Sea at Eilat, Israel. ... found boring in living reef-building corals (Morton. 1983). ... about their reproduction or development.
Marine Biology 115, 245-252 (1993)
Marine Biology .................
@ Springer-Verlag1993
Spawning and development of three coral-associated Lithophaga species in the Red Sea O. Mokady 1, D. B. Bonar 2'3, G. Arazi 1, Y. Loya 1 1 Department of Zoology, Tel Aviv University, Tel Aviv 69978, Israel 2 Department of Zoology, University of Maryland, College Park, Maryland 20742, USA 3 Center of Marine Biotechnology, University of Maryland, 600 E. Lombard St., Baltimore, Maryland 21202, USA Received: 28 July 1992 / Accepted: 21 September 1992
Abstract. Lithophaga date mussels from three species (L. lessepsiana, L. simplex and L. purpurea) were removed from their stony coral hosts in the Red Sea at Eilat, Israel. Spawning, observed in the laboratory on several occasions during 1987-1988, appeared to be closely tied to lunar periods, occurring primarily during the last quarter and the new moon. Embryonic and larval development was typical of that described for other mytilids and, except for pigmentation differences, which could be discerned during embryogenesis, the developmental stages of the three species were indistinguishable. Development to the pediveliger stage took 3 to 4 wk in standard culture conditions, but raising the temperature to 27.5~ increased the growth rate of larvae of L. lessepsiana by as much as three-fold, so that the pediveliger stage was attained in 16 d. Larvae resulting from spawning by L. simplex adults removed from the coral Astreopora myriophthalma grew significantly faster in culture than larvae from adults removed from the coral Goniastrea pectinata (comparison of slopes, p < 0.05). The latter individuals showed a 6-d growth plateau at the early umbone stage. Metamorphically competent larvae were capable of delaying metamorphosis for up to 4mo, which would allow an extended period for dispersion and would increase the chance of finding a suitable substratum in the natural environment.
Introduction Coral reefs host an exceedingly diverse assemblage of invertebrate associates, many of which apparently require the presence of the living corals as obligate symbionts (Patton 1975, Hadfield 1976, Morton 1983). Several members of the family Mytilidae, primarily in the genus Lithophaga (commonly called date mussels), are found boring in living reef-building corals (Morton 1983). The association between the mussel and the coral is established by settlement and metamorphosis of mussel pediveligers on the corals, following which the mussels
are believed to bore into the calcium carbonate skeleton primarily by chemical means (Lazar and Loya 1991). Kleemann (1980) has presented extensive data on the relationships between Lithophaga spp. and corals from the Great Barrier Reef, showing different degrees of coral host specificity. Absolute species specificity is rare, and previous claims for this phenomenon seem to reflect sitespecific situations in different locales. For example, L. lessepsiana is commonly found in both Pocillopora damicornis and Stylophorapistillata on the Great Barrier Reef (Kleemann 1980), but is only found in S. pistillata in the Red Sea (Loya 1981), although both corals are present. The Lithophaga species in the current study demonstrate various degrees of host species specificity. L. simplex has the widest host distribution, being commonly found in Astreopora myriophthalma, Goniastrea pectinata, and Echinopora gemmacea (Gohar and Soliman 1963, Brickner 1985). L. purpurea is found in both Montipora erythraea and Cyphastrea chalcidicum, although the differences in size and spawning season between the bivalves inhabiting the two corals leave some question as to whether the two populations are both the same species (Winter 1985, Brickner and Loya 1990). L. lessepsiana is found only in Stylophora pistillata although it is rarely present in shallow water coral colonies but inhabits coral colonies below 15 m in very large numbers (Loya 1972, 1981). Speculations on the factors influencing the distributions of different lithophagid species on their particular host corals have usually assumed a mechanism of active larval choice of substratum involving chemotaxis by the larvae or chemical induction by the host coral (Highsmith 1980, Kleemann 1980). The varying degrees of host specificity shown by the Red Sea Lithophaga species offer a particularly appropriate experimental system to examine this hypothetical basis for establishing the musselcoral relationship. As is true for most coral-associated infaunal organisms, very little is known about the life history of date mussels. Lithophagids have proven notoriously difficult to spawn in the laboratory (Culliney 1971, Scott 1988), and very little is therefore known
246 a b o u t their r e p r o d u c t i o n or development. As a p a r t o f a study o f host-specific settlement and m e t a m o r p h o s i s ( M o k a d y et al. 1991, M o k a d y et al. in press), it was first necessary to describe e m b r y o n i c and larval development o f these three Lithophaga species.
