Population dynamics and reproductive cycle of Pholoë minuta (Polychaeta: Sigalionidae) in Galway Bay

Pholoë minuta (Fabricius) is a prominent member of the Amphiura filiformis (O.F. Müller) subcommunity in Galway Bay, Ireland. The aim of the present investigation was to study the population dynamics and reproductive cycle of P. minuta in this area. Monthly sampling over a 2 yr period (November 1978–October 1980), at a permanent station (18 m) with a fine sand substrate, revealed a mean density of 96 individuals per 0.1 m². The width of the fifth elytron was correlated with total body length and was used as a basis for analysing population structure. Four distinct year classes were evident and estimates of recruitment levels and growth rates were calculated. P. minuta is polytelic and reproduces for the first time when approaching 3 yr of age. Spawning occurred in March—April and appeared to be associated with a seasonal low in bottom-water temperature.     

Demography and reproductive biology ofMelinna palmata (Ampharetidae: Polychaeta) in Inner Galway Bay on the west coast of Ireland

A high-density population of the polychaeteMelinna palmata Grube was the focus of a two-year study at a shallow-water location in Inner Galway Bay on the west coast of Ireland. Mean densities were highest in the second year of the study following successful recruitment. Size-frequency analysis of population structure (April 1983 to March 1985) revealed the presence of four cohorts, with the 1983 settlement becoming the dominant modal class. Size comparison of this cohort with the 1982 settlement showed that growth rates were lower and mortality higher in the 1983 settlement, which may be related to density dependent intra-specific competition.M. palmata at this location live for 2 to 2.5 yr with a small number surviving to 3 yr of age.M. palmata in Galway Bay is dioecious and has an even sex ratio. Reproductively mature worms were approximately 2 yr old when they first spawned. Spawning took place between May and July, when mature oocytes had a modal size of ca. 290µm. Proliferation of previtellogenic oocytes (ca. 10µm) from the gonads occurred following spawning. Growth over the winter was slow and was followed by a period of rapid growth between March and May, coincident with rising water temperatures. The presence of mature unspawned oocytes, which were resorbed after commencement of new proliferation, was taken as evidence of the species propensity for polytely. However, post-spawning mortality appeared to be high, so that only a small number of individuals survived to spawn the following year. The highest abundance of mature males was encountered between May and August, but lower levels of mature stages were present throughout the year.     

Field survey of the occurrence and significance of regeneration in Amphiura filiformis (Echinodermata: Ophiuroidea) from Galway Bay,west coast of Ireland

The Boreo-Mediterranean amphiurid Amphiura filiformis (O. F. Müller) occurs in high densities within Galway Bay. This burrowing species consistently shows signs of recent regeneration (of both arms and disc “cap”) at the above locality. Specimens were collected by SCUBA in March, April and May 1980, May 1981, and April 1982. Biomass assays have revealed that individuals with up to 50% (mean ca. 25%) of their entire body weight consisting of regenerated tissue are not uncommon. The occurrence of arm breakage and regrowth was found to be more frequent in the proximal (basal) and mid-arm regions than in the exposed arm tip portions. Roughly 3% of the population were found to be in the process of regenerating the disc “cap” or covering. Preliminary results from gut analyses of potential predators at the sampling station indicate that young flatfish (plaice and dab) actively “crop” the arms of A. filiformis. Records from north western Europe of this species occurring in the diets of fish and invertebrates are considered. In this long-lived species, the continual necessity to regenerate is deemed to be a major drain on bodily resources.     

