Discussion
The mean number of colonising B. amphitrite is
significantly different among sites and this
species, even not the most abundant one, is dominant at the power plant outflow. Among five barnacle species found, B. eburneus, B. improvisus,
B. perforatus, B. trigonus and B. amphitrite (Rossi
et al. in preparation), the last is considered the
most pollution resistant (Relini and Relini Orsi
1970).
The lowest recruitment of barnacles, including B. amphitrite, observed at the Industrial
Channel should indicate that, in this station a
severe selection acts before or during larval settling. Indeed, several potential factors may contribute to differences in barnacle settlement and
mortality among sites. According to O'Connor
and Richardson (1998) variability in larval settling behaviour is a general and complex phenomenon. Settlement success of the larvae and
variability in recruitment are influenced by various factors including delayed metamorphosis,
larvae age, water flow, salinity. temperature,
chemical and physical nature of the substratum's
surface that is the presence of dissolved compounds or bacterial film (Judge et ai. 1988;
Pechenik et al. 1993; Anil and Kurian 1996;
Satuito et al. 1996; Yamamoto et al. 1996; Bryan
et al. 1996; O'Connor and Richardson 1998). The
Industrial Channel population is probably subjected to high generalised mortality due, probably, to chemical pollution. Such an effect on an
expected large cypris larvae population might
explain the lack of significant departure from
the Hardy-Weinberg equilibrium as previously
described by Montero et ai. (1994). Our preliminary data indicate a lower rostrocarenal mean
length in B. amphitrite from this site than from
the power plant outflow and from Chioggia. at
least in 40-80 days from panel immersion. This
result might reflect a low growth rate of organisms in this site but could support the hypothesis of high mortality during the precocious
phase of benthic life, a very critical period
(Gosselin and Qian 1996), coupled with continuous larval settlement. The low number of
colonising B. amphitrite at Chioggia station as
well as its dominance at the power plant outflow
should be due to its ecological characteristics. At
Chioggia station B. amphitrite might suffer
interspecific competition, particularly with B.
eburneus that is the most abundant species
Balanus amphitrite in the Lagoon of Venice
327
recorded in this site. On the other hand, B.
amphitrite should outcompete and exclude B.
eburneus from the power plant outflow. Indeed,
even if, according to Scheltema and Williams
(1982), neither larval survival and development
of B. eburneus are affected by increase in temperature to 30°C, this species is more sensitive
to high temperatures and salinity than B.
amphitrite whose settlement is not modified by
thermal pollution (Relini 1980; Relini and Relini
Orsi 1970).
We cannot relate the low genetic variability
observed at GPI locus to the selective effect of
both chemical and thermal pollution because no
significant differences in allele frequencies
among stations have been observed. Low polymorphism at this locus could be a genetic characteristic of the species that, indeed. is poorly
investigated. For B. amphitrite populations from
the inflowing and outflowing canals at the Haifa
(Israel) electrical plant cooling system, Nevo et
al. (1977) reported that values of estimated heterozygosityvary between 0.034 and 0.167. In fact,
our data seem comparable as biased estimated
heterozygosity at GPI locus is between 0.089, at
the power plant outflow, and 0.184, at the
Chioggia station. At the same time, we cannot
exclude that some stabilising factor affects the
genetic structure of the B. amphitrite population
from the Lagoon of Venice. This should contribute to maintaining constant gene frequencies
as shown by the result of the genetic monitoring
carried out periodically over the last 10 years
(Patarnello et aI. 1991; Montero et a1. 1994).
Our results confirm the data by Montero et
al. (1994) indicating that the individuals of B.
amphitrite homozygous at GPI locus present
higher GPI activity than the heterozygotes.
Moreover, we observed that homozygotes activity is higher in organisms from the power plant
outflow than in ones from the other two sites.
The highest temperature recorded at this station
and the highest enzymatic activity observed in
organisms from there might induce an increase
in uptake and accumulation of pollutants
(Barbaro et al. 1978; Montero et al. 1994) and
may affect the survival of organisms. In fact, at
the power plant outflow, total mortality is higher than in other sites and increases from August
to January. As suggested by Montero et al. (1994)
a significantly lower efficiency in heterozygotes
might be coupled with differential fitness
between genotype and produce the excess of
The mean number of colonising B. amphitrite is
significantly different among sites and this
species, even not the most abundant one, is dominant at the power plant outflow. Among five barnacle species found, B. eburneus, B. improvisus,
B. perforatus, B. trigonus and B. amphitrite (Rossi
et al. in preparation), the last is considered the
most pollution resistant (Relini and Relini Orsi
1970).
