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RESEARCH ARTICLE
Age, growth and population structure of Modiolus barbatusfrom the Adriatic
M. Peharda Æ C. A. Richardson Æ I. Mladineo ÆS. Sestanovic Æ Z. Popovic Æ J. Bolotin Æ N. Vrgoc
Received: 26 June 2006 / Accepted: 27 September 2006 / Published online: 20 October 2006� Springer-Verlag 2006
Abstract Age, growth and population structure of
Modiolus barbatus from Mali Ston Bay, Croatia were
determined using modal size (age) classes in length
frequency distributions, annual pallial line scars on the
inner shell surface, internal annual growth lines in shell
sections of the middle nacreous layer and Calcein
marked and transplanted mussels. The length frequency
distributions indicated that M. barbatus attain a length
of ~40 mm in 5–6 years indicating that a large propor-
tion of the population in Mali Ston Bay is <5 years old.
Some mussels of ~60 mm were predicted to be 14 years
old using the Von Bertalanffy growth (VBG) equation.
Up to the first 6 pallial line scars were visible in young
(<6 years) mussels but in older shells the first scars
became obscured by nacre deposition as the mussel
increased in length and age. The age of the older shells
(>6 years) was determined from the middle nacreous
lines in shell section, which formed annually in winter
between February and March; the wider dark incre-
ments forming during summer (June to September).
The oldest mussel, determined from the middle nacre-
ous lines, was >12 years, with the majority of mussels
aged between 3 and 6 years of age. The ages of mussels
ascertained using the growth lines were not dissimilar to
the ages predicted from the length frequency distribu-
tions. Age at length curves produced using modal size
class data were not different from the data obtained
using the pallial scar rings and internal growth lines.
Taken together these data suggest that M. barbatus
attains a length of 40 and 50 mm within 5 and 8 years,
respectively. Eighty one percent of individual M. barb-
atus injected with a Calcein seawater solution (300 mg
Calcein l–1), into their mantle cavity successfully
deposited a fluorescent line, which was visible in suit-
ably prepared shell sections under ultra violet light.
Incorporation of Calcein into the mussel shells was
seasonally variable with the lowest frequency of incor-
poration in mussels marked in February and recovered
in May. Seasonal shell growth was observed with sig-
nificantly higher growth rates in mussels marked in May
and removed in August (ANCOVA, F3,149 = 23.11,
P < 0.001). Mussels (~18 to 22 mm) marked in May and
recovered in August displayed maximal growth rates of
>2.5 mm month–1 compared with a mean mussel
growth rate of 1.2 ± 0.6 mm month–1. At other times of
the year mussel shell growth ranged from immeasurable
to 1.48 mm month–1.
Introduction
The bearded horse mussel Modiolus barbatus (Linnaeus
1758) is a commercially important mussel and is
Communicated by O. Kinne, Oldendorf/Luhe.
M. Peharda (&) � I. Mladineo � S. Sestanovic �Z. Popovic � N. VrgocInstitute of Oceanography and Fisheries,Setaliste Ivana Mestrovica 63,21000 Split, Croatiae-mail: [email protected]
C. A. RichardsonSchool of Ocean Science, University of Wales – Bangor,Menai Bridge, Anglesey LL59 5AB, UKe-mail: [email protected]
J. BolotinInstitute for Marine and Coastal Research,University of Dubrovnik, Kneza Damjana Jude 12,20000 Dubrovnik, Croatia
123
Mar Biol (2007) 151:629–638
DOI 10.1007/s00227-006-0501-3
distributed along the Croatian coastline of the Adriatic
Sea (Benovic 1997; Zavodnik 1997). These mussels oc-
cur in the lower eulittoral–sublittoral fringe and extend
down to depths of 110 m where they attach by strong
byssus threads to rocky substrata. They occur from the
British Isles south to Mauritania, West Africa and are
found in the Mediterranean (Poppe and Goto 2000).
Despite their commercial importance and wide distri-
bution, there are no data on the age, growth and pop-
ulation structure of the species in Croatian coastal
waters. Such information is crucial for estimating sus-
tainable exploitation rates as well as for estimating the
potential of the species for aquaculture production.