Materials and methods Mussel collection and spawning Adult Lithophaga spp. were extracted from healthy coral colonies collected during 1987-1988 at Eilat Israel. In the H. Steinitz Marine Biology Laboratory (MBL) at Eilat, corals were maintained in aquaria with fresh running seawater, until the mussels were removed. Extraction of the mussels by gentle demolition of the coral skeleton with hammer, chisel and forceps was generally done within 1 to 2 h of coral collection. Lithophaga spp. mussels were maintained in clean aquaria in either fresh flowing seawater or in static, filtered water which was changed regularly. The mussels were fed daily a mixture of unialgal cultures (Isochrysisgalbana and Chaetoceros calcitrans) obtained from the Mariculture Centre, IOLR, in Eilat. Later, mussels were transported to the Zoology Department at Tel Aviv University where they were maintained in static cultures with feeding and water change every other day. A crude determination of gonadal state could be made by external examination of Lithophaga lessepsiana and L. simplex adults. The viscera of ripe individuals could be seen through the translucent shell as a homogeneous dense purple (female) or creamy white (male) color. Individuals lacking the dense color or with mottled color were invariably in an early stage of gametogenesis or spawned out. The dense purple pigmentation of the shells of L. purpurea prevented determination of gonadal state by external examination. Hence, in this species, determination of the relative numbers of ripe individuals in a collected population could only be made by dissecting visceral tissues from a subsample and examining them with a compound microscope. A wide variety of techniques failed to induce spawning, including both hot and cold thermal shocks for varying periods of time, incubation with dense algal suspensions, hypo-osmotic shocks, KC1 and serotonin injection, mechanical stimulation of the adductor muscle, exposure to stripped conspecific sperm and/or eggs, and hydrogen peroxide incubations (see Strathmann 1987 for review of bivalve spawning induction methods). Attempts to produce embryos with manually excised sperm and eggs were also unsuccessful, although manually excised sperm were capable of fertilizing naturally spawned eggs. The only viable embryos which were obtained during the present study resulted from adventitious spawnings (see "Results"). Spawnings were usually discovered after the fact, but on two occasions, individuals (of both Lithophaga lessepsiana and L. purpurea) in the act of spawning were removed, briefly rinsed with 0.45 gm filtered seawater (FSW), isolated in separate dishes and the resulting gametes collected. Eggs were rinsed by allowing them to settle through FSW twice and were fertilized with either naturally spawned or mechanically stripped sperm suspensions. For both observed spawnings of L. lessepsiana males began spawning first, followed approximately 15 rain later by the females.
Rearing Larval cultivation was conducted by standard bivalve culture techniques, with minor modifications (Loosanoff and Davis 1963, Chanley 1981). Embryos resulting from adventitious mass spawnings or from the controlled fertilizations were rinsed several times by allowing them to settle through FSW, decanting the supernatant and resuspending them in FSW. Rinsed embryos were maintained in beakers or culture bowls until hatching, at which time the majority of swimming larvae were collected by pipette from just beneath the
O. Mokady et al.: Development of Red Sea Lithophaga species water surface and transferred to larger beakers of FSW. When the straight-hinge stage was reached (approximately 2.5 d) larval density was adjusted to approximately 20 ml- 1 and a mixture of the algae Isochrysis galbana and Chaetoceros calcitrans was added to the cultures to a final density of 25 000 cells ml- 1. On alternate days thereafter, cultures were changed by gently sieving larvae onto an appropriate-sized Nitex mesh and transferring them to fresh culture vessels with new algal cells. Antibiotics (Sigma Chemical Co.) were used in almost all cultures. Both rifampicin at a final concentration of 5 gg ml- 1 and a penicillin/streptomycin/neomycin mixture at 100 units each ml- ~ effectively prevented detectable bacterial activity, showing no obvious negative effects on iarvat development. For scanning electron microscopy, embryonic and larval stages were prepared by sequential fixation in PIPES-buffered glutaraldehyde and bicarbonate buffered osmium tetroxide, dehydrated in an ethanol-step series or by the single-step dimethoxypropane technique (Maser and Trimble 1977, Maugel et al. 1980), dried by the critical point method and coated with gold-palladium before examination in AMRay or Cambridge scanning electron microscopes.