Demography of Pholoe minuta (Polychaeta: Sigalionidae) in Galway Bay,west coast of Ireland,with special reference to settlement and recruitment patterns

Pholoe minuta is a numerically dominant member of the Amphiura filiformis subcommunity in Galway Bay. A longterm (5 yr) assessment of its population structure highlighted the shortfall in information regarding juvenile stages in the post-spawning months. Additional remote and in situ sampling programmes (including the use of a Muus sampling device), covering two successive postspawning periods, were undertaken to provide information on the juvenile recruits. Estimates of density levels (130 to 680 m^-2), growth rates (doubling in size in 2 mo) and mortalities (80%) were calculated for the newly settled P. minuta. Mean monthly densities, seasonal population structure, recruitment levels (21.1%), growth (1 mm per annum) and survivorship patterns of the adult population were evaluated using the dorsal width of the fifth chaetiger, which is shown to be a suitable parameter of total body size. The success of grab sampling as a method for examining early settlement of P. minuta is critically reviewed, and a recommendation for future benthic population studies is proposed.     

Population dynamics of Amphiura chiajei (Echinodermata: Ophiuroidea) in Killary Harbour,on the west coast of Ireland

The infaunal ophiuroid Amphiura chiajei Forbes is a dominant member of the A. chiajei community in Killary Harbour, a fjord-like inlet on the west coast of Ireland. High density populations (700 individuals/ m²) occur in sediments with a silt/clay content of 80 to 90% and organic carbon levels of 5 to 7%. A study of the population dynamics of this species was carried out from November 1985 to October 1988. At the outset, the population consisted of approximately equal numbers of adults and juveniles. By October 1988, the percentage of adults had increased to 95%, with little or no change in the mean annual dersities. However, there was some degree of variability between the mean monthly densities. Such variability may have been due to very low bottom temperatures, resulting in mortality among the older members of the population. Observations suggest that in Killary Harbour the species may attain an age of 10 yr, with an early annual growth rate of 0.5 mm (oral width). Annual recruitment was variable and low, due presumably to intraspecific competition with the adults; recruitment success may be very closely linked to adult mortality. The Killary Harbour findings are considered in the light of what is currently known of similar amphiurid populations in different geographical locations.     

The ichthyoneuston of Galway Bay (Ireland)

The ichthyofauna of the neustal (0 to 10 cm) and immediate subsurface water (12 to 48 cm) of Galway Bay was sampled during the day from June 1983 to September 1984 and at night from May to September 1984. Species diversity (H) and the number of species per sample were significantly higher at night. The density of larvae at night was higher only during May. Thirty-nine of the more common species were grouped by factor analysis resulting in 13 species groups with distinct spatial and temporal distributions. Three main time periods, February to April, May to July and July to September and three spatially defined station categories, each of which had distinct species groups, were also isolated by factor analysis. Station categories were contiguous and hydrographically and topographically distinct. Eight species groups showed evidence of vertical migration. Five groups were most abundant from July to September compared with three during May to June and one to two at other times. Eighty-one species or other taxomonic entity (genus or family) were recorded. The majority were pseudoneustonic and represented by early larvae and post-larvae. Euneustonic species (Ciliata mustela, Ciliata septentrionalis, Crenimugil labrosus, Gaidropsaurus mediterraneus, Gasterosteus aculeatus, Liza sp., Rhinonemus cimbrius, Scophthalmus maximus) were all present as late post-larvae or juveniles and all except Ciliata spp occurred during autumn. Facultatively neustonic species (Ammodytes marinus, Belone belone, Ctenolabrus rupestris, Entelurus aequoreus, Nerophis lumbriciformis, Sprattus sprattus, Trachinus vipera) were, except for Belone belone and Ctenolabrus rupestris, all represented by juveniles. All except Ammodytes marinus and Sprattus sprattus were common only in autumn.     

