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.     

Life cycle of Pseudocalanus acuspes Giesbrecht (Copepoda, Calanoida) in the Central Baltic Sea: I. Seasonal and spatial distribution

The seasonal and spatial distribution of Pseudocalanus acuspes in the Bornholm Basin (Central Baltic Sea) was studied on 16 cruises between March 2002 and May 2003 from stratified (10 m) multinet samples. The highest abundances were reached in May 2002 and April 2003 (618×10³ and 869×10³ ind. m⁻², respectively). Ontogenetic vertical distribution was stage specific with differences of mean annual weighted mean depth >30 m between nauplii and males; it followed closely the hydrography which was characterized by a permanent halocline and a summer thermocline. The vertical distribution showed a positive correlation with salinity especially in the older developmental stages; the relationship to temperature was negative in the nauplii and copepodite stage I (CI). Most of the stages performed a seasonal migration. The consequences of the vertical distribution patterns in relation to the effects of climate and predation are discussed. A stage shift from nauplii in April/May to CIV and CV as overwintering stages indicated slow seasonal development. However, nauplii were observed all the year round, and the resulting stage structure did not allow to distinguish generations. Changes in the prosome length of females seemed to be related to the advection of water masses with different temperatures rather than to different generations. It could not be clarified whether the strong increase of nauplii and adults after an inflow event of cold, saline North Sea water in the beginning of 2003 was a result of advection or improvement in habitat conditions.     

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.     

Simple molecular method to distinguish the identity of Calanus species (Copepoda: Calanoida) at any developmental stage

Diagnostic morphological characteristics of copepods of the genus Calanus are restricted largely to minor variations in secondary sex characteristics. This presents a persistent problem in the identification of individuals to species level, especially for immature stages. We have developed a simple molecular technique to distinguish between the North Atlantic Calanus species (C. helgolandicus, C. finmarchicus, C. glacialis and C. hyperboreus) at any life stage. Using the polymerase chain-reaction (PCR), the mitochodrial large subunit (16S) ribosomal RNA (rRNA) gene was amplified from individual copepods preserved in ethanol. Subsequent digestion of the amplified products with the restriction enzymes DdeI and VspI, followed by electrophoretic separation in 2% agarose (Metaphor, FMC Ltd), produced a characteristic pattern for each species. The versatility of the method is demonstrated by the unambiguous identification to species of any life stage, from egg to adult, and of individual body parts. Received: 11 May 1998 / Accepted: 5 August 1998     

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.     

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.     

Distribution and abundance of eggs of Labidocera aestiva (Copepoda: Calanoida) in the bottom sediments of Buzzards Bay,Massachusetts, USA

Grain-size composition of the sea-bed and density of eggs of Labidocera aestiva in bottom sediments in Buzzards Bay were determined at approximately monthly intervals from March 1983 through April 1984. The results of this study, together with those of Marcus (1984), show that during the fall and winter periods of 1982–1984 the proportion of eggs of L. aestiva occurring in the surficial sea-bottom sediments declined and the proportion of eggs in the deeper sediment layers increased. High positive correlations (r ²=0.72 and 0.92) were observed during the early fall 1983 between egg abundance and the proportion of the total sediments represented by the mud fraction. During late fall, winter, and early spring 1983–1984, the coefficients of determination were much lower. Physical criteria (e.g. sedimentation and transport characteristics) can be used to accurately predict the distribution and abundance of recently spawned eggs on the sea-bottom.     

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.     

Relationship between mouthpart structures and in situ feeding habits of species of the family Pontellidae (Copepoda: Calanoida)

Pontellid copepods were collected from the surface waters of a tidal front region in the Bungo Channel (the Inland Sea of Japan) in June 1986 to examine the relationship between the morphology of cephalic appendages and gut contents. In particular, two dominant species,Labidocera japonica Mori andPontellopsis yamadae Mori, were compared in detail. Large setae on the second maxillae ofL. japonica possessed two rows of setules at right angles to each seta along its inner margin except a terminal part which was serrated, whereas the inner margin of those setae ofP. yamadae was entirely serrated. Judging from the structure of the mouthparts, especially the second maxillae, the former species seems to employ both suspension and raptorial feeding modes, in contrast to the latter, which may use only the raptorial mode of feeding. InP. yamadae, the first maxilla and the maxilliped are also modified for carnivory. Gut content analysis supported the morphological evidence for feeding differences, and revealed thatP. yamadae is a carnivore preying mainly on copepodids whileL. japonica feeds omnivorously on copepod nauplii and phytoplankton particles. Since the mouthpart structures of congeners are quite similar to each other, the feeding behavior and habits might also be similar. Within the family Pontellidae, the generaAnomalocera, Calanopia, Epilabidocera, andPontella have mouthpart structures similar to those ofLabidocera, whereas the genusPontellina resemblesPontellopsis. Morphological similarities would suggest that the first group of genera employs both suspension and raptorial feeding modes, and thatPontellina is a carnivore likePontellopsis.     