Modiolid bivalves are common inhabitants of mangrove
systems in South East Asia where they attach to the
living and dead roots of mangrove plants (Morton
1977). In Panguil Bay, southern Phillippines, for
example the related species, Modiolus metcalfei is found
in sandy and muddy subtidal sediments away from the
fringes of the mangroves (Tumandara et al. 1997) where
they are harvested by local fisherman. Harvesting re-
sults in patchy distributions of the mussels, which reside
with the anterior axis of the shell buried up to two thirds
in sand and mud substrata.
Growth in bivalves is usually described in terms of
an increase in the dimensions of the shell valves (see
Gosling 2003 for review). Several methods for deter-
mining the age and growth rate of European and
Mediterranean bivalves have been developed e.g.
length frequency analysis (Manca Zeichen et al. 2002),
surface growth rings (Peharda et al. 2002; Leontarkis
and Richardson 2005), growth lines in shell sections
(Richardson 2001) and stable oxygen isotopes (Ken-
nedy et al. 2001; Richardson et al. 2004). Each of the
methods has its advantages and disadvantages. Certain
techniques may be more appropriate for one species
than for another, but the best approach would be to
employ a range of methods which providing a robust
estimate of growth than using any one method alone
(Seed and Brown 1978).
Analysis of modal size classes in size–frequency
distributions enables estimates of the population
structure and age structure of bivalve populations to
be assessed under undisturbed natural conditions, but
does not, however, allow individual growth rates to be
determined. The accuracy of the analysis also depends
on how well the collected samples reflect the size
structure of the actual population. Where recruitment
is seasonal and the life span is short and there is little
variability in individual growth rates, individual year
classes can often be identified as distinct modes.
However, in long-lived species with extended recruit-
ment and variable individual growth rates it is usually
not possible to use size–frequency distributions to
measure growth rate due to the merging of year
classes (see Gosling 2003). The use of surface growth
rings for estimating the age and growth of bivalves is
often problematic because of the difficulties of dis-
tinguishing between seasonally produced annual
growth lines and those resulting from predation or
fishing induced disturbances (e.g. Gaspar et al. 1994;
Ramsay and Richardson 2000). The analysis of inter-
nal growth lines in shell sections has been developed
into a powerful tool for investigating the past growth
history of individual bivalve shells and for determining
their age (Richardson 2001). Whilst this method pro-
vides an accurate estimate of age and growth rate,
usually only limited numbers (20–30 shells) can nor-
mally be processed and analyzed routinely. Variations
in the stable oxygen isotopic composition (e.g. Jones
et al. 1983; Krantz et al. 1984; Kennedy et al. 2001)
and elemental ratios (e.g. Richardson et al. 2004) of
bivalve shell carbonate have been used successfully to
determine annual cycles of shell growth in one or two
individual shells. The limitations of these techniques
for determining bivalve population structure are the
number of shells whose age can be determined (e.g.
Richardson et al. 1999). In the assessment of individ-
ual bivalve shell growth rates the marking of individ-
ual bivalves (Richardson 2001; Gosling 2003) by
incorporating the fluorescent marker Calcein into the
mineralizing shell and transplanting them into their
natural environment has been shown to be a reliable
technique for estimating seasonal growth rates in
Perna perna (Kaehler and McQuaid 1999a).
In this paper we applied several methods. Analysis
of length frequency distribution was used for deter-
mining population growth rates. Calcein marking and
transplanting of M. barbatus was applied for analyzing
individual growth rates and seasonal differences in
growth rates, while the analysis of pallial line scars and
growth lines in the middle nacreous layer were applied
for individual age and growth determination.
Materials and methods
Samples of between 189 and 559 Modiolus barbatus
were collected monthly between January and
December 2004 by SCUBA divers from depths of
between 3 and 5 m from Mali Ston Bay in the Adri-
atic Sea (Fig. 1). Mussel length (anterior–posterior
axis) was measured to the nearest 0.1 mm using ver-
nier calipers and monthly length frequency distribu-
tions plotted. To estimate the growth of the
component mussel size classes these distributions
123
630 Mar Biol (2007) 151:629–638
were separated into their component size cohorts and
analyzed using the method of Bhattacharya (1967)
contained in the FiSAT statistical package (FAO-
ICLARM). The method of sum of squared errors
(SSE) was used to determine the curvature parameter
(k) in the modified von Bertalanffy growth equation
Lt = L11� (ekðt�t0Þ). The growth performance index
phi-prime (/¢) was estimated in order to compare the
growth parameters obtained in the present work with
those reported by other authors (Sparre and Venema
1992).