Burrow morphology,biometry, age and growth of piddocks (Mollusca: Bivalvia: Pholadidae) on the south coast of England

Biometry and growth of three piddock species Pholas dactylus, Barnea candida and B. parva, from chalk and clay substrata were investigated between 1999 and 2000 at five low shore locations along the south coast of England. Piddock burrow shape was significantly different (P<0.01) amongst the locations. Burrows at Lyme Regis showed the largest (height/maximum diameter) ratio (4.86±2.00) whilst those at Compton the lowest (3.73±1.62). Using the method of Bhattacharya, the population structure of P. dactylus, B. candida and B. parva was separated into eight, three and five modal size classes, respectively. Age and shell growth were determined from the number and spacing, respectively, of annual growth lines present in acetate peel replicas of shell sections. The von Bertalanffy growth (VBG) equation fitted the size at age data obtained for P. dactylus and B. candida (L and K were 79.3±13.8 mm and 0.0011±0.22 and 29.6±1.5 mm and 1.17±0.47, respectively) whilst the size at age data for B. parva were linearly related and did not fit the assumptions of the VBG equation. Male and female gonads were mature and piddocks competent to spawn between June and September 1999, with settlement of juveniles observed between November 1999 and February 2000. A significant relationship between burrow aperture diameter and age of the occupant piddock was established for the three species. Burrow morphology and spatial distribution of burrows were influenced by substratum hardness and population density. Based on estimates at Lyme Regis, piddocks are capable of removing up to 41% of the shore substratum to a depth of 85 mm over their lifespan (12 years), significantly compromising the structural stability of the soft rock shores they inhabit and contributing to bioerosion.Communicated by O. Kinne, Oldendorf/Luhe     

Population structure and growth of Donax trunculus (Bivalvia: Donacidae) in the western Mediterranean

A comparison of shell growth in Donax trunculus (collected between 1988 and 1990 of Cullera, Spain) has been carried out using an analysis of cohort progression in monthly length frequency distributions, hyaline surface shell growth rings and internal microgrowth bands. In the Mediterranean there are two periods of recruitment of D. trunculus, one in the summer (July to September) and the other in winter (December to February). Clams recruited to the population in winter display a clear cessation in shell growth during the following summer which may possibly be correlated with spawning, whereas individuals of the summer recruited cohort show no growth cessation the following summer and continue to deposit shell during this period. The normally opaque shell of D.trunculus reveals the presence of translucent hyaline growth rings when the shells are backlit by a strong light source, and these have been shown to be laid down in the shell during summer months. Formation of a hyaline ring is accompanied by a narrowing of the microgrowth patterns present in shell sections. Both the hyaline rings and the length frequency distributions have been used to determine the age and growth rate of D. trunculus.     

Population dynamics of Mytilicola intestinalis in Mytilus edulis in south west England

The population dynamics of Mytilicola intestinalis Steuer in mussels (Mytilus edulis L.) from the River Lynher, Cornwall, England, have been studied over 3 years. By transplanting uninfested mussels from the River Erme, South Devon, into the Lynher mussel bed, the study was extended to the growth and development of new infestations under natural conditions. Female Mytilicola intestinalis are shown to breed twice, and two generations of parasites coexist for most of the year, with recruitment taking place in summer and autumn. One generation contributes its first brood to the autumn recruits before overwintering and contributing its second brood to the following summer's recruits. The other generation overwinters as juvenile and immature stages to contribute its two broods successively to the summer and autumn recruits. Environmental temperatures are believed to control the rates of development at all stages rather than acting as triggers in the onset or cessation of breeding at specific times. There is no evidence to support the contention that heavily infested mussels are killed, and parasite mortality is shown to be density-independent.     

Population dynamics and secondary production in Melinna palmata (Polychaeta: Ampharetidae)

A Southampton Water (England) population of Melinna palmata Malmgren was studied from July 1978 to March 1980. M. palmata is a gregarious sedentary polychaete living in muddy substratum. The distribution of the polychaete in Southampton Water was related to the silt content of the sediment. The species occurs in sediments of <60% silt, it is dioecious, and the gametes are shed during a prolonged breeding season throughout the year, with peaks in March and July. Mature oocytes measure between 180 and 240 m in diameter at spawning, and fertilization is external. Unshed oocytes are resorbed. Annual secondary ary production was 0.42 gC m^–2yr^–1 and the P:B ratio was 2.19.     

