Intercolony Coordination of Zooid Behavior and a New Class of Pore Plates in a Marine Bryozoan

10 Pages·1992·4.4 MB·English

by  D F Shapiro

Reference: Biol. Bull 182: 221-230. (April. 1992)
New
Intercolony Coordination of Zooid Behavior and a
Class of Pore Plates in a Marine Bryozoan
DANIEL SHAPIRO
F.
Section ofEcology andSystematics, Cornell University. Ithaca. New York. 14853.
Abstract. This paper describes a mixed allorecognition Introduction
interaction between adjoining colonies ofthe encrusting
cheilostome bryozoan Membranipora membranacea, in Colonial marine invertebrates such as sponges, cnidar-
which characteristics ofboth intercolony fusion and in- ians, bryozoans, and ascidians are capable ofindetermi-
tercolony rejection occursimultaneously. Intercolonyco- nate asexual growth. As a result, contact between con-
ordination of zooid behavior was assayed by applying specific and heterospecific coloniesisextremelycommon
electrical stimuli to one colony of a colony pair while on most marine hard substrata where space is limiting
observingthebehavioroftheadjoiningcolony. Retraction (Dayton, 1971; Stebbing, 1973a; Jackson, 1977; Osman,
offeeding structures (lophophores) by the unstimulated 1977). Many ofthese colonial invertebrates have highly
colony indicated intercolony coordination of behavior. discriminating immune systems capable of allorecogni-
Naturally occurring and artificially created pairs ofgeno- tion the ability to distinguish between genetically iden-
typically identical andgenotypicallydistinct colonieswere tical and genetically distinct tissue (for review see Gros-
examined. Additionally, colony borders were examined berg, 1988). Ifgenotypically identical, or closely related
forthe presence ofpore plates, structures that physiolog- (e.g.. sibling) coloniescome intocontact, theycommonly
ically link zooids within colonies. Contact between ge- fuseintoasinglecolony. Ifgenotypicallydistinctcolonies
netically identical colonies(isocontact)alwaysresulted in come into contact, tissue rejection typically follows, and
a characteristic border morphology, characteristic pore fusion does not occur (Sabbadin, 1982; Scofield et ai,
plates, and intercolony coordination of zooid behavior. 1982; Chancy, 1983; Rinkevich and Loya, 1983a; Shenk
Contact between genotypically distinct colonies(allocon- and Buss, 1991).
tact)alwaysresulted inacharacteristic bordermorphology Recent work describing allorecognition responses of
and intheformation ofcharacteristicporeplatesofatype colonial marine invertebrates has revealed a diversity of
neverbefore described. However, only coloniesthatwere interactions ranging from intercolony fusion to intercol-
young when they first came into contact showed coordi- ony rejection. Colonies ofthe hydroid Hydractinia sym-
nated behavior. Intercolony coordination of zooid be- biolongicarpmmay fuse permanently, fuseandthen later
havior is probably the result ofneural connections made reject, or reject with the subsequent production of ag-
through pore plates. Intercolony behavioral coordination gressive hyperplasitic stolons (Buss and Grosberg, 1990;
between younggenotypically distinctcoloniesispeculiar, Shenk and Buss, 1991). In ascidians, allorecognition re-
because the colonies simultaneously show characteristics sponses include permanent fusion, fusion followed by
of physiological integration (coordinated behavior) and separation, fusion followedbycomplete resorption ofone
tissue rejection (borders and pore plates characteristic of colony, rejection with little furtherinteraction, and rejec-
contact between genetically distinct tissues). This inter- tion with necrosis ofthe tissues ofone or both colonies
action showsthatthepresence ofthe morphological char- (Koyama and Watanabe, 1982; Scofield and Nagashima,
acteristics ofintercolony rejection does not always imply 1983; Rinkevich and Weissman, 1987, 1989).
