Differences in the parasitic effects of a bopyrid isopod and rhizocephalan barnacle on the portunid crab,Charybdis bimaculata

Bopyrid isopods and rhizocephalan barnacles are obligate parasite crustaceans which harm their decapod hosts. However, to the best of our knowledge, studies have not compared which of these parasites has a greater parasitic effect on its hosts. Here, the parasitic effect of the bopyrid isopod, Allokepon hendersoni, and an unidentified sacculinid rhizocephalan species, infesting the same population of portunid crabs, Charybdis bimaculata, was investigated and compared for the first time. Samples were collected from the bycatch of a trawl fishery in Tosa Bay, Japan. A total of 2601 crabs were collected, of which 14 (0.55%) were parasitized by the bopyrid and 21 (0.82%) by the rhizocephalan. One of the two female crabs parasitized by the bopyrid was ovigerous (with much fewer eggs than unparasitized females). No ovigerous crab was found from the eight females parasitized by the rhizocephalan. Because only two female crabs were parasitized by the bopyrid, the following analyses were made using the male crabs. Both parasites reduced the wet weight (crab condition) and the cheliped size (secondary growth) of C. bimaculata, but the impact of the parasitism did not differ between the parasite species. The size of the abdominal flap of male hosts was reduced by the bopyrid infestation; however, rhizocephalan infestation caused enlargement of the abdominal flap, which is an indication of feminization. The present study provides information on how the effect of these two parasitic castrators on the same host crab varies. A moderate decrease in crab condition and cheliped development was common among the parasites, suggesting that the degree of impact might be favorable for the survival of the two parasites species.     

Distribution and abundance of a nemertean egg predator ( Carcinonemertes sp.) on a leucosiid crab,Randallia ornata

The purple globe crab, Randallia ornata (Randall, 1839) (Decapoda: Leucosiidae) is a common crustacean found on sandy beaches from Northern California to Baja California, Mexico. An undescribed species of Carcinonemertes, which shares biological features with a partially described worm from Tasmania was recovered from R. ornata. The worm reaches 10 mm in length and secretes a sturdy tube with a distinctive spiral shape. The total prevalence of the worm on R. ornatawas 70% (85% for ovigerous female crabs, 67% for post-ovigerous female crabs, and 45% for male crabs). Intensities reached 32 worms per crab. Adult worms were found in crab egg masses, regressed adults were recovered from under the abdomen of post-ovigerous females, and encysted juveniles were found on the gill lamellae of non-ovigerous female crabs and occasionally from the gill lamellae of male crabs.     

Population Biology and Breeding Period of the Sand-Bubbler Crab <Emphasis Type="Italic">Dotilla Fenestrata</Emphasis> (Brachyura: Ocypodidae) from Southern Mozambique

The sand-bubbler crab Dotilla fenestrata (Hilgendorf, 1869) is the most common and abundant brachyuran in the intertidal area of many East African sandy shores. Monthly sampling surveys were performed from March 2001 to February 2002 at Praia do Triunfo, Southern Mozambique. At each sampling survey, 15 stations (0.25 m²) were sampled to a depth sediment of 30 cm. Crabs were identified, sexed, and measured for carapace width (mm). A total of 2456 crabs were collected of which 1247 were males (mean ± sd: 5.36 ± 0.04 mm), 938 non-ovigerous females (5.03 ± 0.03 mm) and 271 ovigerous females (5.15 ± 0.06 mm). The overall size frequency distribution showed a unimodal pattern suggesting a stable population with continuous recruitment. A clear sexual dimorphism was observed, with males being larger than ovigerous females, which in turn, were larger than non-ovigerous females. The overall sex ratio did not differ from 1:1. The annual reproductive cycle of Dotilla fenestrata was continuous and ovigerous females were recorded throughout the year, with peaks of occurrence during the warmer months (February–August). Despite of this continuous cycle, few juveniles were observed during the study period. These observations and the maximum size attained by adults supports the hypothesis that for this burrowing crab, recruitment may occur in different habitat than that occupied by adults. However, the studied area is favourable for growth, since the analysed population attains sexual maturity in smaller sizes than that from close areas.     