Genetic evidence of the presence of two species of Crassostrea (Bivalvia: Ostreidae) on the coast of Brazil

 Although oysters are commercially very important in Brazil, there is still much dispute about the number of Crassostrea species occurring on the Brazilian coast. The dispute is centered around C. brasiliana, considered by some authors to be a junior synonym of C. rhizophorae. In this paper we compared, by allozyme electrophoresis, sympatric and allopatric populations of the two putative species. Of the 17 loci analysed, five were diagnostic for the two species in sympatry (gene identity = 0.46 to 0.47), clearly demonstrating that they are distinct biological species. Heterozygosity (h) levels were high for both species (h = 0.24 to 0.28), and no heterozygote deficiencies were observed in any population (local inbreeding, F _IS  = 0.141; P > 0.70). Levels of population structure in C. rhizophorae along 1300 km of coast were very low (population inbreeding, F _ST  = 0.026; P > 0.15), indicating that the planktonic, planktotrophic larvae of these species are capable of long-range dispersal. Received: 14 January 1999 / Accepted: 8 December 1999     

Sexual reproduction of the reef coral Montastreacavernosa (Scleractinia: Faviidae) in the Santa Marta area, Caribbean coast of Colombia

Sexual reproduction of the reef-building coral Montastreacavernosa was studied in the Santa Marta area, Caribbean coast of Colombia, from October 1990 to October 1991. The area is subjected to a seasonal upwelling–outwelling regime. From microscopical analysis of tissue sections sampled each lunar month around the full moon, it was confirmed that this is a gonochoric, broadcasting species, with a single gametogenic cycle per year, and a 1:1 sex ratio. Oogenesis began a few weeks after spawning, and extended for 11 mo. The onset of spermatogenesis occurred just before the full moon of June, later than at other localities where sea-water temperatures during the first half of the year are not as low. The amount of reproductive tissue strongly increased in both sexes after July, in association with an increase in sea-water temperature and a decrease in photoperiod. The gonad index of fully mature female colonies was about four times lower than at other localities, perhaps due to the stressful seasonal regime. The male gonad index was lower than that of the female, indicating sex-related differences in the rates of biomass allocation to reproduction. As in other Caribbean localities, spawning occurred after the full moons of August and September. However, there was some evidence of a third spawning episode after the full moon of October, possibly associated with a delay in the occurrence of maximum sea-water temperatures in near-equatorial localities compared to higher latitudes. The probability of cross-fertilization in this species with a gonochoric breeding system and a broadcasting mode of reproduction is favored by its balanced sex ratio, its usually high within-reef zone abundance, and by localized and repeated spawning episodes, synchronized by lunar phase. Received: 19 April 1996 / Accepted: 24 September 1996     

Annual variation in the growth rate of Yoldia notabilis (Bivalvia: Nuculanidae) in Otsuchi Bay,northeastern Japan,analyzed using shell microgrowth patterns

Annual variation in the growth rate of the protobranch bivalve Yoldia notabilis (collected in Otsuchi Bay, northeastern Japan between 1989 and 1991) was determined by measuring shell length at successive growth lines. Factors affecting growth rates were assessed by examining long-term environmental data. Shell cross-sections of Y. notabilis showed a clear pattern of internal growth lines which formed simultaneously with the annual external lines on the outer shell surface. Hence, they were used as an age marker. A maximum lifespan of 17 yr was determined for this species, and a sigmoidal growth curve was obtained by the internal growth line analysis. Examination of the growth lines also revealed a large annual variation in growth rate, fluctuating as much as 32-fold during the past 9 yr. The variation correlated negatively with water temperature and positively with chlorophyll a content in the water column. It is suggested that the variation in annual growth rate is dependent on food supply during the spring phytoplankton bloom which varies from year to year according to the flow of the cold Oyashio current each spring.     