Marking of individual M. barbatus with a fluorescent
Calcein mark was undertaken to investigate seasonal
changes in shell growth. In a preliminary study 10 live
M. barbatus (size range 30.2–35.1 mm) were marked by
injecting ~5 ml of a Calcein seawater solution (300 mg
Calcein l–1) into the mantle cavity using a small syringe
needle inserted between the shell valves in the area
around the pedal gape (see Kaehler and McQuaid
1999a). This location was chosen for entry into the
pallial cavity because the syringe needle could be in-
serted without opening the shell valves and it did not
damage the posterior shell margin. The mussels were
held in laboratory aquaria and supplied with seawater
for 1 month to allow further shell deposition to occur
beyond the expected Calcein mark. The flesh was re-
moved by immersing the mussel briefly in boiling water
and the shells air-dried and their length measured.
Each right shell valve was embedded in Epoxy resin
(Struers Ltd), sectioned from the umbo to the posterior
shell margin and the cut surfaces ground and polished
(see Richardson 2001). Polished shell surfaces were
observed under ultra violet radiation at a wavelength
of between 460 and 490 nm (U-MWIB Cube) using an
Olympus fluorescence microscope. Narrow band of
fluorescence were observed in all ten-mussel shells thus
demonstrating that M. barbatus incorporates Calcein
into its shell and the fluorescent band can be used as an
internal shell marker in shell growth studies of this
species (see Kaehler and McQuaid 1999a).
Between May 2004 and May 2005 a field experiment
was undertaken in Mali Ston Bay. Every 3 months ~50
to 60 M. barbatus (range 16.3–60.2 mm) were collected
by SCUBA diving and mussels injected in the field with
Calcein to mark the shell. After the injection mussels
were left out of the water for about 30 min to allow
incorporation of Calcein into the shell. Marked mussels
were placed in 2 mm mesh net cages and suspended
from a pier just above the bottom in a water depth of
5 m. After 3 months the mussels were removed and
replaced with another batch of similar size freshly
collected and Calcein marked mussels. The flesh of the
marked cage grown mussels was removed by boiling
and the shells sectioned. Mussel growth, measured
between the bright green fluorescent Calcein mark and
the growing posterior margin was measured using a
calibrated eyepiece graticule following the procedure
described by Kaehler and McQuaid (1999a). These
data were used for construction of a Gulland-Holt plot,
where growth rates were plotted on Y-axis and mean
shell length on X-axis (Gulland and Holt 1959). Von
Bertalanffy growth parameters were estimated from a
numerical value of the slope (k) and x-intercept (L¥)
(Sparre and Venema 1992). Differences in growth rates
Fig. 1 Location of Mali StonBay in the Adriatic
123
Mar Biol (2007) 151:629–638 631
between the quarterly periods were tested with a one-
way analysis of covariance (ANCOVA) with length as
covariate and study period as factor. Prior to the
analysis the growth increment data were log trans-
formed and tested for homogeneity of variance using
Levene’s test.
Further mussel age and growth data were obtained
from an analysis of the pallial line scars (see Peharda
et al. 2002) on the inner shell surface of the shells
marked with Calcein (N = 199) prior to their being
embedded in resin and from growth lines in the middle
nacreous layer of shell sections (Richardson 2001). To
assist in understanding the timing of growth line for-
mation in M. barbatus, small mussel shells (~20 mm)
from each monthly collection obtained for construction
of the length frequency distributions were embedded in
resin and sectioned. The growth lines were viewed di-
rectly from the sections, obviating the need to prepare
acetate peel replicas of the polished sections. The age
of each mussel was determined independently by two
readers and where discrepancies were found between
the ages these were either resolved following discus-
sion or an average estimated age calculated. In 8.5% of
shells, where the growth lines were either not clearly
visible or pronounced disturbance lines were present,
they were omitted from the analysis. Von Bertalanffy
growth curves were fitted using FiSAT to the size and
age data determined from the mussel shells. The dis-
tance between the clearly visible consecutive growth
lines was measured to the nearest 0.1 mm using a cal-
ibrated eyepiece graticule and these data were used in
the construction of a Gulland-Holt plot. In Mali Ston
Bay M. barbatus spawns between June and August,
with a peak in July (Mladineo et al. 2007), therefore 1st
August was chosen as the birth date.