Parasitism of the rhizocephalan <Emphasis Type="Italic">Briarosaccus callosus</Emphasis> on the crab <Emphasis Type="Italic">Paralomis verrilli</Emphasis> (Crustacea: Lithodidae) in the waters of southeast Sakhalin

The crab Paralomis verrilli in the waters of southeast Sakhalin is infected by the parasitic rhizocephalan barnacle Briarosaccus callosus. The prevalence of parasitic infection was on the average 4.36% (6.29% for females, 3.28% for males) and varied between samples from none up to 14.9%. The degree of prevalence was not related to the average carapace width and the sex ratio of crabs in samples. Sterilization of female P. verrilli was caused by the B. callosus infestation or its consequences. No more than two parasite externae per crab were found. Crabs with two externae made up 5.6% of all infected specimens. The infestation of crab hosts with two B. callosus externae negatively influenced the growth of the externae. A positive relationship was found between the width of the crab carapace and the length of the parasitic externae. The survival rate of P. verrilli with either one or two parasite B. callosus externae did not differ substantially. The proportion of crabs with externae and those with “scars” (12.2% in our case) can be taken as the index of survival of the parasitized crabs.     

Locomotor activity pattern in pairs of the subsocial desert isopod Hemilepistus reaumurii

Locomotor activity rhythm of fresh adult of H. reaumurii was recorded in pairs with ovigerous and with non-ovigerous females. According to double-plotted actograms and waveform curves, results showed the presence of different locomotor patterns. In fact, locomotor profiles of pairs with ovigerous females were mainly bimodal whatever the recording conditions, whereas locomotor patterns were trimodal in pairs with non-ovigerous females. Results showed also that locomotor activity rhythm was more stable in the presence of the synchronizer (nLD cycle) than in constant darkness whatever the recording type. Moreover, the most important stability of the locomotor rhythm was observed under constant darkness and individuals were more active in pairs with ovigerous females than in pairs with non-ovigerous females.     

Prevalence and mean intensity of infestation by Carcinonemertes carcinophila imminuta (Nemertea: Carcinonemertidae) in the gills of Callinectes danae and Callinectes ornatus (Decapoda: Portunidae) from São Sebastião,Brazil

The nemerteans of the genus Carcinonemertes live in association with decapod crustaceans. Juveniles of Carcinonemertes carcinophila imminuta Humes, 1942 are found in the gills of their hosts. A total of 281 specimens (52 males and 229 females) of Callinectes danae Smith, 1869 and 373 (332 males and 41 females) of Callinectes ornatus Ordway, 1863 were inspected for the occurrence of Carcinonemertes carcinophila imminuta in their gills. The prevalence and mean intensity of infestation by these nemerteans with regard to sex, maturity, condition of adult female crabs (ovigerous or non ovigerous), size (carapace width) and molt stage of hosts were evaluated. A prevalence of infestation by Carcinonemertes carcinophila imminutaof 39.1% was found in the population of Callinectes danae; the mean intensity of infestation was 12.0±2.7, while intensity ranged from 1 to 268. In this host species, significant differences in prevalence were observed between males and females, juveniles and adults and ovigerous and non-ovigerous adult females. Prevalence of infestation did not differ significantly among crabs in different molt stages. Mean intensity did not differ significantly between males and females nor among crabs in different molt stages. Significant differences in mean intensity were observed between ovigerous and non-ovigerous adult females of Callinectes danae. A positive correlation was observed between prevalence of infestation and the carapace width of Callinectes danae females. The prevalence of infestation in Callinectes ornatus was 8.0%; the mean intensity of infestation was 2.7±0.4 while intensity ranged from 1 to 9. Prevalence and mean intensity of infestation did not differ significantly between males and females and juveniles and adults. Prevalence did not differ significantly among crabs in different molt stages. No correlation was observed between the prevalence of infestation and the carapace width of males and females of Callinectes ornatus.     

