Mating behavior of Labidocera aestiva (Copepoda: Calanoida)

The calanoid copepod Labidocera aestiva Wheeler exhibits a ritualized mating behavior. Sensory hairs and spines and integumental pores on the male and female are believed to play a regulatory role in the mating encounter. Scanning electron microscopy (SEM) revealed approximately 20 unusual pore structures (pit-pores) on the right ventrolateral surface of the female's abdominal segment, the area where the spermatophore coupling device is attached. Prior to spermatophore transfer, the male vigorously strokes this area with the modified endopod of his left fifth leg to prepare the pit-pore region for attachment of a new spermatophore. Transmission electron microscopy (TEM) indicated that the cells underlying each pit-pore may have a secretory function. We hypothesize that the pit-pores secrete a substance which dissolves the cement binding the coupling device to the abdomen and aids the female to remove a discharged spermatophore from her urosome.Contribution No. 116, Harbor Branch Foundation, Inc.     

Sources and concentrations of vascular plant material in sediments of Buzzards Bay,Massachusetts, USA

Samples of surface sediment from Buzzards Bay and creek sediment from Great Sippewissett Marsh were analyzed for lignin and stable carbon isotope composition in 1984. The lack of change in composition of lignins in detritus of Spartina alterniflora over two years of decomposition and similar aldehyde/acid ratios of lignin oxidation products of plant and sediment samples indicated minimal diagenesis of lignins in sediments. Remains of non-woody angiosperm tissues made up the bulk of the vascular plant debris in Great Sippewissett Marsh and Buzzards Bay sediments. These plant remains were evenly distributed over the sampling area in Buzzards Bay. Based on model calculations, salt marshes potentially contributed a significant fraction of the total amount of vascular plant debris in coastal marine sediments. The bulk of the organic matter in Buzzards Bay sediments, however, was derived from phytoplankton; vascular plant remains made up only 5 to 7% of the total amount of organic carbon in these sediments.     

Occurrence,viability and significance of resting eggs of the calanoid copepod Labidocera aestiva

Field and laboratory observations on the occurrence and viability of eggs of Labidocera aestiva Wheeler show that eggs of this temperate species which are laid in late fall remain viable on the bottom and hatch in late spring. These eggs thereby provide the means of repopulating temperate areas with this species after its winter disappearance from the plankton.     

Subitaneous and diapause eggs in Mediterranean populations ofPontella mediterranea (Copepoda: Calanoida): A morphological study

Transmission electron microscope studies on the morphology of subitaneous and diapause eggs in the neustonic copepodPontella mediterranea, collected in coastal waters of the Gulf of Naples from May to December 1986, revealed striking structural differences between the two egg types and the existence of a third egg type that was morphologically similar to a diapause egg but which hatched within 2 to 3 d. In smooth, subitaneous eggs the plasma membrane was surrounded by a thin outer chorion as compared to the thick and highly complex four-layer structure enveloping subitaneous eggs with short spines, and diapause eggs. This coat may be assembled just prior to or after deposition since the morphology of developing oocytes was similar in females laying either subitaneous or diapause eggs. The only other major structural difference between the two egg types was the presence of numerous, disc-like bodies in the ooplasm of diapause eggs. Development was arrested for at least 3 mo, prior to the initiation of the first cleavage, suggesting that these eggs were kept dormant by some endogenous and, possibly, structural mechanism preceeding the termination of diapause.Dedicated to B. Scotto di Carlo, teacher and friend, who died tragically at sea during the course of a cruise on December 15, 1988     

Female genital structures of Metridinidae (Copepoda: Calanoida)

The female genital structures of six calanoid copepod species, belonging to the genera Gaussia, Metridia and Pleuromamma, were studied using light and scanning electron microscopy (SEM). The copulatory pores and seminal receptacles are paired in Gaussia and Metridia, but unpaired in Pleuromamma. A thin epicuticle and a spermatophoral plug are the mechanisms by which the pores are closed before and after copulation. The pores open directly into the receptacles, which are reduced to shallow integumentary depressions in Gaussia. The mode of insemination suggests two matings in females of Gaussia and Metridia, but only one in Pleuromamma. Paired gonopores and egg-laying ducts are present in the three genera, with a characteristic, closed semicircular configuration; these are opened during egg-laying by the action of retractor muscles of the gonoporal plates. The seminal ducts, which open into the gonopores, are long, thin and paired in Gaussia and Metridia, whereas in Pleuromamma they are short, broad and unpaired. The opening of the seminal and egg-laying ducts is synchronized. The shell ducts are paired in Gaussia and Metridia, unpaired in Pleuromamma; these arise from glands situated in the lateral expansions of the last prosomite and lead into the distal part of each egg-laying duct in Gaussia and Metridia and of the egg-laying duct receiving the seminal duct in Pleuromamma. The position and anatomy of these structures are compared to those of other families and genera, and a functional interpretation of their morphology is proposed. The main evolutionary trends of the different structural patterns of female calanoid genitalia are presented in tabular form. Received: 30 December 1996 / Accepted: 11 February 1997     