a lack ofphysiological integration between colonies. All ofthe above examples involve either different in-
tensities of rejection or a temporal separation between
fusion and rejection. This paper describes a mixed inter-
Received 4June 1990;accepted 13 January 1992. action between colonies of the encrusting cheilostome
221
222 D. F. SHAPIRO
bryozoan Membranipora membranacea involvingsimul- sityofCalifornia, LosAngeles. AtFHL,coloniesofAfem-
taneousevidence ofphysiological fusion and tissue rejec- branipora membranacea that had settled on black acrylic
tion. Zooidswithin bryozoan coloniesarephysiologically panelssuspended from the FHLdock, as well ascolonies
integrated through a nerve net that traverses the calcified collected from the field, were used in this study. Colonies
zooidal walls through pore plates (Thorpe et al, 1975; were collected from the field by haphazardly selecting
Lutaud, 1977, 1979), distinctive structures in the zooidal bryozoan-encrusted bladesofthe kelpLaminariasp. from
wall where there is a concentration ofseveral open pores Turn Island and transporting them back to FHL where
(Silen, 1944; Banta, 1969). The most obvious display of the kelp blades were hung from the FHL dock. In Cali-
physiological integration ofzooidswithin abryozoan col- fornia,M. membranaceacolonieswerecollected from kelp
ony is the coordination ofthe lophophore retraction re- beds off the coast of Malibu, California. Bryozoan en-
sponse. In response to a localized disturbance to one or crusted blades ofthe kelp Macrocystispyrifera were hap-
a few zooids, all ofthe zooids within a colony simulta- hazardly removed from the upperpartsofkelpfrondson,
neously retract their feeding structures (lophophores). I andjust below, the surface ofthe water. Bladeswere then
have observed that when genotypically distinct colonies transported backtothelaboratorywheretheyweremain-
ofM. membranacea come into contact, intercolony co- tained in a recirculating seawater system.
ordination of lophophore retraction is frequently ob-
served. Yet, theintercolony bordersofthese samecolonies Inicrcolonycoordination oflophophore retraction
show no morphological characteristics effusion.
Because ofthe mixed nature ofthis interaction, I will Toensurethat agiven intercolony borderwasbetween
avoidthetermsfusionand rejection. Fusioninbryozoans two colonies descended from different larvae rather than
is commonly associated with physiological integration previously separated parts ofa colony decended from a
(Stebbing, 1973b; Humphries. 1979; Nielsen. 1981Cha- single larva, I used only colony pairs for which I could
ncy, 1983); consequently, the term fusion could also be locate both ancestrulae. The ancestrula is a pair ofmor-
applied to colonies that show physiological integration, phologically distinct zooids that develop from the larva
but lack any morphological characteristics offusion. To aftersettlement and metamorphosis(Fig. 1A). Ancestrular
avoid this ambiguity, I will refer to contact between ge- zooids are easily distinguished from younger asexually
netically distinct tissues as "allocontact". and 1 will refer produced zooids because they are rounder, more heavily
to contact between genetically identical tissues as "iso- calcified, and togetherare distinctively heart shaped (Fig.
contact." The physiological consequences and morphol- 1B). Unlessindicated otherwise, wheneverI mention col-
ogyoftheseinteractionscan then bedescribed separately. onypairs, I will be referringtopairsofcoloniesdescended
M. membranacea occurs naturally in dense monospe- from separate larvae.
cific populations where contact between conspecifics is To test for intercolony coordination oflophophore re-
extremely common, if not unavoidable. Larvae of M. traction, I stimulatedcolony pairselectrically. A stimulus
membranacea disperse in the plankton for up to four was applied to one of the two colonies. A colony-wide
weeks (Yoshioka. 1982), thus naturally settled adjoining lophophore retraction response in the adjoining unstim-
colonies are unlikely to be siblings. Consequently, the ulated colony was used as an indication of intercolony
majority ofintercolonyinteractionsarebetween unrelated behavioral coordination. Electrical stimuli were applied
colonies. However, contact between genotypically iden- with an electrode placed on the surface ofthe colony. All
ticaltissuesoccasionallyoccurswhenasinglecolonygrows stimuli were at, orjust above, the threshold stimulus (a
into contact with itselfafter either growing around some single square pulse between 5 and 10 volts for 5-10 ms)
objectorfi ;on resultingfromdamagetothecolony(pers. required toelicit a colony-wide lophophore retraction re-
w
obs.). I 1) examine how intercolony coordination is sponse. In addition toelectrical stimuli, mechanical stim-
related to ; ^ size and age at which genotypically distinct uli were applied to pairs ofvery small colonies (less than
mm
colonies t/.st come into contact, (2) compare the mor- 10 2) to eliminate the possibility that intercolony co-
phologyofthebordersbetween genotypicallydistinctcol- ordination was an artifact resulting from electrical con-
onies to those between genotypically identical colonies, duction ofthe stimulus through the water or across the
and (3) examine both types of borders for pore plates colony surface. Mechanical stimuli wereapplied by lightly
thatcould facilitate intercolonycoordination ofzooid be- touchinga dissecting needle to one ofthe colonies on the
havior. edge opposite the intercolony border.
