REPRODUCTIVE CYCLE AND FECUNDITY OF THE DATE MUSSEL LITHOPHAGA LITHOPHAGA (BIVALVIA: MYTILIDAE)

The reproductive cycle and fecundity of the date mussel Lithophagalithophaga, L. a well-known, ediblespecies has been examined.Sexes are separate and the mean number of eggs per each gonadis 1.894 x 10⁶ ± 1.044 x 10⁶ S.D. Reproduction firstoccurs at an age of 2⁺years and at a shell length greater than0.9 cm. Gonad activity is annual and is observed at all ages.The sex ratio for individuals up to 7 cm is 3:1 in favour ofthe males, whereas it becomes 1:1 for individuals greater than7 cm. The release of gametes by males and females occurs almostsimultaneously and begins immediately after a decline in thehighest water temperature ( 27°C), an increase in salinity(>31) and a decrease in the dissolved oxygen (6.5 ppm). Smallpercentages of mature individuals appear during the first wintermonths thus lengthening the reproductive period. This phenomenonis attributed to the temperature difference in deeper waters,the delay in gamete release by young individuals, tide, waveaction and changes in salinity. The fecundity of Lithophagalithophaga is high because its life-cycle is adversely affectedby environmental factors such as waves and tides. Fecundityincreases with shell length, more so with total wet weight andmainly with the age of the animals. Summer seems to be the suitableseason to exploit the date mussel of shell lengths > 5 cm. (Received 6 September 1993; accepted 10 March 1994)     

Population dynamics of the date mussel,Lithophaga lithophaga (L., 1758) (Bivalvia: Mytilidae), in the Evoikos Gulf (Greece)

Changes with time in aLithophaga lithophaga population, spatial distribution, size-frequency distribution and recruitment were studied. The population was homogenous, without statistically significant differences, down to a depth of 6 m, the depth to which sampling was carried out. Population density showed periodic variations over the year and changed as the gradient of the substratum increased. Dispersion indices indicated that individuals were distributed randomly in clumps during the period of recruitment at different depths. Juveniles, up to a length of 4 mm, formed the highest percentage of the population. Larvae settled at a length of >260μm, and individuals reached a length of 35 mm by the end of their first year. The appearance of newly settled individuals occurred two months after the maximum occurrence of larvae in the water. High percentages of settled spat were first observed in early winter at a depth of between 0–3 m although, later, higher percentages were found at 3–6 m depth.     

Seasonal species composition and distribution of barnacle larvae in Avacha Inlet (Kamchatka)

The annual dynamics of the numbers and distribution of barnaclelarvae in Avacha Inlet were studied. Planktonic cirripede larvaewere found in the area studied from late March at a temperatureof –1–0°C until October. The highest densityof 18 000 m^–3 was observed in mid-April. Balanus crenatusand Semibalanus cariosus were the dominant species, Chthamalusdalli and Balanus balanus larvae being less abundant. Naupliardensity was maximal at 0–5 m and decreased with depth.Maximum densities of B.crenatus larvae were observed in RakovayaBay, where there are many ships and a seawater-cooled powerstation. Most nauplii of the intertidal barnacle S.cariosus,which prefers wave-exposed open coast, were taken in the AvachaInlet narrows Settling on artificial substrata was most intensivefrom late May to early June     

GROWTH AND REPRODUCTION OF THE MINUTE LAND SNAIL PUNCTUM PYGMAEUM (DRAPARNAUD)

Reproductive characters and growth of Punctum pygmaeum wereinvestigated under laboratory conditions. Eggs were laid singlyat intervals of 2–8 days. Egg size averaged 0.41 by 0.50mm. Due to a variable period of egg retention, newly depositedeggs contained differently developed embryos resulting in anincubation period varying from 1 to 34 days. Each snail on averagelaid 6.0 eggs (range 1–16 eggs) during its life. Egg productionrate and hatching success in individuals kept in isolation werethe same as in individuals kept in pairs. Hatchlings measured0.49 mm in shell breadth. Snails grew throughout their life,but the growth rate declined significantly at a shell breadthof 1.3–1.4 mm, the point at which the onset of sexualmaturity occurred. Comparison of reproductive characters ofsnails living in leaf litter revealed that very small-sizedspecies produce few, but in relation to their shell size largeeggs, which they oviposit singly. ^*Present address: Zoological Institute, University of Basel,Rhein-sprung 9. CH-4051 Basel, Switzertand (Received 23 May 1988; accepted 11 July 1988)     

