Comparative Morphology of the Byssi of Dreissena polymorpha and Mytilus edulis

6 Pages·1993·4 MB·English

by  L R Eckroat

Research Note
Comparative Morphology of the Byssi of Dreissena polymorpha
and Mytilus edulis
Larry R. Eckroat and Louise M. Steele*
The Pennsylvania State University at Erie, The Behrend College, Erie, Pennsylvania 16563-0800, U.S.A.
Abstract. Scanningelectron micrographs reveal differences inbyssal stems, threadsandplaquesoftwocommonbiofouling mussels, the zebra mussel
Dreissenapolymorpha(Pallas), andthebluemussel Mytilusedulis(Linnaeus). Thesedifferencesincludeanchorageofthestem, thepatterninwhichthreads
branch from the growing stem, thread surface topography, plaqueorientation, and the morphology ofthe region ofthe thread that extends into the plaque.
Morphological dissimilaritiesarelikely tobe relatedtodifferences inthe mechanicsofbyssus formation andtodifferences inthesurfacetexture and length
oftheventralgroove. Morphologicaldifferencesbetweenbyssiofvariousbiofoulingmusselsshouldbeconsideredasresearchersdevelopandadaptmechanisms
ofcontrol.
Thepost-larvaeofmostbivalvemolluscspossessbyssal quinone tanning process of thread formation, so that the
structuresby whichthey are secured tothesubstratumasthey threads formed were weaker than those of unchlorinated
undergo metamorphosis (Yonge, 1962). Somebivalves retain animals (Holmes, 1970). Therefore, although chlorination
a byssal apparatus into the adult stage. For example, the created a flow dependent distribution of mussels due to
marineblue musselMytilusedulis (Linnaeus), which is well weakening oftheir byssus attachment systems, chlorination
known for its commercial importance, and the freshwater failed to suppress fouling by M. edulis in areas oflow water
zebramusselDreissenapolymorpha (Pallas), arecent invader flow. In D. polymorpha, chlorination is effective in control-
ofthe Great Lakes (see Mackie et al., 1989 for a review of ling zebra mussel larvae, but adult mussels are able to pro-
the zoogeography ofD. polymorpha), are two species that tect themselves from intermittent doses of chlorine by ad-
retain a byssus asjuveniles. Thus, the byssus plays a major duction (Anon., 1991).
role early in the lives of most bivalves and is important Since zebra mussels were first identified in the Great
throughout the lives of some species. Lakes as a two-year-old age class in 1988 (Hebert et al.,
Purchon (1977) lists two major advantages to the 1989), they have been causing fouling problems for users of
presence of a byssal attachment. First, by forming a byssal raw waterbyaccumulatingonexposed surfacesandphysically
attachment, molluscsdo notexpendenergy unnecessarily to blocking intake pipes by attaching to one another to form
maintainpositionona substratum. Inaddition, whenthetide layersasthickas0.3 m (Clarke, 1952; Griffithsetal. 1989).
,
ebbsorthe waterlevel falls, a byssate mollusc can withdraw Because the zebra mussel contributes to the biofouling of
its foot and close its shell valves to prevent desiccation. water supplies primarily by forming byssal attachments, the
Mussels are, therefore, regarded as being among the most study ofbyssus morphology could lead to the development
difficult fouling organisms tocontrol because they are capable of a method of controlling this mollusc.
ofwithstandingconditionsthat eliminate most other species. It has been reported that the byssusofDreissenapoly-
For instance, chlorination has been used in attempts to con- morpha is similar to the byssal apparatus found in Mytilus
trol both Mytilus edulis andDreissenapolymorpha Holmes edulis and other marine bivalves (Moore, 1991). In previous
.
(1970) reports that in M. edulis, one ofthe most serious of- studies, however, the authors (Steele, 1991; Eckroat et al.,
fenders in fouling marine intake systems (Clapp, 1950), the 1992) observed that the structure of the byssus ofD. poly-
principal effect of chlorination was a depression in the ac- morpha differed from the byssal structure ofM. edulis that
tivity of the foot, leading to a reduction in the number of hadbeendescribed inthe literature (Brown, 1952; Smeathers
threads formed. Inaddition, chlorination interfered with the and Vincent, 1979). Therefore, the current study was under-
^Current address: Case Western Reserve University, School ofMedicine, takentoclearly delineatedifferences betweenthebyssi ofD.