Results Spawning Results f r o m previous studies o f reproductive cycles o f R e d Sea lithophagids dictated the m o n t h s during which we could expect to find ripe adults o f the three species examined in the present study (Table 1). At no time did we find large percentages o f adults with fully ripe gametes. Routinely, and for all species, we f o u n d the largest part o f the populations to have g o n a d s containing partially developed or degenerating gametes. A t t e m p t s to fertilize "stripped" eggs and sperm yielded only a few abortive cleavage stages. Table 2 presents a compilation o f spawning d a t a for the three Lithophaga species studied, f r o m several coral hosts, collected over the course o f two years. Several individuals o f a g r o u p o f L. lessepsiana spawned in midD e c e m b e r 1987 after several days in the laboratory, and again in m i d - J a n u a r y 1988, after a full m o n t h u n d e r artificial l a b o r a t o r y illumination. In the latter event, individuals kept in Eilat and in Tel Aviv spawned almost simultaneously. This m o n t h - l o n g interval between spawnings, and the simultaneous spawning o f the bivalves in Eilat and Tel Aviv, p r o m p t e d us to c o m p a r e the dates o f these adventitious spawnings to lunar cycles. Fig. 1 presents graphically the spawning times in relation to the synodical lunar cycle. As can be seen in Fig. I, observed spawnings o f any single species occurred within a few days o f the same lunar phase. F o r all species in general, spawning tended to occur in the last quarter to shortly after the new m o o n . D u r i n g m o s t spawnings, only a small n u m b e r o f individuals (two to six) released gametes, a l t h o u g h the Lithophaga lessepsiana and L. purpurea spawnings on 11 J a n u a r y 1988 included almost two dozen individuals (ca. 20% o f a mixed p o p u l a t i o n o f the two species).
E m b r y o l o g y and larval development The developmental timetables for the three Lithophaga species studied are listed in Table 3. The time and size o f
O. Mokady et al.: Development of Red Sea Lithophaga species S L. lessepslana
6 A
L. s/knplex
E
L. s/mp/ox
J I
(G p e c t/na ta) L, pUfpUf88
F
C
K
{1~ er/thrae~)
I)
L, purpurea (0 chalo/b~fcum}
Days
0 New Lunar cycle 9
D H
c 5
10 1st ()
15 FulF 0
20 3rd (])
25
30 New 9
Fig, 1, Lithophaga spp. Spawning dates of Red Sea Lithophaga
species in the laboratory in relation to lunar phases. Each letter corresponds to a spawning event listed in Table 2. Coral hosts given in parentheses. S. pistillata: Stylophora pistillata; A. myriophthalma: Astreopora myriophthalma; G.peetinata: Goniastrea peetinata; M. erythraea: Montipora erythraea; C. chalcidicum: Cyphastrea chaleidicum
Table I. Lithophaga spp. Reproductive seasons of Red Sea Lithophaga species Mussel species
Coral host
L. lessepsiana Stylophora pistillata L. simplex
L. purpurea
Spawning months
Source
Dec.-Jan.
Loya 1981 Brickner 1985
Goniastreapectinata Jul.-Aug. Brickner 1985 Astreopora myriophthalma Montipora erythraea Aug.-Sep. Brickner 1985 Cyphastrea chalcidicum Nov.-Jan. Winter 1985
Table 2. Lithophaga spp. Spawning dates of Lithophaga spp. removed from Red Sea corals. Letters in parentheses, indicating simultaneous spawning events, correspond to lettering in Fig. 1. 1: Eilat; 2: Tel Aviv Mussel species