Population structure of Brachidontes pharaonis (P. Fisher, 1870) (Bivalvia, Mytilidae) in the Mediterranean Sea, and evolution of a novel mtDNA polymorphism

Brachidontes pharaonis (Fisher P, 1870) is an Indo-Pacific mussel that has colonized the Mediterranean Sea via the Suez Canal. Mussels may have migrated by natural dispersal of pelagic larvae, or they may have been transported on the hulls of ships, or in ballast water, or by some combination of these. Mitochondrial COI sequences (618 bp) from 101 mussels from six localities in the central and eastern Mediterranean Sea and from one site in the Red Sea were used to describe population structure. Analysis of molecular variance indicated that frequencies differed among populations, and that 92% of the variation resided within populations. The majority of haplotypes were private alleles. No simple pattern of longitudinal variation was detected for haplotype frequencies, haplotype diversity, or nucleotide diversity. A variety of tree-building algorithms (NJ, MP, ME) gave consistent results, showing two sister groups. One clade had a leucine (L-form) and the other one a methionine (M-form) at the 30th amino acid. These sympatric forms were detected within all localities and they had similar frequencies in males and females. The M- and L-form were separated by a 7.3% sequence divergence, indicating that this is an ancient polymorphism. Closely related species of mussels had exclusively the L-form, so we infer that the M-form evolved from the L-form lineage of B. pharaonis, probably in the northern portion of the Red Sea. Neutrality tests detected an excess of rare mutations, a pattern of variation expected in expanding populations. Patterns of variation detected by Tajima’s D suggest that some form of balancing selection has maintained this polymorphism.     

Population dynamics of Aurelia aurita medusae in Kiel Bight,Germany (FRG)

Seasonal occurrence, individual growth and development of population biomass of the scyphomedusa Aurelia aurita were studied in 1978–1979 in the western Baltic Sea. First ephyrae of the developing population appear in November, but the great majority are produced in April and May. On the average 0.09 medusae m^-3 with a biomass of 25 g occur between July and September. The standing stock is below 10 mg from December to April. Growth is characterized by a stagnation period in winter and early spring, by rapid increase in early summer and by size reduction in autumn after release of gonadal products. The maximum monthly growth rate was observed in June, when the average diameter increased from 7 to 48 mm and the wet weight from 0.06 to 22.8 g. Mortality is low before maturation. Genetical determination, starvation and parasitization are discussed as causes for morphological reductions and finally for death in autumn.     

Physiological energetics of cultivated mussel (Mytilus edulis) populations in two Scottish west coast sea lochs

Physiological responses of suspended cultured mussels, Mytilus edulis L., in two Scottish sea lochs (Lochs Etive and Leven) were investigated in their native and transplanted environments, after 15 d, 4.5 mo and 1 yr acclimatization, during the main growing season of May to September 1992. These measurements were integrated by means of the balanced energy equation, and scope for growth was calculated to assess the performance of each stock. Transplanted mussels showed clear signs of stress during the first 15 d after transfer, with low clearance rates and energy retention and high rates of respiration and nitrogen excretion. There were significant differences in some of the physiological responses (clearance, respiration and excretion rates), scope for growth and growth efficiency between the native populations, with the responses of mussels in Loch Etive being more favourable than those in Loch Leven. With newly transplanted mussels after 15 d acclimatization, almost all these measurements also differed significantly from native mussels in their host site and the original stocks but, with the exception of ammonia excretion rates, all the variables of cross-transplanted mussels after 4.5 mo acclimatization were the same as those of the native stock at the host site. Good agreement between observed long-term growth rates and estimated scope for growth suggests that, like growth rate, differences in physiological responses are mainly controlled by environmental factors; i.e., stock or origin had no significant influence on variations in physiological response (except ammonia excretion). The results further indicate that scope for growth estimated during the main growing season can be used to assess the actual growth rate and to compare sites for on-growing.     