To determine whether the non-stimulated colony ofa
Materials and Methods
pairofbehaviorally coordinated colonies was responding
Animal collection to the physical retraction of the lophophores of the ad-
Research wasconducted at Friday Harbor Laboratories joiningcolony, I retested 20coordinated colony pairsafter
(FHL), San Juan Island. Washington, and at the Univer- first makinga fine cut with a razorblade alongthe border
INTERCOLONY COORDINATION OF ZOOID BEHAVIOR 223
between theadjoiningcolonies. Cutsweremadesothat no
lophophoresalongthe intercolony bordersweredamaged.
To determine whethercoordination was bidirectional,
a stimulus wasapplied to one colony ofa pairuntil I had
obtained 20 behaviorally coordinated and 20 non-coor-
dinated pairs. A second stimulus was then applied to the
other colony ofeach pair.
The frequency ofintercolony behavioral coordination
inanatural populationofM. membranaceawasmeasured
at Friday Harbor by sampling three bladesofLaminaria.
Both sides of5 X 10 cm rectangles were censused 5 cm
from each edge ofthe blade at 25, 50, 75, 100, and 125
cm from the base ofeach blade (where the stipe meets
the blade). Each colony was recorded as being solitary or
in contact with othercolonies. Ifa colony was in contact
with another colony, it was tested for intercolony coor-
dination of lophophore retraction. In all, 1301 colonies
were sampled.
Intercolony coordination andsize atfirst contact
To determine the relationship between colony size at
firstcontactand intercolony coordination, 92 pairsofM.
membranacea colonies were cultured on black acrylic
panelsin Friday Harbor. Panelswereclearedatleastonce
a week of all other organisms. Each colony monitored
was in contact with only one other colony. The size of
each colony at the time of first intercolony contact was
determined by tracing each colony on acetate paper and Figure1. A.Thefoundingancestrulaofacolonyshortlyafterlarval
settlement and metamorphosis. B. Ancestrulae and intercolony border
calculatingtheareaofthetracingusingavideo-integrated ofa pair ofcolonies that have grown into contact. Small bubble-like
imageanalysissystem. Followingcontact, allcolonypairs structuresvisiblealongtheintercolony borderareallocontactporeplates.
were tested for intercolony coordination of lophophore Abbreviation: a, ancestrula. Sizebars = 0.5 mm.
retraction one to three times each week for five weeks.
Additional dataontherelationshipbetween intercolony
coordination andcolonysizeatfirstcontactwereobtained 3). Because the ancestrula marks the site oflarval settle-
forM. membranacea colonies in California. Densities of ment and metamorphosis, I assumed that the distance
M. membranacea in California tend to be higher than in between theancestrulae oftwocolonieswould bedirectly
Friday Harbor (pers. obs.). As a result, data could be ob- correlated to the size ofthe colonies at first contact. Ad-
tained for adjoining colonies that were typically smaller ditionally, itseemed likelythatcolonieswouldnotbecome
at first contact than those observed in Friday Harbor. In coordinated immediately upon contact, butwould instead
all, 230colony pairswere selected from 10differentMac- require a period oftime for the formation ofintercolony
rocystisblades. Colonieswere selected togive a maximum physiological connections. Consequently, foreachcolony
range ofvalues for size at first contact. Because ofhigh pairIalsoestimated howlongcolonieshadbeenincontact
colony density, colony "pairs" were sometimes in serial by measuring the intercolony border length (Fig. 3). Be-
contact with other colonies (forming linear groups of 3, cause the length ofthe border between colonies increases
4, or more colonies). However, no colony was ever in as both colonies grow, I assumed that the length ofthe
contact with more than two other colonies, and a single intercolony border would be directly correlated to how
colony was never used more than once. Colonies were longthecolonies had been in contact. Aftermakingthese
examined using a dissecting microscope, and all mea- measurements, colonies were tested for intercolony co-
surements were made with an ocular micrometer. ordination oflophophore retraction.