Vertical distribution of pelagic chaetognaths and feeding of Sagitta enflata in the Central Equatorial Pacific

The vertical distribution and feeding of pelagic chaetognathsat 5°S, 160°W in the Central Equatorial Pacific wereinvestigated using a series of 0–500 m vertical haulswith a VMPS net over a 24 h period between 6 and 7 October 1990.The total number of individuals per haul was between 370 and688. Fourteen species in four genera were found at this station.The most abundant species was Sagitta enflata which comprised32.4–61.1% of the individuals collected from the 0–500m layer. Mesopelagic species made up 9.3–15.1% of thetotal number of individuals. Sagitta enflata and Pterosagittadraco were found in the upper part of the thermocline both byday and at night. The fraction of the population containingfood items (FCF) of S.enflata in the 0–50 m layer variedbetween 4.8 and 12.5% (mean 10.8%) and feeding activity washighest between sunrise and noon. The percentages of Copepoda,Foraminifera, crustacean larvae, Chaetognatha, Pteropoda, Ostracoda,fish and unidentified material in the gut of S.enflata were51.9,6.7,3.8,2.9,1.9,1.9 and 30.9%, respectively. Sagitta enflataconsumed food organisms which were mainly between 0.5 and 1.0mm in length. The daily feeding rate of S.enflata was 1.81 preyper individual, which was equivalent to 8.06 mg C m^–2day^–1. This corresponded to     

ASPECTS OF THE LIFE CYCLE, POPULATION DYNAMICS,GROWTH AND SECONDARY PRODUCTION OF THE EDIBLE SNAIL HELIX LUCORUM LINNAEUS, 1758 (GASTROPODA, PULMONATA) IN GREECE

This paper examines the biology and ecology of Helix lucorumL. which lives in mainland Greece, as well as its growth andsecondary production. A demographic study revealed that (a)3 cohorts exist at any time during the year (when adults ofall generations belong to the same cohort) (b) egg-laying andhatching occur during the months of July and August respectively,(c) the most rapid growth takes place during spring. Study ofH. lucorum genitalia in relation to age showed that the snailsaxe sexually mature 3 years after hatching, when the largestdiameter of their shell (D) is equal to or greater than 35 mm Von Bertallanffy's method suggests that Helix lucorum may liveup to 14 years or more in order to reach its possible maximumsize (48.80 mm) The study of relative growth of D in relation to P_s (peristomesurface) of Helix lucorum shows that D grows faster than P_swhen D12.50 mm; juveniles change their growth rate when theirD arrives at 22.05 mm, and growth becomes slower when adultsarrive at 36.27 mm Annual secondary production calculated by the size frequencymethod gave a mean annual density of 3.39 individuals per m²,a mean annual crop (biomass) of 4.04 g-m^–2 and an annualproduction (P) of 5.02 g · m^–2. The annual turnoverratio (P//b) is equal to 1.24 (Received 23 June 1987;     

GROWTH OF JUVENILE BITHYNIA TENTACULATA (PROSOBRANCHIA, BITHYNIIDAE) UNDER DIFFERENT FOOD REGIMES: A LONG-TERM LABORATORY STUDY

The growth of juvenile Bithynia tentaculata (Proso-branchia,Bithyniidae) was measured under controlled laboratory conditionsover 18 months. The animals were fed with different concentrationsof suspended food (Chlamydomonas reinhardii at 2, 000 cells.ml^–1and at 10, 000 cells.ml^–1), solid food (lettuce) and combinationsof both (lettuce with 2, 000 cells.ml^–1 and with 10, 000cells.ml^–1 Chlamydomonas). The growth of all animals wasmeasured weekly, and after 1 years final shell sizes, shelldry weights and ash-free dry weights, as well as soft body dryweights and ash-free dry weights were determined. Survival rateof the animals increased from 20% when fed with 2, 000 algalcells.ml^–1 to 75% with lettuce and 10, 000 cells.ml^–1as food. Final shell sizes and shell weights were not influencedby food, but soft body dry weights were significantly reducedwhen animals were fed on suspended food only. Reproduction (i.e.egg laying) was observed at the end of the second summer whenlettuce (with or without algal addition) was given as food.The ecological consequences of these results are discussed. (Received 7 April 1993; accepted 9 August 1994)     

