Mali Ston Bay Research Articles

Reproductive cycle of <i>Glycymeris nummaria</i> (Mollusca: Bivalvia) from Mali Ston Bay, Adriatic Sea, Croatia

The reproductive cycle of the dog cockle, Glycymeris nummaria (Glycymerididae), was studied using specimens collected monthly from Mali Ston Bay, southeastern Adriatic Sea over the period from January to December 2010. We analysed sex ratios, gonad developmental stages, and oocyte diameters using standard histological techniques. There were no differences in the overall sex ratio and no difference in shell length with respect to sex. Results of a size-frequency analysis of oocyte diameters agreed with a qualitative analysis of gonadal developmental stages, confirming the occurrence of one annual spawning peak from May to July. A significant correlation was identified between mean gonad indices of males and females and condition index. This study provides the first data on the reproductive characteristics of G. nummaria , providing necessary information for the future sustainable management of its potential fishery.

Controlling factors of phytoplankton seasonal succession in oligotrophic Mali Ston Bay (south-eastern Adriatic)

Fine spatial and temporal phytoplankton variability in Mali Ston Bay has been observed for the first time based on physicochemical properties and small herbivorous zooplankton. Extensive year-through research was conducted during 2002 at Usko station which is traditionally an area of intensive shellfish farming. The Neretva River inflow, submarine springs ("vruljas") and precipitation are additional sources of nutrients in the bay. Temperature and salinity, combined with total inorganic nitrogen (TIN) were observed to be the most important environmental factors driving the succession of phytoplankton communities. Orthophosphate was a potential limiting factor for phytoplankton development. The nanophytoplankton abundances, as well as the microphytoplankton diatoms are controlled by herbivorous zooplankton grazing ('top-down' control) more than other groups of microphytoplankton. Nanophytoplankton dominated phytoplankton abundance and its most intensive development was recorded in winter and spring, while increase in microphytoplankton abundance occurred in spring and autumn. Diatoms dominated microphytoplankton abundance mostly in winter and autumn, while dinoflagellates dominated in spring and summer. Considering the number of taxa and abundance, dinoflagellates were the dominant microphytoplankton group during the year and were the main component of the spring blooms. At that time, in conditions of elevated temperature (>16 °C), decreased salinity (34-36) and increased concentrations of TIN, blooms of harmful dinoflagellate Prorocentrum minimum were recorded for the first time in the bay. The results showed a significant difference in environmental conditions, as well as in the annual phytoplankton succession and community structure, as compared with studies carried out more than 20 years ago in this area.

Diets of fan shells (Pinna nobilis) of different sizes: fatty acid profiling of digestive gland and adductor muscle

Pinna nobilis is a large bivalve, endemic to the Mediterranean Sea that was shown in a previous study to ingest significantly different proportions of different food items according to its shell height. Fatty acid profiling of total lipids was used to examine these differences in diet in relation to shell size. Small (shell height 23.0 ± 3.3 cm), medium (SH 41.5 ± 5.5 cm) and large (SH 62.7 ± 4.8 cm) P. nobilis were collected from Mali Ston Bay, Adriatic Sea, in March 2010, and fatty acid analysis was performed on digestive gland and adductor muscle tissues. Based on the analysis of the digestive gland, small P. nobilis were associated with a detrital food chain, characterized by saturated and branched-chain fatty acids, while the diets of medium and large individuals had a greater proportion of polyunsaturated fatty acids. This likely reflects the fact that smaller individuals feed within the benthic boundary layer where the detritus concentrations are high. Fatty acid incorporation into the adductor muscle, likely representative of food taken up over a longer time period, was reversed, that is, larger individuals had lower levels of polyunsaturated fatty acids. This most probably reflects energy expenditure, which typically increases with increasing organism size.

Growth And Longevity ofGlycymeris nummaria(Linnaeus, 1758) from the Eastern Adriatic, Croatia

Growth and longevity of Glycymeris nummaria were determined from samples of shells, collected monthly by scuba divers between January 2010 and December 2010 from Mali Ston Bay, eastern Adriatic Sea. Acetate peel replicas of the shell margin (marginal increment analysis) and hinge region of the shell were used to validate an annual periodicity of growth lines and to determine shell age, respectively. Results indicate that G. nummaria forms a narrow growth line after February, when seawater temperatures are at a minimum. Growth lines, visible in the hinge region of all the shells, were used to estimate the age of 76 G. nummaria and provided an estimate of maximum longevity of the species at 20 y. Construction of a von Bertalanffy curve using shell length at age data produced a value of L∞ = 71 mm, a growth parameter (K) of 0.11/y, and time at length 0 was -3.74 y. The growth performance index was 2.74.