Coral species
Date
Site
L. lessepsiana Stylophorapistillata
22 Jan. 1987 (A) 14 Dec. 1988 (B) 11 Jan. 1988 (C)
1 2 1,2
L. simplex
Astreoporamyriophthalma
21 Aug. 15 Jul. 5 Aug. 7 Aug. 11 Aug.
1 2 2 2 2
L. simplex
Goniastreapeetinata
17 Jul. 1988 (I) 14 Aug. 1988 (J)
2 2
L.purpurea
Montipora erythraea
21 Aug. 1987 (D) 16 Sep. 1988 (K)
1 2
L.purpurea
Cyphastreachalcidicum 11 Jan. 1988 (C)
2
1987 (D) 1988 (E) 1988 (F) 1988 (G) 1988 (H)
247
larvae at different developmental stages are based on observation of hundreds of larvae at a specific stage, from which a subsampte of several tens was measured. For the more advanced stages (early umbone stage onward) the noted times reflect the earliest times at which individual larvae reaching this stage were detected in the cultures. The general developmental pattern of all three species was almost identical, except for minor differences in timing. Newly spawned eggs of the three Lithophaga species were unadorned and possessed a large central germinal vesicle (Fig. 2A). Germinal vesicle breakdown was not spontaneous but followed within 5 min of fertilization (i.e., sperm fusion and entry). Eggs which were not fertilized would remain in the germinal vesicle stage and could still be fertilized with freshly stripped sperm at least 12 h later. Although no elevated fertilization envelope appeared, its presence became apparent when the polar bodies were produced (Fig. 2 C). The two successive meiotic cleavages, resulting in expulsion of two polar bodies (the first polar body did not divide again), began 20 to 45 min after fertilization with an additional 45 min between them. Eggs of Lithophaga simplex and L. lessepsiana appeared brown when viewed with trans-illumination and possessed dense, granular yolk which did not allow detailed observation of the developing internal structures during the pre-straight-hinge stages, even though the small eggs did not contain excessive amounts of yolk. Eggs of L. purpurea, although colored a distinct purple which could be seen even under trans-illumination, contained yolk that was less dense than the other species, so that some characteristics of organogenesis could be discerned during the early developmental period. Mitotic cleavage followed a typical molluscan, oblique or "spiral" pattern. Large polar lobes (40 gm diameter, Fig. 2 D) formed during the first two divisions, resulting in unequal-sized blastomeres at the two-cell stage (Fig. 2E). The blastula which resulted from the early cleavages contained a small blastocoel and possessed two large mesoderm mother cells at the ventral end. Hatching from the fertilization envelope occurred during the blastula stage by a localized rupturing of the envelope. Just prior to hatching, cilia appeared on the blastular surface and the embryos could be seen slowly rotating in their envelopes. Once freed of the protective envelopes, blastulae began rotating much more rapidly and swam actively upwards in the water column, aggregating near the surface. Gastrulation proceeded by epiboly, resulting in a typical molluscan trochophore with a distinct apical tuft of 70 ~m (Fig. 2I). During gastrulation the large, posterior mesodermal mother cells were clearly evident as they were overgrown laterally by the micromeres. Scanning electron micrographs revealed the invaginating, then evaginating dorsal shell gland (Fig. 2 K). Internal structural details became much more apparent only after the appearance of the bivalve shells, and even then were indistinct until the straight-hinge stage was well developed (Fig. 3 A). As the bivalve shell became discernable the prototrochal cells formed a more pronounced dorsal band and the prototrochal cilia lengthened noticeably. Con-
O. Mokady et al.: Development of Red Sea Lithophaga species
248
Table 3. Lithophaga spp. Timetable for development of the studied Lithophaga species. Letters indicating spawning events correspond to Table 2. S. p.: Stylophora pistillata; A. m.: Astreopora myriophthalma; G. p.: Goniastrea peetinata; M. e.: Montipora erythraea Mussel species:
L. lessepsiana
L. simplex
L. purpurea
Coral host:
S.p.
A.m.
G.p.
M.e.
Spawning event:
A
B, C
C
H
J
D
Temperature (~
20
25
27.5
25
25
25
Size (gin) Developmental stage Fertilization I st polar body 2na polar body 1s' cleavage 2na cleavage 3ra cleavage 4~hcleavage Hatching blastula Gastrula Early trochophore Late trochophore Straight hinge Early umbone Umbone Early eyed larva Pediveliger Metamorphosis
0 45 rain 90 min 180 rain 230 min 280 rain 355 rain 14 h 21 h 25 h 45 h 58 h -
0 0 20 min . . 90 rain 135 min 180 min 225 rain < 12 h < 18 h < 18 h 22 h 22 h 38 h 38 h 48 h < 48 h 7d 6d 15 d 11 d 23 d 14 d 27 d 16 d 28 d 18 d
Size (gin)
67 . 65-70 70 90-120 135 170-180 260-280 290-370 290-370
current with shell development to the straight-hinge stage (the P r o d i s s o c o n c h I shell), the ciliated p r o t o t r o c h a l ring inflated laterally to assume its typical shape as the larval velum. A t this stage, larvae showed a p r o n o u n c e d tendency to swim u p w a r d s in counter-clockwise, spiraling circles. U p o n reaching the water surface, swimming would slow or stop, the larvae would sink some short distance in the water c o l u m n and spiraling, u p w a r d swimming w o u l d resume. By the third day o f development, straight-hinge larvae began to ingest the unicellular algae. A complete gut had f o r m e d by this time, and algal cells could be traced on their path f r o m collection by the velar cilia into the s t o m a c h by w a y o f the anterior m o u t h . Algal cells in the larval s t o m a c h were constantly rotated by the cilia-lined s t o m a c h wails. Larvae were initially fed Isochrysis galbana and noticeable disruption o f cells in the larval s t o m a c h was a p p a r e n t only after 20 to 30 min. Cells o f Chaetoceros calcitrans, were n o t noticeably broken until the second week o f development. The vast majority o f the larvae reached the straighthinge stage at 90 to 120 g m in shell length within a p p r o x imately 48 h (Fig. 3 A), but ceased secreting new shell for the next 5 d. D u r i n g this shell-growth plateau the embryonic yolk was depleted and the larval organs (velum, musculature and alimentary tract) became clearly visible (see Fig. 3 B c o m p a r e d with Fig. 3 A). By the seventh day o f development, clear lipid storage droplets began to appear in the velar and alimentary tissues and the " D " - s h a p e d larval shell began to take on an asymmetrical appearance as shell g r o w t h resumed and the u m b o n e became noticeable. G r o w t h over the next 7 to 10 d p r o d u c e d the distinctive P r o d i s s o c o n c h II shell
.