Laboratory and field observations on the scuticociliate Histiobalantium from the pelagic zone of Lake Constance, FRG

Histiobalantium sp. was found regularly in the pelagic zoneof Lake Constance, FRG, over five annual cycles. Maxima of upto 6400 cells l^–1 were recorded in late summer, with similarnumbers in the 0–8 and 8–20 m depth intervals. Onan annual average, the population accounted for 10–17%of the total biomass of planktonic ciliates. In the laboratory,Histiobalantium grew well on a diet of the cryptophyte Rhodomonassp. Maximum growth rates obtained in batch cultures were 0.21and 0.33 day^–11 at 9 and 18°C, respectively. In situexperiments using diffusion chambers yielded positive growthrates in autumn and winter. The highest values recorded at theambient temperatures 5, 14 and 17°C were 0.17, 0.32 and0.40 day^–1, respectively. Comparing these results withthe different seasonal distributions and higher measured growthrates of other ciliates from Lake Constance, we conclude thatHistiobalantium is a superior competitor at relatively low algalfood concentrations. ²Present address: Fisheries & Oceans Canada, 4160 MarineDrive, West Vancouver, BC, V7V 1N6, Canada     

AGE, GROWTH AND MATURATION OF THE SQUID ENOPLOTEUTHIS LEPTURA (OEGOPSIDA: ENOPLOTEUTHIDAE) FROM THE CENTRAL-EAST ATLANTIC

Fifty-nine specimens of the tropical epipelagic eno-ploteuthidEnoplotcuthis leptura were collected in the central-east Atlanticbetween 1986–1988. Statoliths were extracted from allspecimens (mantle length (ML) 4.1–92 mm) and processedunder the statolith ageing technique. The characteristic featureof statolith morphology in E. leptura is a sculpture of therostrum, which is covered by numerous tiny spines and knobs.In the ground statolith it was possible to distinguish fourmain growth zones consisting of narrow growth increments likethose in other squids studied. Allometric growth of statolithsversus ML is negative. E. leptura is a short-lived squid witha half-year life span. Growth rates of E. leptura are high atjuvenile stage (instantaneous rate of growth (G) of body weight(BW) 0.04–0.06). An early maturation of males (at age45–60 days) and females (at 80–90 days) causes asharp decrease of somatic growth of E. leptura, and mature squidhave low growth rates (G of BW - 0.OO3-O.0O5). Spawning takesplace between January and September with two peaks: in Januaryand in June-July. (Received 22 November 1992; accepted 15 February 1993)     

PREDATION ON THE BIVALVE CHOROMYTILUS MERIDIONALIS (KR.) BY THE GASTROPOD NATICA (TECTONATICA) TECTA ANTON

The effects of a population of the boring gastropod Natica tectaon the bivalve Choromytilus meridionalis were investigated atBailey's Cottage, False Bay, South Africa. In July 1979 theN. tecta density on the mussel bed averaged 69 m^–2 andthe population consisted mainly of reproductively mature individualsbetween 20–33 mm shell width. Laboratory experiments on N. tecta showed that prey size selectionis an increasing function of predator size. The prey size rangetaken by large N. tecta is also greater than that taken by smallindividuals. The position of the borehole on the mussel shellis a function of the way in which the shell is held by the footduring the boring process. Consumption rates measured in thelaboratory showed an increase from approximately 1 kJ per weekin 18 mm N. tecta to 4.5 kJ per week in 28 mm individuals. Populationconsumption in the field was calculated as 663 kJ m^–2month^–1. It was estimated that at this rate the standingcrop of mussels in the pool would be eliminated within 10 months.Field measurements showed significant depletion after 6 months. New spat settlement of mussels occur every 4–6 years.The growth curve shows that after one year the population meansize exceeds 30 mm shell length, which is beyond the prey selectionsize range of small N. tecta. It was concluded that at the timeof a new mussel settlement a niche is provided for the simultaneoussettlement and growth of juvenile N. tecta in high densities.However, within one year the increase in prey size, togetherwith depletion due to over-exploitation, limits population growthand density in N. tecta. (Received 14 March 1980;     