The potential for aquaculture of the bearded horse mussel (Modiolus barbatus) and Noah’s Ark shell (Arca noae) in southern Croatia

To assess the potential of the bearded horse mussel (Modiolus barbatus) and Noah’s Ark shell (Arca noae) for aquaculture in southern Croatia, we analyzed their survival and growth rates under two experimental conditions—suspended culture and on-bottom culture. Furthermore, we investigated feeding on zooplankton by these two species in suspended culture conditions and compared them with previously published results of their feeding on zooplankton in the natural benthic environment. Experimental studies were conducted in Mali Ston Bay (Adriatic Sea) from December 2009 to December 2010. Differences were observed in terms of survival of tagged M. barbatus and A. noae—specimens of M. barbatus exhibited much better survival rates both in suspended culture and in on-bottom cages than did A. noae. Growth of A. noae under both culture conditions was essentially negligible, while M. barbatus grew several times faster in suspended culture than on the bottom. Values for the growth constant K and L inf for M. barbatus in suspended culture were 0.45 year−1 and 55.9 mm, while corresponding values for on-bottom culture were 0.09 year−1 and 58.6 mm, respectively. Zooplankters were present in the stomachs of 86.3 % of M. barbatus and 86.4 % of A. noae. Most abundant taxa were bivalve larvae followed by tintinnids, copepods, unidentified eggs and gastropod larvae. In conclusion, this study clearly shows that M. barbatus is a promising candidate for suspended aquaculture in southern Croatia.

Engineering properties of marine sediments in Mali Ston Bay (Croatia) based on “Mainland-Pelješac” bridge investigations

The complete geological characteristics of the wider area of the Peljesac peninsula are presented: lithostratigraphical, structural/tectonic and seismotectonic. The main accent is put on engineering geological characteristics of clastic sediments in Mali Ston Bay encountered during investigations for implementation project of the “Mainland-Peljesac” bridge. The bridge large proportions with 2404 m in length and with 14 pylons under the sea, requested thorough and expensive in-situ and laboratory investigations which enabled development of detailed and reliable engineering model of quaternary clastic sediments at the explored site. The results clearly indicate three different engineering units which are predominantly classified according to the obvious difference in consistency: Unit A – very soft and soft soil; Unit B – soft to firm soil and Unit C – firm soil. Because of great soil total thickness, locally over 100 m, its generally poor engineering properties and great seismicity of the area the foundation surroundings are unfavorable. The mentioned units which are classified according to its engineering properties also tightly correspond to the deposition environments which represent much wider area than just exploration site. Therefore authors presume presence of units A, B and C in wider area with similar geological situations like proximities of the Zrmanja, Krka, and Cetina deltas.

Reaction of the mussel Mytilus galloprovincialis (Bivalvia) to Eugymnanthea inquilina (Cnidaria) and Urastoma cyprinae (Turbellaria) concurrent infestation

In total 480 individuals of Mytilus galloprovincialis were sampled monthly from October 2009 to September 2010, at the shellfish farm in the Mali Ston Bay, south Adriatic Sea (Croatia) in order to assess the extent of pathology imposed by two parasites, Eugymnanthea inquilina (Cnidaria) and Urastoma cyprinae (Turbellaria). Although a deteriorating impact on host reproduction or condition index was lacking, we evidenced ultrastructural and functional alteration in host cells at the attachment site. Ultrastructural changes included hemocytic encapsulation of the turbellarian and cell desquamation in medusoid infestation. Caspase positive reaction inferred by immunohistochemistry (IHC) was triggered in cases of turbellarian infestation, in contrast with hydroids, suggesting that the former exhibits more complex host–parasite interaction, reflected in the persistent attempts of the parasite to survive bivalve reaction. We have evidenced that both organisms trigger specific host reaction that although not costly in terms of host reproductive cycle or growth, results in mild tissue destruction and hemocyte activation. A lower degree of tissue reaction was observed in cases of hydroid infestation, compared to turbellarian.