Cleveland, Ohio 44106, U.S.A. polymorpha and M. edulis by making morphological
American Malacological Bulletin, Vol. 10(1) (1993):103-108
103
104 AMER. MALAC. BULL. 10(1) (1993)
comparisons oftheir stems, threads, and plaques. Accurate showed that the ventral groove extended to the distal tip of
comparisons are especially important because morphological the footofM. edulis, butextendedonly abouthalfthelength
differences between the byssi ofvarious biofouling mussels of the foot in D, polymorpha.
should be recognized as researchers develop and adapt con- Removal ofthe byssus from specimens ofDreissena
trol mechanisms that affect the byssus. polymorpha revealed that most of their byssal threads
branched from one linear location ofthe stem and from all
MATERIALS AND METHODS around the stem's circumference (Fig. 3). Cuffs or sheaths,
formed by the outer laminae, were observed at the bases of
LivespecimensofDreissenapolymorpha, 2.1-2.5 cm the threads in some specimens (Eckroat et a!., 1992). Ex-
in length, were collected at Lampe Marina on Presque Isle, amination ofintact specimens ofMytilus edulis showed that
Erie, Pennsylvania, during October 1990 and maintained in threads, attached tothe stem by overlapping rings, branched
a 15°C freshwater aquarium for approximately one month, from progressively more distal locations alongtwo opposite
where they were fed 200 ml of an alga mixture (3 g of edges of the stem (Fig. 4).
Chlorella spp. blended in 1 Lofwater) daily. Additional live In the proximal region, threads of Dreissena poly-
specimensofD. polymorpha, 1.5-2.3 cm in length, werecol- morpha were cylindrical and smooth (Fig. 5), but the prox-
lected in Thompson's Bay on Presque Isle, Erie, Penn- imal portionsofthreadsofMytilus edulis were flattenedwith
sylvania, during May 1991 and maintained in a 15°C a crimped edge and had corrugations or folds around their
freshwater aquarium overnight before their byssal threads circumferences (Fig. 6). The distal half of threads of D.
were prepared for examination. Live specimens of Mytilus polymorpha became increasingly rough with longitudinal
edulis measuring approximately! 1.0cm and 5.0-6.5 cm were ridges that were most pronounced at the distal ends of the
collectedat Wachapreague, Virginia, andat Rye, New Hamp- threads (Fig. 7). ThreadsofM. edulis, however, were smooth
shire, respectively, and maintained in a 24°C saltwater in the distal region (Fig. 8).
aquarium (31 ppt) for 14 days and fed marine Chlorella spp. Threads ofDreissenapolymorpha and Mytilus edulis
daily as described above. formedobliqueangles withthe substratumandendeddistal-
To prepare specimens for scanning electron micro- ly in thin, oval plaques (bottom portions ofFigs. 9 and 10).
scopy, either the byssi were removed, or the mussels' shells In D. polymorpha, threads became broader, flattened, and
were opened so that the byssi could be studied intact. bifurcated asthey expandedalongtheplaque's wideaxis (Fig.
Specimens were fixed in 5% glutaraldehyde in sodium phos- 9, near top), which was oriented perpendicular to the
phate buffer (pH 7.2) for24-48 hours at 5°C, washed in buf- longitudinal axis of the thread (bottom portion of Fig. 9).
fer alone for 15 minutes, and dehydrated in a graded ethanol Threads ofM. edulis, however, trifurcated and formed thin,
series. After specimens were critical point dried using car- root-like extensions that continued into the plaque (top por-
bon dioxide as the transitional fluid, they were mounted on tion ofFig. 10). The narrow axis ofthe plaque was oriented
aluminum stubs and sputter coated with gold-palladium. perpendicular tothe longitudinal axis ofthe thread (Fig. 10).