The role of faecal material in the particulate organic carbon flux in the northern Humboldt Current, Chile (23{degrees}S), before and during the 1997-1998 El Nino

Sedimentation rates of faecal material, phytoplankton and microzooplanktonand production rates of faecal material from crustaceans andpelagic tunicates were estimated during the austral summer andwinter 1997, and summer 1998, in the northern Humboldt Current(23°S, off Antofagasta, Chile). Sampling periods coveredpre-El Niño (January 1997) and El Niño 1997–98(July 1997 and January 1998). Samples were collected using floatingsediment traps deployed at 65, 100, 200 and 300 m depth in oceanicand coastal areas. Sedimentation rates during January 1997 were,on average, 152 ± 23 and 85 ± 57 mg C m^–2day^–1 at 65 and 300 m depth, respectively. During July,these rates averaged 93 ± 56 mg C m^–2 day^–1at 65 m depth and 35 ± 12 mg C m^–2 day^–1at 300 m depth, while in January 1998 they were 98 and 109 ±37 mg C m^–2 day^–1 at 65 and 200 m depth, respectively.Recognizable faecal material made up the bulk of the sedimentingmatter, accounting for 8 ± 5% (n = 14), 31 ± 26%(n = 16) and 8 ± 5% (n = 5) of the average total organiccarbon recorded from all sediment trap samples collected duringJanuary and July 1997 and January 1998, respectively. However,at300 m depth, the contribution of recognizable faecal materialto total sedimented organic carbon increased to 43 ±33% (n = 4) during July 1997. The remaining sedimenting particlesconsisted mainly of tintinnids, crustacean exuviae, heterotrophicdinoflagellates (both thecated and athecated) and diatom cells.During this study, we estimated that only a minor fraction (average± SD = 5 ± 8%) of the copepod faecal materialproduced within the photic zone sedimented down to 300 m depth,suggesting an efficient recycling within the overlaying watercolumn. On the other hand, an important fraction (47 ±30%) of the euphausiid faecal strings was collected in the 300m depth trap, suggesting that this material would enhance thedownward flux of particulate organic matter (POC). POC fluxesto 65 and 300 m depth traps were in the range of 4–20%and 3–8% of the estimated primary production during thewhole study period. It is postulated that the overall verticalflux of particulates and, in particular, faecal pellets wasdetermined by a combination of three factors. The first wasthe composition of the zooplankton assemblages in the studyarea. When the dominant group was calanoid copepods, their faecesseemed to contribute poorly to the vertical flux of particulates.On the other hand, when the dominant group was euphausiids,a significant proportion of their faecal material was collectedin the sediment trap located at 300 m depth. The second wasthe relatively high abundance of cyclopoid copepods from thegenera Oncaea, Corycaeus and Oithona, which are reported tofeed on aggregates of phytodetritus and faecal pellets producedby calanoid copepods, suggesting that they may act as a naturalfilter to sedimenting particulates. The third was the compositionand size spectrum of the phyto- and microzooplankton assemblageswhich are potential food sources for the meso- and macrozooplankton.These factors were partially modulated by both the 1997–1998El Niño and seasonality.     