The lowest recruitment of barnacles, including B. amphitrite, observed at the Industrial
Channel should indicate that, in this station a
severe selection acts before or during larval settling. Indeed, several potential factors may contribute to differences in barnacle settlement and
mortality among sites. According to O'Connor
and Richardson (1998) variability in larval settling behaviour is a general and complex phenomenon. Settlement success of the larvae and
variability in recruitment are influenced by various factors including delayed metamorphosis,
larvae age, water flow, salinity. temperature,
chemical and physical nature of the substratum's
surface that is the presence of dissolved compounds or bacterial film (Judge et ai. 1988;
Pechenik et al. 1993; Anil and Kurian 1996;
Satuito et al. 1996; Yamamoto et al. 1996; Bryan
et al. 1996; O'Connor and Richardson 1998). The
Industrial Channel population is probably subjected to high generalised mortality due, probably, to chemical pollution. Such an effect on an
expected large cypris larvae population might
explain the lack of significant departure from
the Hardy-Weinberg equilibrium as previously
described by Montero et ai. (1994). Our preliminary data indicate a lower rostrocarenal mean
length in B. amphitrite from this site than from
the power plant outflow and from Chioggia. at
least in 40-80 days from panel immersion. This
result might reflect a low growth rate of organisms in this site but could support the hypothesis of high mortality during the precocious
phase of benthic life, a very critical period
(Gosselin and Qian 1996), coupled with continuous larval settlement. The low number of
colonising B. amphitrite at Chioggia station as
well as its dominance at the power plant outflow
should be due to its ecological characteristics. At
Chioggia station B. amphitrite might suffer
interspecific competition, particularly with B.
eburneus that is the most abundant species
Balanus amphitrite in the Lagoon of Venice
327
recorded in this site. On the other hand, B.
amphitrite should outcompete and exclude B.
eburneus from the power plant outflow. Indeed,
even if, according to Scheltema and Williams
(1982), neither larval survival and development
of B. eburneus are affected by increase in temperature to 30°C, this species is more sensitive
to high temperatures and salinity than B.
amphitrite whose settlement is not modified by
thermal pollution (Relini 1980; Relini and Relini
Orsi 1970).
We cannot relate the low genetic variability
observed at GPI locus to the selective effect of
both chemical and thermal pollution because no
significant differences in allele frequencies
among stations have been observed. Low polymorphism at this locus could be a genetic characteristic of the species that, indeed. is poorly
investigated. For B. amphitrite populations from
the inflowing and outflowing canals at the Haifa
(Israel) electrical plant cooling system, Nevo et
al. (1977) reported that values of estimated heterozygosityvary between 0.034 and 0.167. In fact,
our data seem comparable as biased estimated
heterozygosity at GPI locus is between 0.089, at
the power plant outflow, and 0.184, at the
Chioggia station. At the same time, we cannot
exclude that some stabilising factor affects the
genetic structure of the B. amphitrite population
from the Lagoon of Venice. This should contribute to maintaining constant gene frequencies
as shown by the result of the genetic monitoring
carried out periodically over the last 10 years
(Patarnello et aI. 1991; Montero et a1. 1994).
Our results confirm the data by Montero et
al. (1994) indicating that the individuals of B.
amphitrite homozygous at GPI locus present
higher GPI activity than the heterozygotes.
Moreover, we observed that homozygotes activity is higher in organisms from the power plant
outflow than in ones from the other two sites.
The highest temperature recorded at this station
and the highest enzymatic activity observed in
organisms from there might induce an increase
in uptake and accumulation of pollutants
(Barbaro et al. 1978; Montero et al. 1994) and
may affect the survival of organisms. In fact, at
the power plant outflow, total mortality is higher than in other sites and increases from August
to January. As suggested by Montero et al. (1994)
a significantly lower efficiency in heterozygotes
might be coupled with differential fitness
between genotype and produce the excess of