Results
Length frequency analysis
The shell length of M. barbatus ranged between 1.7 and
65.5 mm. The smallest mussels (~5 mm) appeared in
the population between May and December, sugges-
tive of a prolonged settlement period (see Fig. 2). Only
0.8% of the M. barbatus were >55 mm in length. The
monthly length frequency distributions were polymo-
dal suggesting the presence of several age classes.
When the monthly distributions were separated into
their component size (age) classes and the length at age
data generated using the method of Bhattacharya fitted
to the VBG equation, an asymptotic length (L¥) of
66.11 mm and a k of 0.181 year–1 were obtained
(Table 1). From the VBG equation M. barbatus attain
a length of ~40 mm in 5–6 years indicating that a large
proportion of the population in Mali Ston Bay is <5
years old. However, some mussels were ~65 mm and
the VBG equation predicts that they are ~20 years old.
Analysis of Calcein marked Modiolus barbatus
shells
Two hundred and thirty two mussels were marked with
Calcein, of these 33 died during the 3 month growth
period (~14%) whilst 45 mussels (~19%) did not have a
Calcein mark. The highest mortality (18 mussels)
occurred in mussels marked and transplanted into the
cage in February and recovered in May 2005; for the
remaining periods mussel mortality ranged between 3
and 7 mussels. Mussels marked in February and
recovered in May were the least successful at incorpo-
rating a Calcein mark; 16 mussels failed to incorporate a
Calcein mark. The 154 successfully Calcein marked
M. barbatus demonstrated that growth was seasonal.
Equations obtained from Gulland-Holt plots of mussel
growth in each study period are shown in Table 2. The
estimated asymptotic lengths were variable ranging
between 49.3 mm (August–November) and 57.9 mm
(November–February), whilst k ranged between
0.063 year–1 (November–February) and 0.161 year–1
(May–August) (Table 1). There was no difference in
the length of the Calcein marked M. barbatus between
the four study periods (ANOVA, F3,150 = 1.86,
P = 0.138). Nevertheless statistically significant differ-
ences between the growth rates during the different
study periods (ANCOVA, F3,149 = 23.11, P < 0.001)
were observed. This was due to the high growth rates of
mussels marked in May and removed in August
(Fig. 3). No statistically significant differences in
mussel growth were observed in the three other
periods of the year. During each study period shell
growth was significantly and negatively related to shell
length (ANCOVA, F1,149 = 122.75, P < 0.001); growth
decreasing with increasing shell length. Two mussels,
initial length 17.9 and 21.5 mm, marked in May and
recovered in August, displayed rapid growth
(>2.5 mm month–1) compared with a mean mussel
growth rate of 1.2 ± 0.6 mm month–1. At other times
of the year growth ranged from immeasurable (a Calcein
mark on the shell margin) to 1.48 mm month–1.
Analysis of pallial line scars and middle nacreous
layer growth lines
Pallial line scars on the inner shell surface (Fig. 4) and
growth lines in the middle nacreous layer (Fig. 5) were
123
632 Mar Biol (2007) 151:629–638
counted in 182 mussels (shell length 17.9–61.3 mm).
Each method had its advantages and disadvantages
although the first annual line was difficult to distinguish
using either method. The first six pallial line scars were
only visible in young (<6 years) mussels because in
older shells the first scars became obscured by nacre
deposition as the mussel increased in length and age.
Counting pallial scars in older mussels was not possible
Fig. 2 Monthly lengthfrequency distributions ofModiolus barbatus from MaliSton Bay, Adriatic to showthe appearance of smallmussels in the populationbetween May and October.The distributions are variablefrom one month to another
123
Mar Biol (2007) 151:629–638 633
so instead their age was determined using growth lines
visible in the middle nacreous layer of shell sections.