Mating behavior of Centropages typicus (Copepoda: Calanoida)

Observations were made on approximately 300 mating couples of Centropages typicus Krøyer. Mating behavior consists of 4 distinct stages: pre-copulation, spermatophore extrusion, copulation, and post-copulation. Successful copulation requires proper positioning of the spermatophore complex on the female urosome and is dependent upon precise movements and orientation of the male during the mating encounter. Sub-optimal mating encounters, resulting in the improper placement of the spermatophore complex, were observed and adaptations tending to reduce spermatophore wastage are discussed.Contribution No. 62, Harbor Branch Foundation, Inc.     

Longevity of subitaneous and diapause eggs of Centropages hamatus (Copepoda: Calanoida) from the northern Gulf of Mexico

Copepod resting eggs are abundant in the seabed of many bays and estuaries where they provide a potential source of recruits for growth of planktonic populations. In the northeastern Gulf of Mexico the copepod Centropages hamatus (Lillejeborg) occurs in the water column only during the late fall, winter and early spring. The species produces subitaneous and diapause eggs, and both egg types have been found in the seabed. We determined the longevity of these two egg types to ascertain their potential for contributing to the growth of the planktonic population and for sustaining a persistent egg bank. Eggs were collected from females and incubated in the laboratory under temperature and oxygen conditions chosen to simulate field conditions. The diapause eggs were also exposed to sulfide. The total hatching success of subitaneous eggs in two experiments declined from highs of 78 and 97% to zero after 60 and 90 d of exposure to anoxia. The total hatching success of diapause eggs that were exposed to anoxia for 90 d however was typically greater than 80%. Some diapause eggs hatched after being incubated under anoxia for 437 d. Diapause eggs survived longer at ambient field temperatures when incubated under anoxia (437 d) compared to normoxia (118 d). Exposure to sulfide did not result in greater mortality of diapause eggs compared to anoxia alone. Diapause eggs that were incubated at ambient field temperatures did not hatch when exposed to normoxia until the temperature dropped to <20 °C. The results of this study suggest that C.␣hamatus sustain a short-term reserve of subitaneous eggs in the seabed that provides recruits for the current year's population. The greater longevity of diapause eggs suggests that they sustain the seasonal reappearance of the species year after year in the northeastern Gulf of Mexico. However, the contribution of diapause eggs of C. hamatus from the Gulf of Mexico to a persistent egg bank is questionable since hatching ceased after 437 d. Received: 30 July 1997 / Accepted: 18 January 1998     

Distribution and relative abundance of Pseudocalanus moultoni and P. newmani (Copepoda: Calanoida) on Georges Bank using molecular identification of sibling species

The sibling species, Pseudocalanus moultoni (Frost, 1989) and P. newmani (Frost, 1989), occur sympatrically on Georges Bank. Taxonomic discrimination of the species relies on subtle morphological characteristics, making routine identification of the species very difficult. DNA sequence variation of two mitochondrial genes, 16S rRNA and cytochrome oxidase I (COI), reliably discriminated P. moultoni and P.␣newmani. Levels of DNA sequence variation for both genes were consistent with those between species of calanoid copepods. A molecular systematic protocol (based on allele-specific PCR amplification) was designed from the COI sequences and used to discriminate females of the two species. The distributions and relative abundances of the two species were mapped for April 1996 based on samples of 15 to 30 females from 12 collections across Georges Bank. The results of this study indicated that P. moultoni females predominated along the northern flank of Georges Bank, while P. newmani females were common on the southern flank, deeper than the 60-m isobath. Received: 16 May 1997 / Accepted: 26 April 1998     

Mating behavior in three species of Pseudodiaptomus (Copepoda: Calanoida)

Quantitative studies of mating behavior between conspecific and heterospecific pairs of three closely related species in the genus Pseudodiaptomus reveal differences in the timing of life-history events, species-specific attraction of mates, distinctive tactile cues, multiple spermatophore attachments, and gametic incomptibility or hybrid inviability. These differences may function as reproductive isolating mechanisms among calanoid copepods. Differences in breeding behaviors and associated morphological features are greater between sympatric than between allopatric species, indicating that selection reinforces both behavioral and morphological divergences.     