Reproductive Stage of the Life Cycle in the Rhizocephalan Barnacle Polyascus polygenea (Crustacea: Cirripedia)

The specific features of the reproductive stage of the life cycle have been studied in the rhizocephalan barnacle Polyascus polygenea, a parasite of the coastal crab Hemigrapsus sanguineus. It is shown that a single crab can bear 1 to 8 externae of P. polygenea. The fecundity of the parasite depends on the size of the externae and their number on the host and may reach as much as 50000 eggs for one externa. In Peter the Great Bay, Sea of Japan, this species repeatedly reproduces during the entire spring–autumn period; externae with developing embryos in the mantle cavity occur from May to September, and planktonic larvae occur from June to October. One externa produces during the season of reproduction no less than three generations of larvae. Thus, the reproductive strategy in P. polygenea comprises a three-stage cascade of reproduction: asexual reproduction via budding of the interna; the development of several generations of one or several externae; and several reproduction cycles of each externa. This allows the parasite to produce a very great number of larvae and ensures the parasitization of a significant proportion of the host crab population. The structure of the ovaries and oogenesis in rhizocephalans and free-living cirripede barnacles have many common features, which provides evidence for integration of these two groups within one monophyletic taxon.     

Effects of three organophosphorus pesticides on population growth and sexual reproduction of rotifer Brachionus calyciflorus Pallas

Effects of organophosphorus pesticides including dichlorvos, triazophos and chlorpyrifos on population growth and sexual reproduction of freshwater rotifer Brachionus calyciflorus were studied by 3-d population growth and 4-d resting egg production tests. The results showed that all the three organophosphorus pesticides influenced significantly the population growth rate, the ratio ovigerous females/non-ovigerous females in the rotifer populations and the resting egg production of the rotifers. Both dichlorvos and chlorpyrifos influenced markedly the mictic rate of the rotifers, but triazophos did not. Compared to the controls, both dichlorvos at 10.0–1000.0 μg/L and chlorpyrifos at 0.01–100.0 μg/L increased the population growth rate, but the reverse was also true for dichlorvos and triazophos both at 10000.0 μg/L. Chlorpyrifos at 10000.0 μg/L made the rotifers dead after 24-hr exposure. Dichlorvos and triazophos both at 10000.0 μg/L, and chlorpyrifos at 1000.0 μg/L all increased the ratio ovigerous females/non-ovigerous females. Both dichlorvos at 10000.0 μg/L and chlorpyrifos at 0.1–100.0 μg/L increased the mictic rate. Dichlorvos at 10.0 μg/L and 100.0 μg/L, and triazophos and chlorpyrifos both at 0.1–100.0 μg/L increased the resting egg production. Both population growth rate and ratio ovigerous females/non-ovigerous females are suitable endpoints for assessing the effects of dichlorvos, triazophos and chlorpyrifos, and mictic rate is a suitable endpoint for monitoring the effects of dichlorvos and chlorpyrifos on the reproduction of the rotifers. Both population growth rate and ratio ovigerous females/non-ovigerous females are more sensitive to dichlorvos and chlorpyrifos than mictic rate.     