Physico-chemical characteristics of the peloid mud from Morinje Bay (eastern Adriatic coast, Croatia): suitability for use in balneotherapy

The study conducted in the Morinje Bay (Central Dalmatia, Croatia) included the investigation of the sedimentological, physico-chemical and geochemical properties of the Morinje Bay sediments in order to assess the suitability of the material for wellness-related purposes including medicinal use. The sedimentological characteristics of the Morinje mud are in accordance with the geological origin of the material; the sediment is fine grained, composed mostly of carbonates, quartz and clay minerals. The composition of the sediment determines the physico-chemical properties of the mud. The high content of carbonate minerals diminishes adsorbing the characteristics of the mud to some extent; however, they are still comparable to some commercial mud already successfully used for medicinal purposes. Geochemical analyses have shown that trace metals concentrations in the Morinje Bay samples were mostly in the range with similar material from unpolluted Adriatic environments. Only the concentrations of Cu, Pb, Zn and As were slightly higher than in the reference material, but still within acceptable limits.     

Interannual, seasonal, local and body size variations in reproduction of the prawn Penaeus (Marsupenaeus) japonicus (Crustacea: Decapoda: Penaeidae) in the Ariake Sea and Tachibana Bay, Japan

 The reproductive biology of female Penaeus japonicus Bate was investigated in the Ariake Sea and Tachibana Bay (located outside the Ariake Sea), Japan from 1994 to 1996. Interannual, seasonal, individual female body size and spatial influences on the incidence of spawning were examined. The proportion of inseminated females, increased with increasing body size up to 170 mm body length (BL) and decreased thereafter. The minimum size at maturity was similar between years; 130 to 140 mm BL. The minimum size of individuals with developing ovaries or spermatophores differed from that of ripe females by 20 to 25 mm. Spawning occurred in the central part of the Ariake Sea and Tachibana Bay but rarely in the inner part. P. japonicus had a clear reproductive cycle. Spawning started earlier and ended later, occurring from mid-May to mid-October, in the eastern central part of and outside of the Ariake Sea compared with the western central part, where spawning occurred from mid-June to mid-September. Spawning dynamics differed across seasons, body sizes and areas but not across years. The seasonal peak in the proportion of ripe females varied with body size. In small individuals (130 to 169 mm BL), no peak was observed, whereas in large individuals (>170 mm BL) the proportion of ripe individuals peaked in June. The proportion of ripe individuals increased with increasing body size and was high outside the Ariake Sea. Factors causing the variation in spawning dynamics are discussed. Received: 30 November 1998 / Accepted: 12 October 1999     

Co-ordinated rhythms of digestion,absorption and excretion in Mytilus edulis (Bivalvia: Mollusca)

Sequential alternation of extracellular digestion in the stomach and intracellular digestion in the diverticula appears widespread among bivalves. The present study documents some physiological consequences of such processes in Mytilus edulis L. collected during 1981 from Whitsand Bay, Cornwall, England. Pronounced temporal fluctuations in faecal deposition are described that relate, in terms of amplitude and period, to both sinusoidal rhythmicity established for ammonia excretion and changes in the morphology of digestive tubules. Although at least partially synchronised among replicate groups of mussels, these cycles bore no consistent relationship with exogenous influences. Hourly fluctuation in the net absorption efficiency for nitrogen, as evidenced by the mean percentage ±2 SE, measured over 24 h sampling periods, was considerable (16.0±53.7, 49.3±10.9 and 52.8±6.6 for mussels acclimated in March, June and October, respectively). This variation in absorption derived from an inverse relationship between the percentage nitrogen within faeces and the rate of faecal egestion. Accordingly, peaks of faecal deposition presumably represented the pulsed remnants of intracellular digestion. Co-ordinated rhythms of digestion, absorption and excretion were thus evident in M. edulis. These processes displayed seasonally dependent periodicities of approximately 8, 3 and 4 h in March, June and October, respectively. It was concluded that, at least for M. edulis, this previously unquantified rhythmicity of physiological processes warrants careful consideration during assays commonly undertaken in the complication of nutrient and energy budgets.