Nutrition studies in the nauplius larva of Calanus pacificus (Copepoda: Calanoida)

Nauplii of Calanus pacificus were raised on a mixture of algae. Details of the mouth-parts, such as denticles, labial palps and lobes, setations and structure of the masticatory teeth were examined by scanning electron microscopy (SEM). Under the experimental conditions (15°C and 300 gC l^-1), exponential growth coefficients for the period Nauplii II–VI were 0.179 for carbon and 0.228 for nitrogen. C:N ratios dropping from 5.1 to 4.7. Growth was isochronous, each stage lasting 1.5 days. Respiratory losses were 15 to 19.6% of body carbon daily. Nauplii raised on a given alga showed higher rates of ingestion in the presence of this food, compared to nauplii switched to other algae. Minimal threshold concentrations for feeding were found, depending on the size of the food offered and ranging from 5.8 gC I^-1 for Lauderia borealis (28.7 m spherical diameter) to 47.1 gC 1^-1 for Chlamydomonas sp. (11.0 m). Unlike the Copepodite I stage, Nauplii II–VI larvae were not able to ingest small cells such as Isochrysis galbana (4.3 m), or very large ones such as Ditylum brightwellii (47.5 m) at more than maintenance rations. Below the critical concentration for maximal feeding, ingestion was clearly dependent on size of the cells offered, but the size-dependent relationship was different for diatoms and non-diatoms. Filtering rates increased from a threshold concentration to a maximal rate at about 50 gC 1^-1, and decreased at higher concentrations. Critical concentrations ranged from 125 gC 1^-1 for L. borealis to 1000 gC 1^-1 for Chlamydomonas sp. Maximal daily rations ranged between 100 and 150% of body carbon.     

Community interactions and zoogeography of the Indian Ocean Candaciidae (Copepoda: Calanoida)

Eighteen species of Candaciidae have been identified from collections made at 339 stations in the Indian Ocean. Most species are zonally distributed; however, on the eastern and western sides of the Indian Ocean, species ranges are extended north or south by boundary currents. Factor analysis was used to cluster phenotypically similar species based on 130 characters taken from the maxilla (a feeding appendage), the first swimming foot, and the fifth foot (a secondary sexual structure). Four morphological clusters were extracted. Clusterings based on separate factor analyses limited to characters from feeding or from sexually adapted appendages are in substantial agreement with the clusters based on composite morphology. Two geographically recurrent groups of species were also identified, one from equatorial waters, one from the central gyre. Each recurrent group is composed of species exceptionally different in body size and belonging to different morphological clusters. Niches of the species are compared using Hutchinson's multidimensional hypervolume as a model. An attempt is made to describe the niche of each species in terms of environmental variables measured at stations where the species was abundant. Environmental variables (dimensions) included in this study are space, time, temperature, food, oxygen and salinity. Mean niche adaptations of most of the species are separable from congeners along at least one niche dimension. It is proposed that community relationships among the Candaciidae developed to prevent both gamete loss and to avoid trophic competition. It is postulated that newly evolving species underwent contemporaneous displacement of both secondary sexual and trophic characters as a condition for sympatry, or diverged in their physiological adaptations to escape sympatric interference.Contribution No. 3686 of the Woods Hole Oceanographic Institution. This research was supported by NSF Grants GB 27405 and GA 43126 and is based on a doctoral dissertation submitted to the Curriculum in Marine Sciences of the University of North Carolina at Chapel Hill.     

Fecundity studies on Acartia tonsa (Copepoda: Calanoida) in standardized culture

Acartia tonsa Dana longevity and egg production data were studied over a 4-year period under standardized culture conditions. Egg-laying patterns and daily rates were evaluated as bioassay tools for measuring the nutritional value of various copepod foods, as well as assessing other aspects of environmental quality. Altogether, 337 females were observed in various aspects of this work. Experiments were performed with newly matured females in loosely covered crystallizing dishes. Fresh water and a standard concentration of algae were supplied daily, and a running daily tally was kept of the eggs produced. The effects of male copepods on egg production were analyzed. Males were either removed after the first eggs were observed or they remained with the female throughout her life span. Daily egg-laying rates of this latter group of females appeared to be more stable, but abnormal embryo development occurred if the males were not removed after fertilization. Likewise, the results of females which were not mated were studied. Life spans were shorter and egg production rates were lower than those of contemporary mated females. Unmated females were subjectively observed to be less active than their mated counterparts. The effects of continuous and temporary starvation on egg-laying were also observed. Under both normal and starved conditions, males appeared to have shorter life spans than either mated or unmated females. During starvation there were no real differences between the life spans or egg productions of mated or unmated females. Non-nutritional particulate matter was evaluated as a food source. It was ingested by the females, but did nothing to prolong the egg-laying period. Finally, the three algal components of the standard ration were evaluated. Females produced equally well with a mixed diet or with Thalassiosira pseudonana (100 m³) or Chroomonas salina (140 m³), the two larger species of this diet, but when Isochrysis galbana (40 m³) was fed, females reacted as if in a starved situation.     