Effects of three organophosphorus pesticides on population growth and sexual reproduction of rotifer Brachionus calyciflorus Pallas

Effects of organophosphorus pesticides including dichlorvos, triazophos and chlorpyrifos on population growth and sexual reproduction of freshwater rotifer Brachionus calyciflorus were studied by 3-d population growth and 4-d resting egg production tests. The results showed that all the three organophosphorus pesticides influenced significantly the population growth rate, the ratio ovigerous females/non-ovigerous females in the rotifer populations and the resting egg production of the rotifers. Both dichlorvos and chlorpyrifos influenced markedly the mictic rate of the rotifers, but triazophos did not. Compared to the controls, both dichlorvos at 10.0–1000.0 μg/L and chlorpyrifos at 0.01–100.0 μg/L increased the population growth rate, but the reverse was also true for dichlorvos and triazophos both at 10000.0 μg/L. Chlorpyrifos at 10000.0 μg/L made the rotifers dead after 24-hr exposure. Dichlorvos and triazophos both at 10000.0 μg/L, and chlorpyrifos at 1000.0 μg/L all increased the ratio ovigerous females/non-ovigerous females. Both dichlorvos at 10000.0 μg/L and chlorpyrifos at 0.1–100.0 μg/L increased the mictic rate. Dichlorvos at 10.0 μg/L and 100.0 μg/L, and triazophos and chlorpyrifos both at 0.1–100.0 μg/L increased the resting egg production. Both population growth rate and ratio ovigerous females/non-ovigerous females are suitable endpoints for assessing the effects of dichlorvos, triazophos and chlorpyrifos, and mictic rate is a suitable endpoint for monitoring the effects of dichlorvos and chlorpyrifos on the reproduction of the rotifers. Both population growth rate and ratio ovigerous females/non-ovigerous females are more sensitive to dichlorvos and chlorpyrifos than mictic rate.     

Sex and reproductive stage differences in the growth,metabolism, feed,fecal production,excretion and energy budget of the Amazon River prawn (Macrobrachium amazonicum)

The aim of this study was to evaluate differences in various physiological measures (growth, fecal production, feed intake, nitrogenous excretion, oxygen consumption, energy substrate used, and energy budget) among males, ovigerous females and non-ovigerous females of the freshwater prawn Macrobrachium amazonicum. This species exhibits pronounced sexual dimorphism and different male morphotypes and has the potential for use in aquaculture. Males and non-ovigerous females were studied for 30?days. Ovigerous females were studied for 10?days. Prawns were fed commercial prawn food, and all males were of the Translucent Claw (TC) morphotype. The results demonstrate physiological differences both between males and females and between females of different reproductive stages. Males had higher rates of ingestion, growth and oxygen consumption and less fecal loss than females. We postulate that in the absence of other morphotypes, TC males may exhibit increased growth rates. Males and females used protein as an energy substrate. Males channeled approximately 9% of their energy budget into growth, whereas non-ovigerous and ovigerous females channeled only 1.4?±?0.4 and 0.07?±?0.07%, respectively. Whereas males and non-ovigerous females channeled 9.0?±?9.74 and 61.8?±?3.0%, respectively, of the energy ingested into metabolism, ovigerous females channeled 97.7?±?4.7% into metabolism, likely due to the frequent beating of their pleopods, which oxygenates and cleans the eggs. As reported for marine prawns, males and non-ovigerous females of M. amazonicum lost approximately 5% of their ingested energy in exuviae. The physiological differences observed between the sexes and between females of different reproductive stages might reflect corresponding differences in patterns of activity, growth, and reproduction.     

Population and reproductive biology of the fiddler crab Uca thayeri Rathbun, 1900 (Crustacea: Ocypodidae) in a tropical mangrove from Northeast Brazil