0 20 min . 55 min 90 rain 130 min 150 min 11 h 17 h 21 h 38 h < 55 h 6d 11 d 18 d 21 d 22 d
0 20 rain . 55 min 90 min 130 rain 150 min 11 h 17 h 21 h 38 h < 55 h 6d 16 d 23 d 27 d 28 d
64 .
. 65-70 90-120 260 270-300 300-350 300-350
Size (gin) 0 170 min 255 min 11 h 16 h 21 h 38 h 53 h 6d 16 d 290 ~m (competent pediveligers) at 27.5 ~ (32.0 ~tm d - 1) was over three times the rate at 25 ~ (9.6 ~tm d - 1), and occurred in only 6 d as c o m p a r e d to 17 d at the lower temperature. Larvae in b o t h treatments reached a g r o w t h plateau within 4 to 5 d following competence, with the faster growing larvae plateauing at a slightly larger size. Larval g r o w t h was also c o m p a r e d between offspring o f Lithophaga simplex adults which had been r e m o v e d
O. Mokady et al.: Development of Red Sea Lithophaga species
Fig. 2A-K. Lithophagalessepsiana. Photomicrographs of L. lessepsianadevelopmental stages: oocyte to trochophore. (A) Unfertilized egg with clear, central germinal vesicle;bar = 20 gin. (B) SEM (scanning electron micrograph) of first cleavage (trefoil stage) viewed from animal pole; bar = 10 p,m.(C) Early trefoil stage showing polar lobe formation; bar=20 gm. (D) Trefoil stage with polar lobe; bar=20 gm. (E) Two-cell stage showing unequal blastomeres;
from the corals Goniastrea pectinata and Astreopora myriophthatma (Fig. 5). In this case, conditions under which larval cultures were reared were identical. Overall, larvae from mussels inhabiting A. myriophthalma grew significantly faster (comparison of slopes, p < 0.05), with rapid growth of the umbone larvae beginning at Day 7, in comparison to rapid growth beginning at Day 13 for larvae from mussels inhabiting G. pectinata. It is particularly interesting that growth of both populations from their inflection points (7 or 13 d) was very similar (11.7 gm d -1 and ~1.2 gm d - l , respectively) up to the growth plateau at the pediveliger stage. Therefore, it is because of the earlier onset of the rapid growth phase of larvae of the A. myriophthalma population that these lar-
249
bar = 20 gm. (F) Four-cell stage; bar = 20 gm. (G) Eight-ceU stage; bar=50gm. (H) SEM of hatched, ciliated blastula (ca. 13h); bar=10gm. (I) SEM of swimming trochophore (ca. 22h); bar= 10 gin. (J) SEM of prominent apical tuft of cilia on animal pole of trochophore; bar=2 gm. (K) SEM of anterior view of trochophore showing shell gland (arrow); bar = 10 gm
vae reached the metamorphically competent pedNeliger stage by Day 22, as opposed to Day 27 for larvae of the G. pectinata population. Regression analysis o f both data sets also revealed that larval growth of both populations to the pediveliger stage is linear, whether measured from the time of fertilization or from the rapid growth inflection (r values between 0.971 and 0.998).