Suspected mechano- and chemosensory structures of Temora longicornis (Copepoda: Calanoida)

The antennulary (A1) setae of Temora longicornis Müller are suspected of being mechanosensory. In the present study, the fine structure of the antennule, setae and pegs was examined by scanning and transmission electron microscopy. Two setae arise proximal to each antennule segment junction. The supraaxial (Type 1) seta of each pair is innervated by a pair of ciliary dendrites and has the morphological characteristics of a mechanoreceptor. The sub-axial (Type 2) seta may be chemosensory, and is innervated by a varying number of dendrites, but always more than two. The antennulary pegs do not appear to be innervated. T. longicornis is directionally sensitive to water disturbances, and this is consistent with the physical structure of the setae. Integumental structures on the body and other non-feeding appendages are described. These observations support previous behavioural experiments which suggested the antennule to be the major site for mechanoreception in copepods; however, mechanoreceptors must also occur on other parts of the body.     

Resting eggs of zooplankton (Copepoda and Cladocera) from the Kiel Bay and adjacent waters (southwestern Baltic)

Resting eggs of four species of calanoid copepods and three species of cladocerans were collected from sediments up to 5cm depth from the Kiel Bay and adjacent waters in the southwestern Baltic Sea during April–May 1994. All but one species of cladoceran was successfully hatched/reared in the laboratory. In the Kiel Bay, egg abundances varied from 1.8x10⁵ to 7.4x10⁵ m^-2. Hatching success of copepod eggs collected from all depths was high (49 to 94%), but was 0 to 79% for cladoceran eggs. Darkness did not seem to affect hatching. Eggs found in the 4 to 5 cm layer of sediment were estimated to be about 15 yr old, showing the presence of an egg bank in the Baltic. Formation of resting eggs may be a genetic trait acquired during the ice ages.     

Molecular systematics of six Calanus and three Metridia species (Calanoida: Copepoda)

The discrimination of species of the copepod genus, Calanus (Copepoda; Calanoida), is problematical-especially in regions of sympatry. Although the species of Calanus exhibit exceptional morphological similarity, they are quite distinct in genetic character. The DNA base sequences of the mitochondrial large subunit (16S) ribosomal RNA (rRNA) gene unambiguously discriminated C. finmarchicus (Gunnerus 1765), C. glacialis (Jaschnov 1955), C. marshallae (Frost 1974), C. helgolandicus (Claus 1863), C. pacificus (Brodsky 1948), C. sinicus (Brodsky 1965), and C. hyperboreus (Kroyer 1838). Sequence differences among Calanus species for this gene portion range from 7.3% (between C. glacialis and C. marshallae) to 23.9% (between C. glacialis and C. sinicus). Differences among conspecific individuals were approximately 1 to 2%. [These sequence data were determined between April and November 1993; the sequenced domain is similar to that published previously in Bucklin et al. (1992) but are derived from analysis of additional individuals.] Statistical analysis of the sequence data using a variety of tree-building algorithms separated the taxa into one group of species corresponding to the C. finmarchicus group (C. finmarchicus, C. marshallae, and C. glacialis) and another ungrouped set of species corresponding to the C. helgolandicus group (C. helgolandicus, C. pacificus, and C. sinicus). The C. helgolandicus group may be older than the C. finmarchicus group, making the tree topology less reliable in this area. Calanus hyperboreus was an outlier; Nannocalanus minor (Claus 1863) was the outgroup. Similar analysis of Metridia species confirmed that M. lucens (Boeck 1864) and M. pacifica (Brodsky 1948) are distinct species; M. longa (Lubbock 1854) was still more divergent. These sequence data will allow the design of simple, molecular tools for taxonomic identifications. Diagnostic characters, assayed by rapid molecular protocols, will enable biological oceanographers to answer important questions about the distribution and abundance of all life stages (as well as patterns of reproduction) of morphologically similar species, such as those of Calanus.     