This method, however, was not without its difficulties
as the mussel shells had a highly variable shell color-
ation, which made it difficult to distinguish seasonal
growth lines from the dark or light patterns of color-
ation. Inspection of smaller mussels (<20 mm) col-
lected monthly showed that the narrow white middle
Table 1 Comparison of the estimated Von Bertalanffy growth parameters using different age determination methods
Method of analysis L¥ (mm) k year–1 t0 (year) /¢
Length frequency analysis 66.11 0.181 –0.071 6.67Calcein marking May–August 55.59 0.161 NA 6.21Calcein marking August–November 49.33 0.134 NA 5.79Calcein marking November–February 57.94 0.063 NA 5.35Calcein marking February–May 52.87 0.077 NA 5.37Pallial line scars and growth lines 59.78 0.210 –0.100 6.62Growth increment analysis (Gulland-Holt) 67.26 0.168 NA 6.63
L¥ asymptotic length, k growth constant, t0 initial condition parameter, /¢ growth performance index, NA not available
Table 2. Gulland-Holt equations estimated from measurements of shell growth in Calcein-marked Modiolus barbatus shells during 4periods between May 2004 and May 2005
Period of study N Gulland-Holt equation r2 P
May–August 41 Y = 8.967 – 0.161 X 0.450 <0.001August–November 39 Y = 6.605 – 0.134 X 0.394 <0.001November–February 47 Y = 3.673 – 0.063 X 0.456 <0.001February–May 27 Y = 4.148 – 0.081 X 0.420 <0.001
Y growth rate (mm/year), X mean total shell length (mm)
Fig. 3 Predicted Von Bertalanfy growth curves estimated fromthe modal classes in length-frequency (a) pallial line scars andgrowth lines in the middle nacreous layer (b) and analysis ofincrements between lines in the middle nacreous layer (c)
Fig. 4 Photomicrograph of pallial line scars (arrows) on theinner shell surface of Modiolus barbatus. Scale bar = 1 cm
Fig. 5 Photomicrographs of polished sections of the shell ofModiolus barbatus from Mali Ston Bay to indicate the appear-ance of the internal growth lines (arrows) in the middle nacreouslayer. Scale bar 200 lm
123
634 Mar Biol (2007) 151:629–638
nacreous lines formed in the winter period between
February and March when sea water temperatures
were minimal (10�C) whilst the wider dark increments
represent summer shell growth.
The oldest mussel, determined using the middle
nacreous lines, was >12 years (L¥ = 60 mm), although
the majority of mussels were between 3 and 6 years of
age. The ages of mussels ascertained using the internal
growth lines were not dissimilar to the ages predicted
from the length frequency distributions; the VBG
curve predicted a mussel of ~60 mm to be ~14 years of
age (Fig. 6). When the shell scar and growth line age
data were fitted to the VBG equation an asymptotic
length of 59.78 mm and k of 0.21 year–1 were obtained
(Table 1); values not dissimilar from those estimated
when the length frequency distributions were sepa-
rated into their component modal size classes (see
Table 1). These data suggest that M. barbatus attains a
length of 40 and 50 mm within 5 and 8 years, respec-
tively. For comparison, the growth increment data used
in the construction of a Gulland-Holt plot produced an
elevated estimate of L¥ (67.26 mm), and a lower k
value of 0.168 year–1 (Gulland-Holt plot N = 317,
Y = 11.307 – 0.168 X, r2 = 0.313, P < 0.001) (see
Fig. 3) compared to the values determined using the
VBG equation (see Table 1). The Phi-prime index
(Table 1) overcomes the problem of correlation be-
tween k and L¥ and enables the comparison between
the different methods for determining the VBG con-
stants. The values of /¢ suggests no differences in the
growth parameters obtained using length frequency
distributions, pallial line scars and middle nacreous
growth lines, however the values obtained using the
Calcein marked mussels is smaller and is suggestive of
a difference.
Discussion
The appearance of juvenile Modiolus barbatus
(~5 mm) into the Mali Ston Bay population occurred
between May and December indicating a prolonged
settlement period. Settlement of a related species
M. metcalfi in Panguil Bay, southern Phillippines,
however, occurred during a discrete period between
May and July (Tumandara et al. 1997). Spat were
found attached to the byssal threads and shell surfaces
of the adult sized shells. In Mali Ston Bay, newly set-
tled M. barbatus spat were found amongst the clumps
of adults which were attached to each other.