Differential ingestion of zooplankton by four species of bivalves (Mollusca) in the Mali Ston Bay, Croatia

This study provides information about differences in composition of ingested zooplankton amongst bivalve species coexisting in the same area in a period from May 2009 to December 2010. The study was conducted at the Mali Ston Bay (42°51′ N, 17°40′ E)—the most important bivalve aquaculture area in the eastern Adriatic Sea. Stomach content analysis was performed on cultured species—Ostrea edulis and Mytilus galloprovincialis, and commercially important bivalve species from their natural environment—Modiolus barbatus and Arca noae. Results confirmed carnivory in bivalves, both from natural and cultured populations, but cultured species had higher numbers of zooplankters than those living on the seabed. The most abundant taxa were bivalve larvae, followed by tintinnids, copepods, unidentified eggs and gastropod larvae. Recorded numbers of bivalve larvae in M. galloprovincialis stomach were the highest so far reported and show that mussels impact the availability of natural spat.

Seasonal fatty acid profile analysis to trace origin of food sources of four commercially important bivalves

Knowledge of bivalve food sources has great importance in aquaculture areas where cultured species usually occur at higher densities than in natural populations. Except phytoplankton, which is considered as their primary food sources, bivalves can use other food sources such as detritus, bacteria and zooplankton. The objective of this study was to determine food sources of four commercially important bivalve species co-existing in the Mali Ston Bay — most important aquaculture area of the eastern Adriatic Sea. Food sources were investigated by analyzing the fatty acid profiles of digestive gland and muscle tissues of the European flat oyster Ostrea edulis and the black mussel Mytilus galloprovincialis, cultured in the water column, and the bearded horse mussel Modiolus barbatus and Noah's ark shell Arca noae from natural populations. Digestive gland fatty acid analysis revealed that during the period of higher plankton abundance in spring and summer, all species ingested mainly phytoplankton followed by zooplankton and detritus. During the low plankton abundance of the autumn–winter period zooplankton and/or detritus became more important in a bivalve diet. Fatty acid composition of muscles indicated differences in selective incorporation of polyunsaturated fatty acid (PUFA) from their diets. While A. noae and M. barbatus accumulated significantly PUFAs during all and O. edulis during spring–summer period, in M. galloprovincialis this tendency was not observed at all. This suggested better utilization of ingested food in bottom-dwelling species. Because all four species consumed similar food sources they were in competition. However, as investigated species in Mali Ston Bay exist in high density, primary production plus the availability of other food sources in this area was probably sufficient to satisfy demands for all species. Aquaculture in the study area depends wholly on natural spat. This suggests that larviphagy, observed for all species, might constitute a self-regulating restriction to natural populations and/or commercial aquaculture growth.

Potential food sources of <em>Glycymeris nummaria</em> (Mollusca: Bivalvia) during the annual cycle indicated by fatty acid analysis

Seasonal changes of food sources were investigated by analysing the fatty acid profiles of digestive gland and muscle tissues of the naturally occurring clams Glycymeris nummaria in Mali Ston Bay, Croatia. Total lipids in the digestive gland and the adductor muscle showed parallel changes, with a maximum after the main spawning event in September. In the digestive gland saturated fatty acids were highly dominant (up to 82%), indicating detritus as the main food source for this species. This type of food prevailed during the autumn/winter period, in contrast to the spring/summer period when detritus was enriched with phyto- and, to a lesser extent, zooplankton. Fatty acid composition of muscles indicated highly efficient utilization of ingested food through significant retention of polyunsaturated fatty acid from the clams’ diet during the entire period investigated.

Growth of the fan musselPinna nobilis(Linnaeus, 1758) (Mollusca: Bivalvia) in experimental cages in the South Adriatic Sea

Juveniles of the fan mussel Pinna nobilis were collected in Mali Ston Bay from October to December 2006. Cages with juveniles were placed at three depths: 1, 3 and 5 m. At the end of a 2-year growth period at 1 m depths, the average length was 244.1 ± 22.9 mm, at 3 m depths, specimens averaged 244.0 ± 25.3 mm, and at 5 m depths, the average length was 231.1 ± 22.5 mm. The average monthly growth of shell length for the total experimental period was 8.7 ± 5.3, 8.4 ± 4.5 and 7.6 ± 4.4 mm, at 1, 3 and 5 m depths respectively.