SPAWNING SEASON AND EGG PRODUCTION IN FORTH ESTUARY RETUSA OBTUSA (MONTAGU) (GASTROPODA: OPISTHOBRANCHIA)

Retusa obtusa held individually in the laboratory carried largeoocytes in the gonad but did not spawn in December-late January.In late Jan-early Feb, however, the first few individuals producedbatches of 8–17 eggs and, by early February, most individualswere producing means of 2–4 eggs d^–1 and maximain late Feb of <7.9 d^–1. Spawning declined to low levelsin few individuals in late March. By early April, spawning hadvirtually ceased prior to the drastic loss of adults in thepopulation by May. Larger R. obtusa generally produced more eggs than smaller onesand the few specimens shorter than 3.8 mm produced markedlyfewer than those of 3.8–5.0 mm shell length. Twenty of the 34 R. obtusa held in the main breeding period(4 sets started 29 Jan-3 Mar) spawned in 3 or more successiveweeks whereas only 6 of them failed to spawn in 2 consecutiveweeks, so indicating that several successive batches of eggsare commonly produced. Yet in virtually all individuals (exceptat the start of breeding) more eggs were produced in the firstweek of captivity than subsequently. These first-week valuesimply total production by individuals in the spawning seasonof some 112-305 eggs comprising about 2.8-7.6 mg dry weightor, by the Forth estuary population, of 26880-73200 eggs m^–2comprising 0.67-1.83 g dry weight m^–2. (Received 19 August 1988; accepted 30 September 1988)     

Marine cladocerans in Toyama Bay, southern Japan Sea: seasonal occurrence and day-night vertical distributions

Marine cladocerans in the zooplankton samples collected vertically(0–500 m) with Norpac nets at 2–4 week intervalsfrom February 1990 through January 1991 at a station in ToyamaBay, southern Japan Sea, were examined. From mid-March throughearly December, cladoceran populations exhibited five distinctpeaks. in which seven species occurred and showed a species-specific pattern of seasonal distribution. In mid-July. Peniliaavirostris exhibited a prominent peak in population density.Three species of Evadne, E.nordmanni, E.spinifera and E.tergestina,showed a clear ecological separation in time. The first twospecies had a peak density in April and July. respectively,but the last species demonstrated three distinct peaks betweenJune and September. Two congeneric species of the genus Podon,P.leuckarti and P.schmackeri, also did not co-exist in time.Simultaneous horizontal hauls with MTD nets made in June andSeptember 1986 revealed a strikingly marked surface distributionof all species. Evadne nordmanni occurred in June and demonstrateda clear ‘reverse’ did vertical migration, residingin the 30 m depth during the night time and ascending up tothe uppermost water column of 0–10 m during the daytime.Among the four species which occurred in September, E.spinifera,E.tergestina and P.schmackeri also showed a reverse migrationbetween the 30 m depth during the night and the very surfaceduring the day. A different pattern was observed in P.avirostrzs,which was distributed at a depth of 30 m during both the dayand the night with no sign of diel vertical migration. Amongthe more prevalent parthenogenetic individuals, some gamogeneticforms were found to exist in most species occurring in ToyamaBay.     

Egg capsules, eggs and embryos of the southwestern Atlantic gastropod Coronium coronatum (Gastropoda: Muricidae)

The egg capsules, eggs and embryos of the muricid gastropodCoronium coronatum are described for the first time. Capsulesare sessile, bulliform, semi-circular, with a plug in the dorsalcenter. Sutures split the capsule into two asymmetrical halves.Recently laid capsules showed the presence of 3639 (n = 2) uncleavednurse eggs with a diameter of 180–210 µm (mean= 197.4 ± 8.9). The number of early embryos was 9–11.The size of the embryos was 320 x 320 to 820–880 µm.Nine pre-hatching embryos of 3.94 mm (n = 8, SD = 0.32)were found inside the older capsule. SEM illustrations of embryosand radulae are provided. Comparison of shell and radula ofembryos with the protoconch and radulae of adults of C. coronatumrevealed that the capsule belongs to this species. (Received 18 March 2006; accepted 10 October 2006)     

Annual pattern of micro- and mesozooplankton abundance and biomass in a subtropical estuary