Because I was unable to culture colonies in California,
direct measurements ofcolony size at first contact were Transplant experiment
not possible. Instead, I estimated colony size at first con- Although unlikely, I cannot be sure that naturally set-
tactby measuringthe intercolonyancestruladistance(Fig. tled adjoiningcolonies are notgenetically similarsiblings
224 D. F. SHAPIRO
thathavesettledincloseproximity. Todeterminewhether tact, only single colonies that had grown around some
behavioral coordination can occur between colonies that object and back into contact with itself were used; for
areclearly notsiblings, 1 pairedM. membranaceacolonies allocontact, only colonies with both ancestrulae present
from Turn Island with colonies from Rocky Point, San were used. Colonieswerepreparedbydissolvingawaythe
Juan Island, a siteapproximately 10 miles northwest from tissues ofcolonies in 2.5% sodium hypochlorite for 12 h
Turn Island. Bryozoan encrusted blades of the red alga to expose the calcium carbonate skeleton.
Iridea were collected from the two sites. I removed 48 Isocontact and allocontact borders ofnaturally occur-
mm
small colonies(<25 2) from the algal bladesbygently ring colonies were also examined histologically for pore
stretching the blade until the colony detached. Twenty- plates. I examined isocontact borders (n = 3) and allo-
fourcolonypairs,eachconsistingofonecolonyfromTurn contact borders between behaviorally coordinated (n = 6)
Islandandonecolonyfrom RockyPoint, werethen placed and non-coordinated (n = 6) colony pairs collected in
mm
onacrylic panels. After24 h, colonies had attached to the California. Approximately 2-3 long sections ofbor-
panels that were subsequently suspended below the FHL ders, along with the kelp substrate, were removed with a
docks. Following contact, all colony pairs were tested for razorblMade. Sampleswere first fixed in 3% glutaraldehyde
intercolony coordination oflophophore retraction twice in 0.1 sodium cacodylateMbuffer. pH 7.4 for 1 h, and
each week for four weeks. then in 4% osmium in 0.1 sodium cacodylate buffer
foran additional hour. Samples were then dehydrated in
Size reduction experiment: allocontact and isocontact agraded seriesofethanol dilutions, treated with propylene
To distinguish the effects ofcolony age from those of eoxmibdee,ddainndginmfeildtirautme.d ovAefrtnerighptoliynmMereidzciansgt lovoewrnviisgchotsitayt
colony size and to establish unambiguous examples of
70C, samples were sectioned (approximately 3 ^ thick)
cioslocoonnyt,aIctrebdeutcweedenlarcgoempcloleotneileyssgerpoawriantegdopnaratcsryolficapsainnegllse and viewed using a light microscope.
at Friday Harbor to pairs ofsmaller subcolonies. Usinga
Results
razorblade tocleanly cut a square ofthe appropriate size
inthecolony. I created paimrsmofeithersmmalml orlargesquare Intercolony coordination oflophophore retraction
sAlulbcootlhoenriepsartthsatowfetrhee 1c6olony2woerre10t0hen s2c,rraepsepdectoifvfetlhy.e A cut between behaviorally coordinated colony pairs
panel with a small spatula. A 1-mm strip of space was always completely eliminated intercolony coordination
also scraped between each colony pair. of lophophore retraction. Thus, colonies were not re-
Allocontact pairs were created by making subcolonies sponding to the physical disturbance created by the re-
on both sides ofthe intercolony border between pairs of traction ofthe lophophores ofadjoining colonies.
non-coordinated colonies(aftertesting forbehavioral co- For all behaviorally coordinated colony pairs tested,
ordination). In all. eight small and seven largeallocontact intercolony coordination was always bidirectional. Stim-
pairs were established. Isocontact pairs were created by ulation of either colony resulted in a colony-wide lo-
reducing single colonies into two smaller subcolonies. In phophore retraction response in the non-stimulated col-
all, seven small and eight large isocontact pairs were es- ony. Unstimulated colonies ofnon-coordinated pairs al-
tablished. In addition to providing an unambiguous ex- ways failed to respond regardless of which colony was
ample ofisocontact, this latter treatment also served as a stimulated. No colony pairs were found in which infor-
control forpossibleeffectsofdamageon theestablishment mation flow was unidirectional.
ofbehavioral coordination, because adjoining parts ofa Intercolony coordination ofbehavior is frequently ob-
single colony should become physiologically integrated served in natural populations. Ofthe 1301 colonies sam-
when they meet. Regeneration and growth ofthecut bor- pled from Laminaria blades, 568 (44%) were in contact
derswas rapid; all colony pairs had grown back into con- with another colony. Ofthese, 408 (72%) were behavior-
tactin approximatelyaweek. Aftersubcolonieshadgrown ally coordinated with at least one neighbor.