Size-differential feeding in Pinna nobilis L. (Mollusca: Bivalvia): Exploitation of detritus, phytoplankton and zooplankton

The endangered fan shell Pinna nobilis is a large bivalve mollusc (<120 cm shell length) endemic to the Mediterranean that lives one-third buried in soft substrata, generally in shallow coastal waters. We hypothesised that P. nobilis of different sizes would ingest different food sources, because small fan shells will inhale material from closer to the substratum than do large fan shells. We studied stomach contents and faeces of 18 fan shells, 6 small (mean 23.0 cm length), 6 medium-sized (mean 41.5 cm length) and 6 large (mean 62.7 cm length) living in a small area of a low-energy coastal detritic bottom characterised by mud, sand and macroalgae at Mali Ston Bay, Croatia. We found that all P. nobilis ingested copious quantities of undetermined detritus (probably at least 95% of ingested material), phytoplankton, micro and mesozooplankton and pollen grains. Large P. nobilis stomach contents showed a preponderance of water column calanoid copepods, while small fan shells had higher numbers of bivalve larvae. All fan shells took in high numbers of harpacticoid copepods that are benthonic, feeding on microbial communities of detritus and benthic vegetation. There was also a significant selection of phytoplankton species, some apparently occurring between inhalation and ingestion. The stomach contents of small P. nobilis had a higher organic matter content than either medium-sized or large fan shells; this indicated that small fan shells ingested detritus of higher organic content than did larger P. nobilis. As the faeces of all P. nobilis had similar organic matter content, this also indicates higher assimilation efficiencies in small fan shells. The demonstration of differential dietary selectivity by different sized animals has implications for future trophic studies of this endangered species. This study also provides the first demonstration of predation on zooplankton by P. nobilis.

First Report ofCryptosporidiumsp. (Coccidia, Apicomplexa) Oocysts in the Black Mussel (Mytilus galloprovincialis) Reared in the Mali Ston Bay, Adriatic Sea

ABSTRACT Cryptosporidium spp. are obligate intracellular apicomplexan parasites that infect epithelial cells of the gastrointestinal systems of a wide range of vertebrate hosts, including humans. Its importance as a serious public health threat was recognized only since the HIV pandemic. Because of the particular filter feeding behavior of bivalves, these marine organisms are susceptible to the accumulation of Cryptosporidium spp. oocysts from the environment and their retention for a certain time, acting as potential zoonotic reservoirs. To preliminary evaluate the presence of Cryptosporidium spp. oocysts in cultured bivalves from the Mali Ston Bay, Adriatic Sea, we have analyzed individuals from a black mussel (Mytilus galloprovincialis) population by immunofluorescence, over a one year period at four different locations. Overall one-year prevalence of the Cryptosporidium spp. was 16.8%, and was correlated with the presumptive number of E. coli in the shellfish and seawater and abiotic factors (temperat...

Levels of Organochlorine Compounds in the Mediterranean Blue Mussel from the Adriatic Sea

The distribution of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) was investigated in Mediterranean blue mussel collected at four locations in Mali Ston Bay few times a year in 2005–2007. OCPs were found in all samples and levels ranged between 0.07 and 7.58 ng g−1 dry wt. Levels of PCBs ranged between 0 (below detection limit) and 21.55 ng g−1 dry wt. For most analyzed compounds there were no significant level changes between the 3 years. Exceptions are decreased levels of β-HCH, DDD, and PCB-138 and increased levels of γ-HCH and DDT in 2007. However, mussels from this area are applicable for human diet.

Risk assessment of parasitic/symbiotic organisms of the commercially important mytilidModiolus barbatus(Linnaeus, 1758)

A parasitological survey of the edible and commercially valuable mytilid horse-bearded mussel (Modiolus barbatus Linnaeus, 1758) was carried out in the Mali Ston Bay, Adriatic Sea, in 2004 and 2005 to assess the potential effects of parasites/symbionts on this bivalve population. Data sets of abundance and prevalence were used to evaluate the structure and dynamics of infracommunities and component parasitic/symbiotic communities, along with evaluated histopathological changes, to assess the overall risk that these organisms pose to the mytilid. Based on moderate diversity indices and expected seasonal oscillation between the values of prevalence/abundance, parasitic/symbiotic fauna of M. barbatus was shown to be relatively stable, although moderately impoverished assemblages, dominated by two species, turbellarian Paravortex cardii and gregarine Nematopsis sp. Histopathological changes indicated only three species as pathogenic –Gymnophallus sp. sporocysts, Nematopsis sp. and a haplosporidian, although their pathological risk was assessed to be very low or low. From the epizootiological point of view, parasites are not a limiting factor for the successful introduction and farming of the M. barbatus.