Physiological responses ofCalanus finmarchicus andMetridia longa (Copepoda: Calanoida) during the winter-spring transition

The effect of pH on the biological availability of copper to the brine shrimp Artemia franciscana was studied with acclimated and non-acclimated individuals for the pH range 5.5 to 8.5. A chemical speciation model was used to calculate the speciation of copper in a chemically-defined saline solution as a function of pH. The lipid solubility of inorganic copper species was determined in hexadecane/saline and octanol/saline extraction systems. Copper is absorbed across the gut epithelium and accumulation is proportional to time over a 120 min experimental period. The biological availability of copper decreases with a decrease in the pH of the salt solution and a concomitant increase of the cupric ion concentration. Acclimation to the experimental pH has a marked effect on the uptake process, which depends on the buffer used. There is no extraction of copper in hexadecane, but a small amount goes in octanol. There is, however, no direct relation between the accumulation of copper in the shrimps and the extraction of copper in octanol. Multiple regression of the accumulation rates for non-acclimated animals on the calculated copper species concentrations shows that much of the variation in accumulation rate with pH is explained when copper hydroxide and/or copper carbonate species are considered to be the biologically available forms. Alternatively, the observations can be interpreted as the result of competitive binding of protons and copper species for carrier systems.     

Life cycles ofCalanoides acutus,Calanus simillimus andRhincalanus gigas (Copepoda: Calanoida) within the Scotia Sea

The life cycles and distribution of three dominant copepods,Calanoides acutus, Calanus simillimus andRhincalanus gigas were studied from the Discovery collections in the Scotia Sea earlier this century.C. simillimus is a sub-Antarctic species which mates in the top 250 m mainly in spring. The rapid development of the summer generation may allow a second mating period and a smaller second generation to appear in late summer.C. simillimus remains in the surface layers for a longer period thanCalanoides acutus orR. gigas, and its depth distribution is bimodal throughout the winter.R. gigas is most abundant in sub-Antarctic waters to the north of the Polar Front. It mates within the top 750 m later in spring, and development seems less synchronised than that of the other two species, with egg laying and the growth season being more protracted. Stages CIII and CIV are reached by the first autumn and further development resumes very early the following spring. It is not clear whether the majority then spawn or whether a further year may be needed to complete the life cycle. The predominantly Antarctic species,C. acutus mates below 750 m in middle to late winter and the summer generation develops rapidly to either CIV or CV. Its lifespan seems typically 1 yr, but some of the CVs which fail to moult and spawn in winter survive into their second summer, and their subsequent fate is uncertain. The cold-water speciesCalanus propinquus is comparatively rare in the Scotia Sea and aspects of its distribution and life cycle are briefly described for comparison. Regional variations in the timing of these events were apparent forC. simillimus and possiblyCalanoides acutus, but were not seen inR. gigas. Their geographic and vertical separation, together with their asynchronous life cycles support the concept of habitat-partitioning of these dominant herbivores.     

Importance of food quality in determining development and survival of Calanus pacificus (Copepoda: Calanoida)

Mixed zooplankton were collected in June and July of 1985 and 1986 from La Jolla Bay, California, USA, and experiments were conducted to determine how selected dinoflagellates affect development and survival of nauplius larvae of Calanus pacificus. We raised nauplii from eggs on nine species of dinoflagellates at concentrations generally >300 g C l^-1, and compared their development and survival to controls reared using the diatom Thalassiosira weissflogii or filtered seawater. Experiments were conducted for 6 d at 17°C. Development and survival rates of the nauplii fell clearly into one of two groups, depending upon the phytoplankton used as food. The first group was characterized by high development rate (0.46 to 0.84 stage d^-1), and by >27% of the original cohort surviving to at least Nauplius IV or V. The five species producing this result were Gymnodinium simplex, G. splendens, Exuviaella marie-lebourae, Gyrodinium dorsum, and T. weissflogii. The second group was characterized by a development rate similar to that in filtered seawater (0.21 to 0.34 stage d^-1), and by nauplii generally failing to molt past the first feeding stage (Nauplius III), often accompanied by high mortality. The five species producing this result were Gyrodinium resplendens, Ptychodiscus brevis, Glenodinium sp., Amphidinium carterae, and Gonyaulax grindleyi. Development rate and survival were not related to cell size or cell carbon, nor to shape or texture (thecate vs athecate dinoflagellates). Poor growth could be related to the absence of some important, but unidentified, nutritional factors. Alternatively, it could be caused by the presence of plant secondary metabolites which are deleterious to growth, a factor we suspect in P. brevis in particular. Prefeeding nauplii exposed to P. brevis lost neuromuscular control prior to becoming lethargic and dying; nutritional deficiencies may not explain these effects. Methods employed in this study provide useful bioassays for detecting chemical interactions between marine plants and animals. Lethal or sublethal effects of dinoflagellates on their most likely potential predators — copepods — may partially explain why they form significant blooms.