The pattern of biomass and abundance of microzooplankton andmesozooplankton were studied over an annual cycle in the NuecesEstuary, Texas. Zooplankton samples and associated hydrographicdata were collected at four locations at biweekly intervalsfrom September 1987 through October 1988. This is a broad, shallowbay system with an average depth of 2.4 m. The concentrationof chlorophyll a in the surface waters averaged 7.4 µgl^–1with 85% passing through a 20 µ mesh. Microzooplankton(20–200 µ in length) were extremely abundant throughoutthis study. Abundances of ciliates (including both aloricateciliates and tintinnids) ranged from 5000 to 400 000 l^–,with a mean of 38 000 l^–1 of seawater over the entirecourse of the study. Mesozooplankton (200–2000 µmin length) abundance averaged 6100 m^–3 for samples collectedduring the day and 10 100 m^–3 for samples collected atnight. Mesozooplankton were dominated by Acartia tonsa whichmade up {small tilde}50% of the total. Biomass estimates formicrozooplankton (based on volume estimates) were often higherthan measured biomass of mesozooplankton. Given the shortergeneration times and higher metabolic rate of microzooplanktoncompared to mesozooplankton, microzooplankton should have agreater effect on the trophic dynamics of the Nueces Estuarythan mesozooplankton.     

RADULAR PRODUCTION RATES IN TWO SPECIES OF LACUNA TURTON (GASTROPODA: LITTORINIDAE)

The molluscan radula is a dynamic organ, both in terms of itsuse and production. New rows of teeth are constantly producedat the posterior end of the radula, while older, worn teethare shed anteriorly, producing a dynamic equilibrium. We useda cold-shock to mark the radular ribbon and measure tooth rowproduction rates in two gastropod species, Lacuna vincta (Montagu)and L. vanegata Carpenter. We found that the average tooth rowproduction rate at 10–11°C did not differ betweenthese two species, and was 2.94 (SE = 0.002) rows per day forLacuna vincta and 2.97 (SE = 0 002) for L. vanegata Inter-individualvariability in production rate was very low, and was correlatedwith shell length, smaller individuals had slightly higher productionrates. The total length of the radular nbbon varied greatlyamong individuals, ranging from 47 to 94 (2.57 to 5.68 mm) rowsin L vincta and 53 to 99 rows (2.80 to 7.14 mm) in L vanegata,and was only somewhat correlated with the length of the shelLThis great variability will result in large differences amongindividuals in the time it takes to replace the radula totally,from 14.96 to 35.44 days in L vincta and from 17 43 to 39 69days in L. vanegata. (Received 1 September 1995; accepted 20 November 1995)     

Winter-spring variability of size-fractioned autotrophic biomass in Concepcion Bay, Chile

The temporal variability of size-fractioned autotrophic biomassat three depth levels (1, 8 and 25 m) was studied during thewinter-spring transition at two oceanographic stations in ConcepciónBay. Size spectra were obtained on eight occasions by two differentmethods: (i) determining the biomass of seven autotrophic sizefractions by in vivo fluorescence; and (ii) measuring the filamentlength of chain-forming diatoms through direct microscopy. Aclear vertical gradient of biomass was found in all profiles,with maximum values in the surface layer (1 and 8 m levels).Values of chlorophyll were on average 6.2 (range 1.08–25.67)times higher at 1 m than at 25 m, and 7.4 (range 1.15–26.83)times more at 8 m than at 25 m. On a temporal basis, total biomassincreased from low average values in winter (2.5 mg chl-a m^–3)to high values in late spring (11.6 mg chl-a m^–3). Duringthe whole sampling period (June 8-November 19), the nano- andnet-plankton (1.8–40 µm and 40–335 µmsize fractions respectively) were more abundant near the surface(1 and 8 m depth) than close to the bottom (25 m depth); however,the picoplankton fraction (<1.8 (µm) showed an inverserelationship, with a slight trend to increase near the bottomtoward spring. The highest absolute biomass was concentratedin the net-plankton fraction during the whole period and therelative importance of the picoplankton decreased from winter(6.50 and 15.5% for shallow and bottom levels) to spring (1.5and 10.3% for shallow and bottom levels). This relative effectis caused by the higher absolute values of biomass observedin the net-plankton fraction toward spring. These changing patternsshould have an impact in the size-composition and abundanceof higher trophic levels, mainly through grazing, in particularby modifying food availability to microfJagellates, ciliatesand filter-feeding zooplankton.     