Biofouling and a covering of periostracum obscured
any potentially visible external growth rings on the
shell surface of M. barbatus precluding their use as a
method for determining their age. Seed and Brown
(1978) reported difficulties in reading the surface
growth rings in the horse mussel Modiolus modiolus
and found that the thick black periostracum had to be
removed with dilute acid before the rings became vis-
ible and age determination was possible. Age estimates
of M. modiolus, based on surface rings demonstrated
high variability with ages varying from 6 to >20 years
of age for mussels of similar size (Seed and Brown
1978). In their study they encountered problems in
identifying different modal size (age) classes from size
to frequency distributions and noted that the distribu-
tions showed relatively little change throughout the
year with no clear regular modes. M. modiolus is a
species of known longevity (40–50 years) (Anwar et al.
1990) and the older year classes merge so it is therefore
perhaps not surprising that there is little evidence of
polymodal distributions within horse mussel length
frequency distributions. By contrast length frequency
distributions of M. barbatus, which we have shown to
be a younger mussel species (5–12 years), showed
evidence of polymodal size distributions in some
monthly population samples, which could be reliably
separated into their modal size classes by the method
of Bhattacharya, and evidence of a prolonged seasonal
recruitment between May and December.
Anwar et al. (1990) estimated the age of M. modi-
olus from internal growth lines in acetate peels of shell
sections what overcame the difficulties associated with
the use of surface rings. Shell sections revealed an
alternating pattern of light and dark annual growth
lines within the middle nacreous layer; lighter, more
translucent increments were deposited during the
summer (May–October) whilst the darker lines were
laid down during the winter (January–March). In older
slow-growing mussels, or those in which the outer shell
layer had been badly abraded, the regular alternating
Fig. 6 Von Bertalanfy growth curve fitted to the length and agedata determined from the pallial line scars on the inner shellsurface and growth lines in the middle nacreous layer
123
Mar Biol (2007) 151:629–638 635
pattern of summer and winter growth could still be
resolved even at the posterior margin of the oldest (48
years old) shells where the growth increments were
narrow and the lines compressed. Shell sections of
M. barbatus revealed a series of annually deposited
growth lines, although the first and second growth lines
were difficult to locate and the lines were narrow at the
margin in larger shells due to shell thickening and
compression of the most recently deposited lines. The
majority of the M. barbatus from Mali Ston bay were
<5 years old, although an occasional individual col-
lected was >12 years of age. The inner shell surface
pallial scars were clear for up to the first 6 years of life
but became progressively overlain with nacre deposi-
tion in older mussels. A combination of both methods,
pallial line scars and internal middle nacreous lines,
was therefore used to age the shells of M. barbatus.
The method used by Kaehler and McQuaid (1999a)
in which flourochrome Calcein is deposited into the
newly mineralizing shell to act as an in situ growth
marker in the brown mussel Perna perna was success-
fully used to mark M. barbatus. No mussel mortality,
resulting from Calcein injection, was observed by Ka-
ehler and McQuaid (1999a) who found that both
juveniles and adults incorporated Calcein into their
shells when it was administered at a concentration of
~80 mg l–1. By contrast we found ~14% mortality in
marked M. barbatus held in cages, with the highest
mortality occurring between February and May, and
~20% of mussels had not produced a fluorescent line
within the shell. The fastest growth rates in M. barba-
tus, (2.5 mm month–1), were about a quarter of the
rates observed by Kaehler and McQuaid (1999a) for
small P. perna (>10 mm month–1) and greater than
that achieved by larger mussels (<1 mm month–1). The
observed high mortality in M. barbatus between Feb-
ruary and May was not caused by Calcein injection but
was the result of successful attacks by the predatory
whelk Hexaplex trunculus. The cage containing the
mussels had been accidentally placed 0.5 m lower in
the water column during this spring period and H.
trunculus had been able to drill the mussels attached to
the mesh of the cage, from outside of the cage. The
dead shells had both drill holes and chipping marks
characteristic of H. trunculus (Peharda and Morton
2006). Caging marine organisms has its disadvantages
(Seed and Brown 1978) since there can be issues of
space and non-normal behavior and water flow through
the cage can be affected by the accumulation of epi-
bionts on the outside of the cage. Whilst we tried to
minimize these potential problems, the growth rates of
the caged mussels had a lower maximum value of k
than those determined using other methods in the
natural environment (Table 1). The clumping together
of caged mussels might have reduced their shell growth
through competition for food resources. Nevertheless
the data are a first contribution to understanding
seasonal growth rates in this little studied bivalve.