Spatiotemporal variation of some metal concentrations in oysters from the Mali Ston Bay, south-eastern Adriatic, Croatia - potential safety hazard aspect

The concentrations of cadmium, lead and zinc were determined in the soft tissue of oysters (&lt;i&gt;Ostrea edulis&lt;/i&gt; L.) from three locations in the Mali Ston Bay on the south-eastern coast of the Adriatic Sea. The sampling was performed during two periods of breeding cycle, in summer (age ~1 year) and winter (age ~20 months, consumption size). The concentrations of cadmium, lead and zinc measured in the oyster soft tissue showed considerable spatiotemporal variations that could be attributed to seasonal differences in the freshwater inflow which varied between the study locations. Median concentrations measured at different locations and in different time periods ranged from 0.303 to 0.969, 0.13 to 0.32 and 208.9 to 650.0 µg/g wet weight for cadmium, lead and zinc, respectively. Although metal concentrations do not exceed the maximal legally approved limits according to the legislation of the Republic of Croatia and EU, more knowledge of their spatiotemporal distribution – of cadmium in particular – would contribute to the sustainable future development of oyster farming in Croatian waters.

Impact of varying cultivation depths on growth rate and survival of the European flat oyster Ostrea Edulis, L.

European oysters, Ostrea edulis, L. were kept in plastic mesh cages at three depths at two commercial oyster farm sites within Mali Ston Bay, Croatia, between April 2004 and August 2005. Oysters from each cultivation depth were sampled at both sites at approximately 3-month intervals for growth parameters, mortality rate control and condition index calculation. The growth parameters of experimental groups from both sites only revealed a considerable distinction for shell length growth (P=0.011). Total weight gain and soft tissue weight gain were the highest for groups cultivated in the middle of the water column as compared with the top and bottom positions for both studied sites. Survival rates were very similar in all experimental groups, situated at different depths of both experimental sites, and differences in the mortality peak during the summer months were not significant. Condition indices fluctuated throughout the trial period and were the highest in spring and summer, with a reduction during the winter months and improvement again in the spring.

The reproductive cycle, condition index and biochemical composition of the horse-bearded mussel Modiolus barbatus

The horse-bearded mussel Modiolus barbatus (Linneus, 1758) is an important edible bivalve in the Adriatic Sea; its population is especially large in the Mali Ston Bay area, where the species is present at depths up to 8 m. In order to assess the sustainable exploitation rate for this species, as well as to estimate its potential capacity for a sustainable aquaculture production, we determined the species’ reproductive cycle along with its nutrient storage strategy, employing histological and biochemical methods. The population shows significantly more females than males, and no hermaphrodites. The smallest adult individual, an active male, was 16.0 mm in length, suggesting that sexual maturation starts around this length. While the period between January and February is characterized by sexual repose, early and late stages of gametogenesis were found between March and May, and spawning peaked from June till August. The increase of oocyte diameter followed the same trend. A significant positive correlation was observed between gonad index and temperature, and a negative correlation between gonad index and salinity. Oscillations of stored nutrients were tightly coupled with the gametogenic cycle.

Age, growth and population structure of Modiolus barbatus from the Adriatic

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 proportion of the population in Mali Ston Bay is 6 years) was determined from the middle nacreous lines in shell section, which formed annually in winter between February and March; the wider dark increments forming during summer (June to September). The oldest mussel, determined from the middle nacreous 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 distributions. 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. barbatus injected with a Calcein seawater solution (300 mg Calcein l−1), into their mantle cavity successfully deposited a fluorescent line, which was visible in suitably prepared shell sections under ultra violet light. Incorporation of Calcein into the mussel shells was seasonally variable with the lowest frequency of incorporation in mussels marked in February and recovered in May. Seasonal shell growth was observed with significantly higher growth rates in mussels marked in May and removed in August (ANCOVA, F3,149 = 23.11, P 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.

HISTOPATHOLOGY OF MODIOLUS BARBATUS

Infection of Gymnophallus sp. in the bearded horse mussel (Modiolus barbatus) was studied in samples collected during 1 year in the Mali Ston Bay (eastern Adriatic Sea). Mean prevalence of sporocyst was low (3.5%), which peaked in February (7.1%), whereas the mean metacercariae prevalence was 8.1%, peaking in September (16.7%). Histologically the Gymnophallus sp. sporocyst infection in bearded horse mussel resulted in clear reaction of bivalve tissue, where the induced changes depend strongly on the site of trematode infection. Retardation of gametogenesis, necrosis of connective tissue and hemocytic infiltration are the main histological features of infection in this new host.