Delay o f metamorphosis A small number of larvae of Lithophaga simplex which spawned in August 1987 were maintained in the laboratory with routine feeding and water changes until mid-Jan-
O. Mokady et al.: Development of Red Sea Lithophaga species
250
Fig. 3 A - E Lithophaga lessepsiana. Photomicrographs ofL. lessepsiana developmental stages: straight-hinge to metamorphosis. (A)
Early straight-hinge stage larva (ca. 48 h); b a r = 2 0 gm. (B) Late straight-hinge stage larva (ca. 5 d); bar = 20 txm. (C) Early umbone stage larva with prodissoconch II shell (ca. 8 d); bar = 30 gin. (I))
A
400
400 / O
27.5
i'
5 200--
E
350
v~
300-
~| c (b
250-
- -
- -
1 5 0
9 z d)
10o
(O3
5o o
A. myr/ophthalma
"
"'"'"
c
v
0 @
Early eyed larva nearing metamorphic competence (arrow indicates eye; ca. 23 d); b a r = 4 0 ~tm. (E) Pediveliger competent to metamorphose (ca. 27 d); bar = 70 gin. (F) Juvenile mussel 1 mo after metamorphosis; bar = 100 pm
I
I
I-
L
I
?-
F-
4
8
12
16
20
24
28
Age
I
I
32
36
,,'"'"'V ~
200-
(d)
Fig. 4. Lithophaga lessepsiana. Growth of L. lessepsiana larvae at two water temperatures. Data represent averages of 10 to 20 larvae at each sampling time (longest shell dimension). Standard deviations ranged +_5 to -t-10 ~tm. The following equations for the rapid growth period (see "Results - Larval growth") derived from linear regression analysis. 25~ Y=9.6X+38.5 (r=0.972); 27.5~ Y= 32.0X- 182.5 (r = 0.976)
G. pect/bata
150100500
4-0
_ / ~
--
0
I
L
I
~
I
I
t-
i
5
10
15
20
25
30
35
40
Age
L --
45
50
(d)
Fig, 5. Lithophaga simplex. Growth of L. simplex larvae obtained from adult mussels extracted from the corals Astreopora myriophthalma or Goniastreapeetinata. Data represent averages of 10 to 20 larvae at each sampling time (longest shell dimension). Standard deviations ranged + 5 to _+10 lam. The following equations derived from linear regression analysis. Larvae originating from A. myriophthalma: Days 0 to 24, Y = I I . 0 X + 5 8 . 7 (r=0.994); Days ? to 24, Y= 11.7X+47.2 (r=0.998). Larvae originating from G. peetinata: Days 0 to 31, Y=8.1X+54.7 (r=0.971); Days 13 to 31, Y = l l . 2 X - 17.7 (r = 0.989)
O. Mokady et al.: Development of Red Sea Lithophaga species A
Q3
lO00j
Induction of Metamorphosis
6OO +
~@
400
z@ d)
200 ~ = 0' o
i
I
20
40
t
~
I
"
i
r
i
60 80 lOO 12o Age (d)
14o
leo
Fig. 6. Lithophaga lessepsiana. Larval and post-metamorphic
growth of L. lessepsiana. Larvae maintained in pristine cultures for 75 d following aquisition of metamorphic competence and then induced to metamorphose by living branches of the coral Stylophora pistillata
Table 4. Lithophaga lessepsiana. Spontaneous metamorphosis of L. lessepsiana larvae during extended larval culture Age (d)
No. of inspected larvae
Metamorphosis (%)
42 52 59 63 66 108 112
24 112 284 300 254 69 67
0 3.6 2.5 3.7 1.7 4.4 3.2
uary of 1988 (143 d). This extended period of larval life prompted a closer examination of the potential for delay of metamorphosis. Consequently, a group of larvae of L. lessepsiana which spawned in January 1988 were regularly inspected for evidence of spontaneous metamorphosis (Table 4). No attempt was made to clear the cultures of metamorphosed individuals, so that the numbers presented in Table 4 represent cumulative metamorphosis over the 112-d period. Except for the first sampling (Day 42, no metamorphosis), the percent of metamorphosed individuals is more or less constant, suggesting that the low level of spontaneous metamorphosis occurs early in the delay period, by Day 52 in this experiment. Larvae which have been subjected to this extended larval period typically showed reduced swimming activity as they aged and spent most of the time crawling on, or swimming near, the bottom of the culture vessel. These larvae retained their ability to metamorphose when presented with an appropriate coral substrate and grew well following metamorphosis (Fig. 6).