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.     

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.     

Taxonomic value of minute structures on the genital segment ofPontellina females (Copepoda: Calanoida)

Scanning electron micrographs of the female genital segment ofPontellina spp. reveal fine-scale structures of systematic and phylogenetic interest. The structures are integumental organs and patches of spinules usually overlooked during routine examinations with a light microscope. Number and arrangement of these structures provide a set of character states and delineate a morphocline in accord with phylogenetic relationships hypothesized previously on the basis of other characters. A basic feature of patchiness shown by the spinules is of ontogenetic interest: arrangement of these patches appears to demarcate individual epidermal cells. The adaptive value of spinules and integumental organs is considered in the light of communicative requirements between partners engaged in courtship and copulation. The specimens used were collected on various expeditions in the warm-water belt of the world oceans and mostly studied during 1977.     

Two sibling species of Tisbe (Copepoda,Harpacticoida): Tisbe reluctans and T. persimilis n. sp. Research on their morphology and population dynamics

Two new forms of the copepod genus Tisbe, one of which was found in the Lagoon of Venice near Chioggia (Italy), and the other in the port of Malta, have been studied. Although showing a striking resemblance in morphology, all attempts to cross representatives of the 2 populations have failed. They may, therefore, be considered as sibling species. Even on closer study, no qualitative structural differences between them were found. However, careful analysis of some biometrical traits revealed significant differences. A certain differentiation of another kind emerges from the comparison of the life-cycle and population dynamics. Under standard laboratory conditions the following main parameters have been considered: (1) survival; (2) mean and minimum generation time, T; (3) net reproduction rate, R ₀; (4) intrinsic rate of natural increase, r _m. The significance of the findings is discussed.     

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.     

Effects of parental acclimation temperature and egg-incubation temperature on egg-hatching time in Acartia tonsa (Copepoda: Calanoida)

Experimental results indicate that both parental acclimation temperature and egg-incubation temperature have significant effects on the egg-hatching time of Acartia tonsa Dana between 15° and 25°C. Effects of parental acclimation temperature and egg-incubation temperature are additive if the long-term parental acclimation temperature is constant. A. tonsa are sensitive to temperature and changes of temperature during development (in vivo) and after they are laid (in vitro), up to the time of hatching. A temperature change of the parent culture for time periods of 86 h to 8 d is sufficient to change egg-hatching times. The time required for a temperature change to affect egg-hatching times depends on the magnitude and direction of the temperature change. Field collected A. tonsa demonstrate that changes in egg-hatching times occur in the field populations and can be predicted by mean weekly water temperature.     

Comparative ultrastructure of lipid storage sites in femaleEuchaeta marina andPleuromamma xiphias (Copepoda: Calanoida)

FemaleEuchaeta marina (Prestandrea, 1833) have one large, thin-walled lipid sac, whereas femalePleuromamma xiphias (Giesbrecht, 1889) have two separate and morphologically distinct lipid storage sites. One lipid site inP. xiphias corresponds to the mesenteric tissue that surrounds the anterior region of the midgut. The morphology of these cells resembles that of mammalian brown adipocytes. The cytoplasm is filled with extensive smooth endoplasmic reticulum, numerous mitochondria and several deposits of intracellular lipid. The second lipid site ofP. xiphias lies in the posterior region of the metasome and resembles the thin-walled lipid sac ofE. marina. Both lie adjacent to, but are not contiguous with, the narrow mesenteric tissue surrounding the last region of the midgut. Both sacs contain a single, large deposit of intracellular lipid enclosed by a very thin rim of cytoplasm and resemble mammalian white adipocytes. The different habitats and reproductive processes of these two copepod species may relate to the observed variations in lipid cell morphology. The reserve lipid inE. marina plays a primary role in reproduction and is linked closely with the continuous cycle of oocytic maturation. The lipids synthesized and stored byP. xiphias, a strong vertical migrator, may be influenced by food availability, a function of their mesopelagic habitat. The primary role of the reserve lipids in this copepod may be to provide energy during migrations and between feeding periods, with relatively less lipid being allocated to reproduction.     

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