Summer vertical distribution of paralarval gonatid squids in the northeast Pacific

The vertical distribution patterns of paralarvae from severalabundant cephalopod taxa were examined from depth-stratifiedtows in the northeast Pacific (44–56°N, 145–165°W)during three summer surveys in 1999–2001. A total of 309cephalopods representing 10 taxa in three families were collected.Gonatid squids composed 97% of the total catch, and the mostnumerous taxa were Berryteuthis anonychus (59% of the totalcatch), Gonatus spp. (21%) and Gonatopsis borealis (17%). B.anonychus and Gonatus spp. were both most abundant in the upper20 m; catches of both taxa varied significantly with depth andwere significantly higher above the thermocline than in andbelow the thermocline. Gonatopsis borealis was collected mostlybetween 20 and 50 m, and catches were significantly higher inthe thermocline than above and below the thermocline. Paralarvaeof the three major taxa showed no evidence of diel verticalmigration. Mantle lengths of Gonatus spp. and G. borealis eachvaried significantly with depth, and Gonatus spp. showed a strongpositive correlation between mantle length and depth.     

Nitrate Effects on Pre-emergence Growth and Emergence Percentage of Wheat (Triticum aestivum L.) from Different Sowing Depths

The effects of a range of applied nitrate (NO₃^–) concentrations(0–20 mol m3) on germination and emergence percentageof Triticum aestivum L. cv. Otane were examined at 30, 60, 90and 120 mm sowing depths. Germination percentage was not affectedby either sowing depth or applied NO₃^– concentration whereasemergence percentage decreased with increased sowing depth regardlessof applied NO₃^– concentration. Nitrate did not affectemergence percentage at 30 mm sowing depth, but at 60 to 120mm depth, emergence percentage decreased sharply with an increasedapplied NO₃^– concentration of 0 to 1·0 mol m^–3then decreased only slightly with further increases in appliedNO₃^– of about 5·0 mol m^–3. Root and shoot growth, NO₃^– accumulation and nitrate reductaseactivity (NRA) of plants supplied with 0, 1·0 and 1·0mol m^–3 NO₃^– at a sowing depth of 60 mm were measuredprior to emergence. The coleoptile of all seedlings opened withinthe substrate. Prior to emergence from the substrate, shootextension growth was unaffected by additional NO₃^– butshoot fr. wt. and dry wt. were both greater at 1·0 and1·0 mol m^–3 NO₃^– than with zero NO₃^–.Root dry wt. was unaffected by NO₃^–. Nitrate concentrationand NRA in root and shoot were always low without NO₃^–.At 1·0 and 10 mol m3 NO₃^–, NO₃^– accumulatedin the root and shoot to concentrations substantially greaterthan that applied and caused the induction of NRA. Regardlessof the applied NO₃^– concentration, seedlings which failedto emerge still had substantial seed reserves one month afterplanting. Coleoptile length was substantially less for seedlingswhich did not emerge than for seedlings which emerged, but wasnot affected by NO₃^–. It is proposed that (a) decreasedemergence percentage with increased sowing depth was due tothe emergence of leaf I from the coleoptile within the substrateand (b) decreased emergence percentage with additional NO₃^–was due to the increased expansion of leaf 1 within the substrateresulting in greater folding and damage of the leaf. Key words: Triticum aestivwn L., nitrate, sowing depth, seedling growth, seedling emergence     

SIZE-ASSORTATIVE MATING IN A NATURAL POPULATION OF VIVIPARUS ATER (GASTROPODA: PROSOBRANCHIA) IN LAKE ZURICH, SWITZERLAND

The number of mating pairs, the size of the mating partners,and the distribution of individuals of Vivi-parus ater on agrid in Lake Zürich were recorded during one breeding seasonin 1990. There was positive assortative mating with respectto shell size. The proportion of copulating individuals rangedfrom 1% to 6% (average 3%) of the active population at any onetime. Individual snails copulated 60 times on average from Apriluntil November. Snails were abundant and copulated in shallowwater close to the shore in Spring. They moved towards deeperareas in Autumn. V. ater copulated on all substrates at anydepth (1–9 m)of the grid. The spatial distribution ofcopulations throughout the summer reflected the pattern of snailabundance. (Received 29 January 1993; accepted 14 November 1994)