into contact, I tested for intercolony behavioral coordi-
nation. Intercolony coordination andsize at first contact
Intercolony coordination ofzooids was observed most
Poreplates frequently when two colonies were small at the time of
A scanning electron microscope was used to examine first contact (Fig. 2). When the areas ofeach colony in a
isocontact borders (n = 2) and allocontact borders ofco- pair at the time of first contact were summed, the com-
ordinated (n = 2) and non-coordinated (n = 2) colony bined area ofcolony pairs with coordinated behavior (n
pairs forthe presence ofpore plates. Coloniesgrowing on = 15; mean = 1.02 cm2, S.D. = 1.51) was significantly
Laminuria were collected at Fridav Harbor. For isocon- smallerthan the combined area ofcolony pairs that were
INTERCOLONY COORDINATION OF ZOOID BEHAVIOR 225
^
226 D. F. SHAPIRO
found in the borders between both coordinated and non-
coordinated colonies. Whereas all previously described
- poreplatesconsistofasingleperforatedcalciumcarbonate
dome, allocontact pore plateswerecomposed oftwo per-
forated calcium carbonate domes placed base to base
formingasingle sphereembedded in the intercolonybor-
der. Allocontact pore platesalso differed from otherpore
plates in that they generally had three or fewer pores. In
contrast, other types ofpore plates generally had four or
more pores. There were no obvious morphological dif-
ferences between allocontact pore plates ofcoordinated
and non-coordinated colonies.
Discussion
The results ofthis study show that allorecognition re-
sponses following contact between colonies of the bry-
ozoan Membranipora membranacea vary depending on
the genetic similarity and age of interacting colonies.
Contact between genetically identical colonies is always
vs^JSSs^Vv^^&MV*?
characterized by an isocontact border, isocontact pore
plates, and coordinated behavior of zooids. Contact be-
tween genotypically distinct colonies is always character-
ized by allocontact borders and allocontact pore plates.
However, only colonies that are young when they first
come into contact, show coordinated behavior.
Intercolony coordinated behaviorappearsto bethe re-
sult ofintercolony neural integration. Thorpe etal. (1975)
demonstrated the presence ofelectrical signals that con-
ducted across colonies ofM. membranacea at the same
rate as the spread of lophophore retractions. Electrical
signals similar to those described by Thorpe et al. (1975)
havebeen found topassbetween behaviorallycoordinated
colonies but not between non-coordinated colonies
(Shapiro and Mackie, unpub. data), providing direct ev-
Figure4. A. Isocontactborderbetweengenotypicallyidenticaltissues idence ofintercolony neural linkage.
ofacolonythathasgrownaroundanothercolonyandbackintocontact Thepresenceofporeplatesprovidesmorphologicalev-
withitself. B. Allocontactborderbetween genotypicallydistinctcolonies. idence for intercolony neural linkage. The time required
A=bb1r.e0vimamt.ions: ab, allocontact border; ib, isocontact border; Size bars for the formation ofisocontact or allocontact pore plates
following initial intercolony contact would explain why
colonies did not become coordinated immediately upon
plates) in shape and slightly raised to form a perforated contact and why colonies with short intercolony border
calcium carbonate dome or lens, the base ofwhich is at- lengthsdid notshowcoordinated behavior. However, the
tached to the zooidal wall (Fig. 5A-C). presence of allocontact pore plates does not necessarily
Poreplates foundin isocontactborders, herein referred indicate behavioral coordination because allocontact pore
toas"isocontact pore plates" (Figs. 5C, 6B), weresimilar plates were also found between non-coordinated colony
to lateral pore plates in that they consisted of a single pairs. Thus, there may be morphological differences on a
round perforated dome. However, whereas lateral pore finerscale (e.g., presence orabsence offunctional nerves)
plates tended be ofa uniform size and regularly spaced between the allocontact pore plates of behaviorally co-
in lateral walls, isocontact pore plates were variable in ordinated and non-coordinated colonies.
size and occurred irregularly, occasionally in groups, in Allocontact pore plates represent a new, morphologi-
the walls formed between genotypically identical colonies. cally distinct class of pore plates never before described
Pore plates found inallocontact borders, herein referred in the Bryozoa. This is the first time pore plates between
to as "allocontact pore plates" (Figs. 5D; 6C, D) were unrelated bryozoan colonies have been described. Chancy
1NTERCOLONY COORDINATION OF ZOOID BEHAVIOR 227
Figure 5. Scanningelectron micrographs ofthedifferent typesofpore plates found in Membranipora
membranacea. A. Basal viewofcalcined zooidal walls showing transverse and lateral pore plates between
zooidswithinacolony.(lOOx). B. Lateralporeplatebetween zooidswithin acolony(500X).C. Isocontact
pore plates(500X). D. Allocontact bordershowingallocontact poreplates(100 ). Abbreviations: ab. allo-
contact border; ap. allocontact pore plate: Kv. lateral wall: tw. transversewall.