Population and reproductive biology of Uca thayeri Rathbun, 1900 were studied for the first time in a tropical mangrove. Absolute density, sex ratio, population structure, handedness, breeding season and fecundity were investigated. Seven transects were delimited in a mangrove area of the Pacoti River, Northeast of Brazil (3° 43′ 02″ S/38° 32′ 35″ W). On each transect, ten 0.25 m² squares were sampled on a monthly basis during low tide periods from September 2003 to August 2004. A total of 483 crabs were obtained, of which 250 were males, 219 non-ovigerous females, and 14 ovigerous females. The U. thayeri population presented bi-modal size frequency distribution, with males and non-ovigerous females not differing significantly size-wise. Ovigerous females were larger than males and non-ovigerous females. The overall sex ratio (1:1.07) did not differ significantly from the expected 1:1 proportion. The major cheliped was the right one in 50% of the males. The observed density was of 8.5 individuals/m², with the specimens being found mostly in shaded areas. Ovigerous females were found in 5 months of the year, coinciding with the rainy season, suggesting that the population of U. thayeri presents seasonal reproductive events. Juvenile crabs were more abundant during the dry period, while larger crabs were found mainly during the rainy period. The fecundity of the studied population was much smaller than that of subtropical populations of this species. The regression analysis shows that the number of eggs increases linearly with the increase of carapace width.     

Anatomy of virgin and mature externae of Loxothylacus texanus,parasitic on the dark blue crab Callinectes rathbunae (Crustacea: Cirripedia: Rhizocephala: Sacculinidae)

Rhizocephalan parasites are dioecious organisms, in that one or several dwarf males are implanted into the external part of the female parasite soon after it emerges from the interior of the host animal. The structure of the female externa and its resident males is crucial for understanding both the reproductive biology and the taxonomy of these specialized parasites. We use scanning electron microscopy and histological methods to study the anatomy of juvenile and the mature externae of the rhizocephalan barnacle Loxothylacus texanus parasitizing the blue crab Callinectes rathbunae. We put emphasis on the implantation of males and the histology of the female reproductive organs. In the virgin externae, male cyprids attach around a cuticular hood covering the mantle aperture, which is partially blocked by a plug of cuticle so only trichogon larvae, not cyprids, can access the mantle cavity. This resembles the situation known from Sacculina carcini. The mature externa is characterized by a visceral mass that contains the ovary, paired colleteric glands, a single male receptacle, but paired receptacle ducts. The proximal attachment of the visceral mass is located at some distance from the basal stalk, as is characteristic for the genus Loxothylacus. The internal anatomy of the mature externa of L. texanus is in most features similar to that seen in other species of the Sacculinidae, which comprises the majority of rhizocephalan species. However, the single receptacle creates a situation where the two implanted males cannot be kept separate as in most other rhizocephalans, but pass through spermatogenesis in a common chamber. This may have unknown effects on the reproductive biology such as male–male competition. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

The influence of visibility and escape ability on sex-specific susceptibility to fish predation in Eudiaptomus gracilis (Copepoda,Crustacea)

The susceptibility to fish predation in males, ovigerous, and non-ovigerous females of the freshwater copepod Eudiaptomus gracilis is investigated in the laboratory by direct observation of the predation sequence of zebrafish, and by studying mortality when confronted with zebrafish or roach. Ovigerous females had the highest encounter rate indicating that the highly visible egg-clutch is a major determinant of their susceptibility. Males were least successful in escaping, the main reason being their inability or disinclination to react fast enough when attacked. This difference in escape reaction may have evolved because of sex-specific requirements in mate encounter and mate location. Ovigerous females and males had higher mortality than non-ovigerous females in all experiments while ovigerous mortality was higher than male mortality only in the zebrafish experiments. Neither experiment showed any difference in male and overall female mortality but as a consequence of the abrupt change in susceptibility between the ovigerous and non-ovigerous condition, it follows that sex-specific mortality rates may be dependent on the reproductive condition within diaptomid populations. A consequence of the sex-specific difference in escape ability is that the sex-specific mortality may be influenced by variation in the attack efficiency within and among predator populations.     