Discussion
The data shown in Fig. I and Table 2 suggest that there is a relationship between lunar phase and spawning for
251 each species. This is especially emphasized by the simultar~eous spawning of Lithophaga tessepsiana in both Eitat and Tel Aviv on 11 January 1988 (Table 2). More comprehensive data will be required, however, to determine just how rigorously spawning is tied to lunar period for each species. The only other lithophagid for which information is available, L. bisulcata, also would not respond to routine efforts to induce spawning (Culliney 1971, Scott 1988) and may also be responsive to lunar period. Scott (1988), for example, was only able to obtain spawning in L. bisulcata from Jamaica during the second or third weeks of October in three different years. Our observations that only a portion of the adults examined by dissection contained fully ripe ova suggest that the general population of each species may have several spawnings over the reproductive season. The spawnings of small percentages of individuals in laboratory populations of L. lessepsiana and L. simplex on several occasions, support this supposition. Development of the Lithophaga species examined in the present study follow the typical pattern for mytilids reported by other authors (e.g. Schweinitz and Lutz 1976, see Bayne 1976 and Sastry 1979 for reviews). For the three species reported here, it is not possible to distinguish visually between the species once past the early trochophore stage, when pigmentation differences disappear. The only other lithophagid for which development has been described, L. bisulcata, also appears to be indistinguishable from the Red Sea species until the late urnbone stage (Culliney 1971). At this stage, the larval shells ofL. bisulcata look more like those ofModiolus demissus, while the larval shells of the Red Sea lithophagids described here look more like those of Mytilus edulis. The two temperature regimes under which Lithophaga lessepsiana were raised revealed dramatic differences in growth rates after the early umbone stage. Larvae at 27.5 ~ grew more than three times faster than those at 25 ~ reaching the competent pediveliger stage in only 16 d instead of the 27 d required at the lower temperature. These results are very similar to those described for Mytilus edulis by Bayne (1965). Bayne's reported QXO values for the rate of larval growth at 2 ~ increments of temperature demonstrated that these values were much higher between 10 and 12 ~ than between 14 and 20 ~ (at these higher temperatures development was essentially temperature independent). Since L. lessepsiana at Eilat is at the extreme northern end of its otherwise tropical distribution, it is likely to be developing at the lower limit of its thermal tolerance range. Consequently, the temperature rise between 25 and 27.5~ probably falls within the steepest part of the temperature dependence curve for L. lessepsiana. The other major developmental rate difference noted in the present study was between the two populations of Lithophaga simplex, in which larvae derived from Astreopora myriophthalma associated adults did not show the growth plateau at the early umbone stage, but grew linearly from fertilization to pediveliger. At this time, we do not know whether genetic differences exist between the two populations. They differ in their ability to settle and metamorphose on the coral hosts (Mokady et al. in
252 press), which leads us to hypothesize that the two populations m a y be reproductively isolated. However, confirm a t i o n will await future, planned research on allozyme distribution and mitochondrial D N A R F L P analysis. The ability o f Lithophaga spp. larvae to delay the onset o f m e t a m o r p h o s i s was n o t surprising, as Bayne (1965) had first reported this p h e n o m e n o n for mytilids, and Culliney (1971) had described it for L. bisulcata. D u r i n g the extended, 4-mo larval period no additional spontaneous m e t a m o r p h o s i s was observed after the seventh week, and larvae retained their capacity to respond to m e t a m o r p h i c induction ( M o k a d y etal. 1991, u n p u b lished data). These d a t a suggest that in the natural envir o n m e n t , m e t a m o r p h i c a l l y c o m p e t e n t Lithophaga spp. larvae are potentially capable o f remaining in the plankton for extended periods o f time, during which chance encounters with coral substrata would offer the o p p o r t u nity for host-specific settlement-site selection.
Acknowledgements. We thank the Israeli Oceanographic and Limnological Research Center in Eilat for providing facilities and algal cultures. Thanks are also due to the H. Steinitz MBL of The Interuniversity Institute of Eilat, for providing laboratory and diving facilities. This research was supported by the USA-Israel Binational Science Foundation (BSF), Jerusalem, Israel.