Seed and Brown (1978) observed that file-marked
M. modiolus had somewhat slower growth rates par-
ticularly amongst older, larger individuals than natu-
rally growing mussels in Strangford Lough, Northern
Ireland. They suggested that the slower growth rate of
caged animals might be due to the conditions created
by the cages themselves and that the presence of mesh
plus the periodic fouling by algae probably impaired
feeding conditions within the cages by reducing water
circulation through them. In positioning the cage it
may not be possible to replicate the natural bed con-
ditions particularly if the cage is suspended in the water
column above the seabed, as was the case in this study.
It is known that water currents differ along the vertical
water column profile and that current speed affects
bivalve growth (Dame 1996). Bivalves living in current
dominated environments are the beneficiaries of
increased food availability and faecal waste removal
(Dame 1996).
Some M. barbatus shells collected during the study
were deformed. Cross-sections of the deformations
under SEM revealed the presence of endolithic cyano-
bacteria (Harper, personal communication). Kaehler
and McQuaid (1999b) found that endolith infestations
significantly reduced the strength and lowered the shell
growth of P. perna in South Africa. Differences in shell
growth rates of individual M. barbatus therefore may
have been partially attributable to endolith infestation.
The relationship between shell growth rates and inci-
dence, distribution and infestation of M. barbatus shells
by endolithic cyanobacteria would be an interesting
area for future study (see also Gray et al. 1999).
The VBG equation estimated a maximum shell
length of ~60 mm and a VBG growth constant (k) of
~0.21 year–1 using the length frequency distribution
data and the shell scar and growth line age data. These
data suggest that M. barbatus attains a length of
between 40 and 50 mm within 5 and 8 years, respec-
tively. The growth rate of M. metcalfei in Panguil Bay
in the Philippines, determined using length frequency
distribution analysis, yielded mean k and L¥ values of
2.04 year–1 and 62.5 mm, respectively (Tumandra et al.
1997); growth rates that were considerably higher than
those observed in M. barbatus from rocky substrates in
Croatian coastal waters. The Phi-prime (/¢) indices,
which ranged between 5.53 and 6.67 for M. barbatus in
this study were lower than those determined for
M. metcalfei (/¢ = 8.98). Tumandra et al. (1997), again
123
636 Mar Biol (2007) 151:629–638
indicating that the sediment dwelling Philippine spe-
cies is faster growing than the Croatian modiolid.
Owing to the rapid growth rate of M. metcalfei it is
therefore likely to be a successful species for maricul-
ture (Tumandra et al. 1997). The Croatian M. barbatus
takes about 12 years to attain a length of ~60 mm,
although the majority of mussels were smaller
(30–40 mm) and between 3 and 6 years of age. This
indicates that in Mali Ston Bay this mussel is a much
slower growing species than M. metcalfei in Philippine
coastal waters.
Bivalve shell growth can be site specific and it is
possible that M. barbatus grows faster in other parts
of the Adriatic. Previous study showed that the
Noah’s Ark shell (Arca noae), that often lives in
clumps with M. barbatus, has different growth rates at
three sampled locations in the eastern Adriatic Sea.
Individuals collected in the Mali Ston bay had the
intermediate growth rates (Peharda et al. 2002) and
they were similar to values recorded for M. barbatus
in the same area. Growth under culture conditions
can be faster than growth in the wild, and therefore,
it is necessary to conduct the experimental growth
study under aquaculture conditions in order to assess
the aquaculture potential of M. barbatus along the
Croatian coastline.
Acknowledgments This research was financed by the CroatianMinistry of Science and Technology. The authors are grateful toZeljko Bace, Marko Zaric, Nika Straglicic, Lovorka Kekez andMark Prime for technical assistance. Special thanks to BarbaraZorica for help with statistical analysis and Professor C.D.McQuaid for helpful suggestions for Calcein marking the musselshells. The experiments conducted comply with the current lawsof Republic of Croatia.
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