The Parasitic Fauna of Common Species of Hermit Crabs of Vostok Bay (Sea of Japan)

The parasitic fauna of common species of hermit crabs was investigated in Vostok Bay (the northwestern Sea of Japan). Pagurus proximus and Areopaguristes hirsutimanus had no parasites. The average prevalence of rhizocephalan infestation in other five species of hermit crabs ranged from 5.67 to 20.83%. The prevalence of the isopod Parapseudione lata on hermit crabs did not exceed 1%, the prevalence of infestation with the isopod Athelges takanoshimensis was significantly higher, reaching 14.58% in Pagurus middendorffii. Parasitism by the isopod P. lata on the hermit crabs Pagurus minutus and P. ochotensis, as well as parasitism by the rhizocephalan Peltogaster reticulata on P. minutus were recorded for the first time. Several variants of simultaneous infestation with two parasitic species were found. The decline in the abundance of P. middendorffii accompanied by the high prevalence of parasites, primarily the colonial rhizocephalan Peltogasterella gracilis, suggests that parasite invasion may play a more significant role in regulating the abundance of hermit crabs than was previously assumed.     

Organization of Interna in Sacculina polygenea (Crustacea: Rhizocephala)

We performed histological studies on trophic and reproductive systems of colonial interna in Sacculina polygenea, a parasite of the coastal crab Hemigrapsus sanguineus. The trophic system that performs functions of absorption, accumulation, and transportation of nutrients from the hemolymph of the host comprises the trophic epithelium of distal canals and transporting trunks. The reproductive system of interna consist of nuclei (early stages of development of the primordia of externae) and the primordia of externae in later stages of development. It has been shown that during morphogenesis of the nucleus two primordia arise, a primordium of the externa itself and a primordium of its trophic system. In the primordium of the ovary, we found oogonia; early oocytes and vitellogenic oocytes were found in the ovaries of the late primordia of the externae. The damaging effects of the interna on the ovaries and testes of the crab host are discussed. Thus, we have found numerous elements of reproductive and trophic systems in the colonial interna of S. polygenea. The term individual is proposed to be used for the externa in rhizocephalan barnacles with its trophic system.     

Effects of the Briarosaccus callosus infestation on the commercial golden king crab Lithodes aequispina

Commercial crab populations off the Kamchatka coasts are infested to a considerable degree by the rhizocephalan parasite Briarosaccus callosus: of 769 Lithodes aequispina males examined, 43 (5.7%) were parasitized. Infestations result in the feminization of the crabs, a significant decrease in the cheliped length, and a significant decrease in the carapace length and width. We suggest that commercial selection of healthy males, and the returning of unsuitable crabs, including infested ones, back into the sea, results in an increase of the proportion of infested crabs in the population, their elimination from reproduction, and, eventually, the gradual degradation of a whole population. To minimize as far as possible the negative effects of commercial crab harvesting, all infested crab specimens caught must be destroyed, either aboard or elsewhere, instead of throwing them back into the sea.     

Control region sequences indicate that multiple externae represent multiple infections by Sacculina carcini (Cirripedia: Rhizocephala)

The rhizocephalan barnacle, Sacculina carcini, is a common parasite of the European shore crab, Carcinus maenas, in which it causes significant detrimental physical and behavioral modifications. In the vast majority of cases, the external portion of the parasite is present in the form of a single sac‐like externa; in rare cases, double or even triple externae may occur on the same individual host. Here, we use a highly variable DNA marker, the mitochondrial control region (CR), to investigate whether multiple externae in S. carcini represent infection by multiple parasites or asexual cloning developed by a single parasite individual. Sequences for multiple externae from C. maenas hosts from the Danish inlet, Limfjorden, and from the mud flates at Roscoff, France, were compared. In almost all cases, double or triple externae from an individual host yielded different haplotypes. In the few cases where identical haplotypes were identified from externae on a multiple‐infected host, this always represented the most commonly found haplotype in the population. This indicates that in Sacculina carcini, the presence of multiple externae on a single host reflects infection by different individual parasites. A haplotype network of CR sequences also suggests a degree of geographical partitioning, with no shared haplotypes between the Limfjorden and Roscoff. Our data represent the first complete CR sequences for a rhizocephalan, and a unique gene order was also revealed. Although the utility of CR sequences for population‐level work must be investigated further, the CR has proved a simple to use and highly variable marker for studies of S. carcini and can easily be applied to a variety of studies in this important parasite.     