(1983) examined the borders between unrelated colonies ( 1983) did find pore plates between sibling colonies ofT.
ofthe cheilostome bryozoan Thalamoporella califomica, califomica. Theseporeplates, which hecalledfusion pore
but found no evidence ofpore plates. However, Chancy plates, consisted ofa single rather than a double calcium
228 D. F. SHAPIRO
B
Figure6. Light micrographsofthedifferenttypesofporeplatesfoundinMembraniporamembranacea.
SectionsA through Cwere made parallel tothe plane ofthecolony. A. Transverse and lateral pore plates
between zooids within a colony. B. Isocontact plates. C. Allocontact plate. D. Section perpendicular to
allocontactborderand planeofcolonyshowinganallocontactplate. Abbreviations:ab.allocontactborder;
ap,allocontactplate;ib,isocontactborder;ip,isocontactplate;k,kelp;Iw,lateralzooidalwall;tp,transverse
poreplate; tw, transversezooidal wall. Sizebars = 20^m.
carbonate dome and thus resemble the isocontact pore It is usually assumed that colony pairs that have the
plates described in this study and not allocontact pore morphological characteristics offusion are physiologically
plates. Additionally, fusion pore plates, like isocontact integrated, and unfused colonies are not (Humphries,
pore plates, were variable in size and occurred irregularly 1979;Stebbing, 1973b; Buss, 1982;Chaney, 1983). How-
in the walls formed by contact between two colonies. T. ever, assays for physiological integration are rarely per-
californica larvae settle within hours ofrelease from the formed (Hidaka, 1985; Rinkevich and Loya, 1983a, b).
parental colony (Chancy, 1983), thus indicating the po- When Rinkevich and Loya(1983a) used SEM toexamine
tential for substantial inbreeding in natural populations the allocontact borders between colonies ofthe Red Sea
(Jackson, 1986). Consequently, although sexually pro- coral Stylophora pistillata with the morphological char-
duced, siblingcoloniesmaybe nearlygeneticallyidentical. acteristicseffusion, they found thatthecolonieswere not
Thus, fusion pores plates are probably the same as iso- physiologically connected. In contrast, this study has
contact pore plates, both being characteristic ofcontact demonstrated that M. membranacea colonies with the
between genetically similar tissues. morphological characteristics of rejection can be physi-
INTERCOLONY COORDINATION OF ZOOID BEHAVIOR 229
ologically connected. Thus, unlessadequate testsare per- implying that colonies had recognized their neighbors as
formed, it may not always be safe to use morphological being genotypically distinct.
evidence offusion or rejection to imply the presence or
absence ofphysiological integration.
Acknowledgments
Allorecognition responses are important in intra- and
interspecific interactions. Rejection responses to contact I thank Jim Morin for generously providing lab space
with colonies often result in the induction ofaggressive and materials in California and Andrea Huvard for in-
structures used to fight,damage, orsurround neighboring structing me in the techniques oflight microscopy. This
colonies (e.g., Ivker, 1972; Francis, 1973; Rinkevich and manuscriptbenefited fromcommentsbyLizFrancis,Jim
Loya, 1983a; Sebensand Miles, 1988; Harvell and Padilla, Morin, Drew Harvell, Josh Nowlis, Jordan West, Staci
1990). On the other hand, fusion responses may benefit Eisner, and an anonymous reviewer. Discussions with Liz
interacting colonies by increasing competitive ability, in- Francis were extremely helpful in organizing the final
creasingfecundity, decreasingprobability ofmortality, or draft. This research was supported in part by the Lerner-
decreasingageoffirstreproduction(Buss, 1982). However, Gray Fund for Marine Research and NSF-OCE-8817498
it may be erroneous to always associate fusion with co- to C. Drew Harvell. I also thank Dennis Willows for pro-
operation and rejection with aggression. Rinkevich and vidingspace and facilitiesat Friday Harbor Laboratories.
Weissman (1987, 1989) found that fusion between geno-
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