Literature cited
Bayne, B. L. (1965). Growth and the delay of metamorphosis of the larvae of Mytilus edulis. Ophelia 2:1-47 Bayne, B. L. (1976). The biology of mussel larvae. In: Bayne, B. L. (ed.) Marine mussels: their ecology and physiology. Cambridge University Press, New York, p. 81-121 Brickner, I. (1985). Reproductive and settlement strategy of the boring bivalve Lithophaga purpurea in living corals. M. Sc. Thesis. Tel Aviv University, Tel Aviv, Israel Brickner, I., Loya, Y. (1990). Reproductive strategy of the boring bivalve Lithophaga purpurea in living corals. Proc. Jt U.S.-Israel Workshop on Marine Symbioses, Eilat, Israel. 32 Chanley, M. H. (1981). Laboratory cultivation of assorted bivalve molluscs. In: Marine invertebrates, laboratory animal management. National Academy Press, Washington, D. C. p. 233-249 Culliney, J. L. (1971). Laboratory rearing of the larvae of the mahogany date mussel Lithophaga bisulcata. Bull. mar. Sci. 21: 591 - 602 Gohar, H. A. E, Soliman, G. N. (1963). On three mytilid species boring in living corals. Publs mar. biol. St Ghardaga 12:65-98 Hadfield, M. G. (1976). Molluscs associated with living corals. Micronesica 12:133-148 Highsmith, R. C. (1980). Burrowing by the bivalve mollusc Lithophaga curta in the living reef coral Montipora berryi and a
O. Mokady et al.: Development of Red Sea Lithophaga species hypothesis of reciprocal larval recruitment. Mar. Biol. 56:155162 Kleemann, K. H. (1980). Boring bivalves and their host corals from the great barrier reef. J. mollusc. Stud. 46:13-54 Lazar, B., Loya, Y. (199t). Bioerosion of coral reefs - a chemical approach. Limnol. Oceanogr. 36:377-383 Loosanoff, V. L., Davis, H. C. (1963). Rearing of bivalve molluscs. Adv. mar. Biol. 1:1-136 Loya, Y. (1972). Community structure and species diversity of hermatypic corals at Eilat, Red Sea. Mar. Biol. 13:101-123 Loya, Y. (1981). Life history strategies of the boring bivalve Lithophaga lessepsiana in the coral Stylophora pistillata. Proc. 4th int. coral Reef Syrup. (1981). [Gomez, E. D., Birkeland, C. E., Buddemeier, R. W., Johannes, R. E., Marsh, J. A. Jr, Tsuda, R. T. (eds) Marine Science Center, University of the Philippines, Manila] 2:756 Maser, M. D., Trimble, J. J. III (1977). Rapid chemical dehydration of biologic samples for EM using 2-2 dimethoxypropane. J. Histochem. Cytochem. 25:247-251 Maugel, T. K., Bonar, D. B., Creegan, W. J., Small, E. B. (1980). Specimen preparation techniques for aquatic organisms. Scanning Electron. Microscopy 2:57-75 Mokady, O., Arazi, G., Bonar, D. B., Loya, Y. (1991). Coral host specificity of settlement and metamorphosis of the date mussel Lithophaga lessepsiana (Vaillant, 1865). J. exp. mar. Biol. Ecol. 146:205-216 Mokady, O., Arazi, G., Bonar, D. B., Loya, Y. (in press). Settlement and metamorphosis specificity of Lithophaga simplex Iredale 1939 (Bivalvia: Mytilidae) on Red Sea Corals. J. exp. mar. Biol. Ecol. Morton, B. (1983). Coral-associated bivalves of the Indo-Paeific. In: Russell-Hunter, W. D. (ed.) The mollusca, Vol. 6. Academic Press, New York, p. 139-224 Patton, W. K. (1975) Animal associates of living coral reefs. In: Jones, O., Endean, R. (eds.) Biology and geology of coral reefs, Vol. 3. Academic Press, New York, p. 1-36 Sastry, A. N. (1979). Pelecypoda (excluding Ostreidae). In: Giese, A., Pearse, J. (eds.) Reproduction of marine invertebrates, Vol. 5. Academic Press, New York, p. 113-292 Schweinitz, E. H., Lutz, R. A. (1976). Larval development of the northern horse mussel Modiolus modiolus (L.), including a comparison with the larvae of Mytilus edulis L. as an aid in planktonic identification. Biol. Bull. mar. biol. Lab., Woods Hole 150: 348-360 Scott, P. J. B. (1988). Initial settlement behavior and survivorship of Lithophaga bisuleata (Mytilidae: Lithophaginae). J. mollusc. Stud. 54:97-108 Strathmann, M. E (1987). Reproduction and development of marine invertebrates of the Northern Pacific Coast. University of Washington Press, Seattle Winter, E. (1985). The life history strategy of the bivalve Lithophaga purpurea boring in the coral Cyphastrea chalcidicum. M. Sc. Thesis, Tel Aviv University, Tel Aviv, Israel C o m m u n i c a t e d by O. Kinne, O l d e n d o r f / L u h e