Specimens were examined and photographed with a Hitachi
S-570 scanning electron microscope. DISCUSSION
RESULTS Byssus morphology is relatedtothe mechanical events
that occur during byssus formation. One of the first
Scanningelectron micrographsprovided moredetailed mechanical events involves the movement ofthe foot tightly
views of byssal structures that have been diagrammatically againstthe substratum sothatthe mussel can secrete aplaque
represented by other workers (Brown, 1952; Smeathers and (Waite, 1983; Eckroat etal. 1992), which is molded by the
,
Vincent, 1979). Comparisons betweenthe byssi ofDreissena distal depression of the ventral groove of the foot (Brown,
polymorpha andMytilusedulis were limitedto noting morph- 1952; Waite, 1983). After a plaque has been formed, an in-
ological differences because age differences of specimens dividual thread, which is ofa fluid consistency, is extruded
make size comparisons of structures meaningless. from the ventral groove (Smyth, 1954; Waite, 1983). The
In intact specimens of Dreissena polymorpha, the plaquesofMytilus edulis andDreissenapolymorpha are very
byssal stem was concealed inside a collar-like structure that thin at their peripheral regions, but their orientations on the
wascontinuous with the foot, andthe region where thebyssal substratum differ. In addition, the structure of the portion
threadsbranched fromthe stem was not visible (Fig. 1). The ofthe thread that continues into the plaque is dissimilar in
stem of Mytilus edulis, which emerged from a raised area the two species. These differences in plaque orientation and
at the base of the foot was, however, visible in intact distal thread morphology could result because the ventral
specimens, and the locations at which the threads branched groove does not extend to the distal tip of the foot in D.
from the stem wereexposed (Fig. 2). Additional observations polymorpha as it does in M. edulis, which could affect the
ECKROAT AND STEELE: COMPARISON OF BYSSI 105
Fig. 1. Intact byssus ofDreissenapolymorpha with threads emerging from a collar-like structure that conceals the stem (scale bar = 1.00 mm). Fig. 2.
IntactbyssusofMyliluseduliswiththestememergingfromaraisedstructurenearthebaseofthefoot(scalebar = 0.30mm). Fig.3.StemofD.polymorpha.
Threadsbranchfromaroundthecircumferenceofoneproximal-distal locationofthestem(scalebar = 0.60mm). Fig.4. StemofM. edulis. Threadsbranch
from progressively distal regions along two opposite sides ofthe stem and are attached by overlapping rings (scale bar = 0.50 mm).
106 AMER. MALAC. BULL. 10(1) (1993)
Fig. 5. Smooth, proximal portion ofthreadofDreissenapolymorpha (scale bar = 10.0/im). Fig. 6. Flattened proximal portionofthread ofMytilusedulis
with crimpededgeandfoldsaroundcircumference (scalebar = 43.0/im). Fig. 7. Distal portionofthreadofD. polymorphawith longitudinal ridges (scale
bar = 23.1 /an). Fig. 8. Smooth, distal portion ofthread ofM. edulis (scale bar = 38.0 jim).
ECKROAT AND STEELE: COMPARISON OF BYSSI 107
shape ofthe distal depression as the foot is pressed against concerning structural characteristics ofthe byssus could be
asubstratum. Inaddition, differencescouldariseintheshape usedtogether with knowledgeofthreadformationandattach-
ofthe distal depression that could affect the morphology of menttodeveloporadaptmechanisms ofcontrollingbiofoul-
thedistal ends ofthe threads ifthe muscle action ofthe foot ing mussels. It is therefore important to realize that among
differs in the two species. differentmolluscan species, thebyssal apparatushasdiverse
As threads are secreted as a liquid, they are molded designs. As control mechanisms are proposed, researchers
tothewallsoftheventralgroove (Brown, 1952; Smyth, 1954; should recognize such morphological dissimilarities andthe
Bairati and Vitellaro-Zuccarello, 1974; Waite, 1983). Our possibledifferences inthemechanicsofbyssusformationthat
results, which agree with those of other workers (Brown, they could reflect.
1952; Smeathers and Vincent, 1979), show that the threads
ofMytilus edulis are corrugated in the proximal third and ACKNOWLEDGMENTS
smooth in the distal two-thirds. The threads of Dreissena
polymorpha, whicharesmoothintheproximalhalfandrough TheauthorsthankDr. BruceBarberofTheVirginiaInstituteofMarine
with longitudinal ridges in the distal half, therefore, differ Sciences and Dr. Nadine Folino ofPhilips Exeter Academy forcollecting
fromthoseofM. edulis. Thesethread surfacedifferences sug- specimensofMytilusedulis. William S. Barbourforcollecting specimens
ofDreissenapolymorpha, and Kathy Mauro for typing the manuscript.
gest that the walls of the ventral groove ofD. polymorpha
havea different surface topography than those ofM. edulis. LITERATURE CITED
Once a thread has been completed, specimens of
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108 AMER. MALAC. BULL. 10(1) (1993)
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