Microbial communities are indicators of parasite infection status

Growing evidence suggests that microbiomes have been shaping the evolutionary pathways of macroorganisms for millennia and that these tiny symbionts can influence, and possibly even control, species interactions like host–parasite relationships. Yet, while studies have investigated host–parasites and microbiomes separately, little has been done to understand all three groups synergistically. Here, we collected infected and uninfected Eurypanopeus depressus crab hosts from a coastal North Carolina oyster reef three times over 4 months. Infected crabs demonstrated an external stage of the rhizocephalan parasite, Loxothylacus panopaei. Community analyses revealed that microbial richness and diversity were significantly different among tissue types (uninfected crab, infected crab, parasite externae and parasite larvae) and over time (summer and fall). Specifically, the microbial communities from parasite externae and larvae had similar microbiomes that were consistent through time. Infected crabs demonstrated microbial communities spanning those of their host and parasite, while uninfected crabs showed more distinctive communities with greater variability over time. Microbial communities were also found to be indicators of early-stage infections. Resolving the microbial community composition of a host and its parasite is an important step in understanding the microbiome's role in the host–parasite relationship and determining how this tripartite relationship impacts coevolutionary processes.     

Hepatopancreas alteration of the blue crab Callinectes sapidus by the rhizocephalan barnacle Loxothylacus texanus

A histological study of the hepatopancreas of the blue crab Callinectes sapidus parasitized by the rhizocephalan barnacle Loxothylacus texanus was conducted to explore if the degree of development of the parasite’s rootlet system was correlated to its maturation process as seen by external characters of its reproductive body or externa. Four types of crabs were examined: control, with virgin and mature externa, and scarred. A clear progression with an increase in number and size of the parasite’s rootlets in the hosts’ hepatopancreas can be seen. Although the hepatopancreatic tubules remain functional, the hepatopancreas appears as a loose structure, completely infiltrated with L. texanus rootlets, in advanced stages of the parasitism.     

Predation by juvenile roach on the calanoid copepod Eudiaptomus gracilis and the cyclopoid copepod Cyclops vicinus: a laboratory investigation with mixed and single prey

We tested the outcome of predation by juvenile roach on the calanoid copepod Eudiaptomus gracilis and the similar sized cyclopoid copepod Cyclops vicinus in laboratory experiments with mixed and single-prey. When ovigerous females and adult non-ovigerous females of the calanoid copepod and the cyclopoid copepod were offered in equal numbers to the fish in the mixed-prey experiments, the cyclopoid copepods were significantly more heavily preyed upon than the calanoids. Between 14 and 16 females of the cyclopoid copepod and only between 2 and 4 calanoids had been consumed after the observation period of two hours. The single-prey experiments revealed that the conspicuousness of the calanoid vs the cyclopoid depended on sex and gravidity. Ovigerous females of the cyclopoid copepod were slightly earlier detected by the fish than ovigerous females of the calanoid, probably a result of the highly visible egg-sacs carried by the cyclopoid females. Females without eggs and males of the cyclopoid copepod were recognized later by the predator than females without eggs or males of the calanoid, probably a result of the different behaviours of the different copepod taxa. Cyclopoids frequently congregated near the aquarium bottom while calanoids were closer to the water surface where they were better visible. The calanoid copepod could better escape the fish's attacks than the cyclopoid copepod. Egg-bearing females of both the calanoid and the cyclopoid copepod could significantly better escape than non-ovigerous females or males. Probably ovigerous females react very early to water disturbances caused by the predator. Activity measurements showed that the cyclopoid copepod displayed 2 to 4 times more hops per time unit than the calanoid. Probably the high number of jerky movements displayed by the cyclopoid attracted attention of the predator and contributed to its greater vulnerability.