Impact of the crayfish Procambarus clarkii on Schistosoma haematobium transmission in Kenya

The Louisiana red swamp crayfish, Procambarus clarkii, which was introduced into east Africa in the 1950s or 1960s, has since widely dispersed. Previous work by our group has shown that P. clarkii can reduce populations of the molluscan intermediate hosts of human schistosomes through predatory and competitive interactions. Here, we investigate whether crayfish can reduce populations of Bulinus africanus and consequently, Schistosoma haematobium prevalence in school children. Children from 6 primary schools in the Machakos and Kitui Districts of Kenya were selected for study. Schools were divided into 3 experimental-control pairs. At experimental schools, crayfish were introduced into nearby aquatic habitats harboring Bulinus africanus snails and serving as S. haematobium transmission sites. Snail habitats near control schools did not receive crayfish. Six months after crayfish introduction, all infected children were treated with praziquantel. Children were then monitored quarterly for 2 years, at which time infection and reinfection rates were compared statistically between the paired schools. In one such pair, crayfish failed to establish, resulting in neither snail control nor a reduction in transmission. At the second pair of schools, the numbers of snails were decreased by the presence of crayfish, but a clear difference in infection rates in children could not be detected, primarily because drought conditions kept overall transmission rates low. At the third school pair, crayfish established well in experimental habitats, snail numbers decreased precipitously, and children at the experimental school were significantly less likely to acquire S. haematobium infections than children at the control school. Our results indicate that under certain environmental circumstances, P. clarkii exerts a significant impact on the transmission of human schistosomiasis in Kenya. Important questions remain regarding the impact of P. clarkii on Kenyan freshwater ecosystems, not the least of which is its potential to significantly influence the epidemiology of schistosomiasis in east Africa.

Morphological and biological notes on Polymorphus (Profilicollis) sphaerocephalus and Corynosoma stanleyi (Polymorphidae: Acanthocephala)

Polymorphus (Profilicollis) sphaerocephalus (Bremser in Rudolphi, 1819) Van Cleave, 1947 (Polymorphidae) cystacanths were recovered from 5 species of grapsid crabs (Paragrapsus gaimardii (Milne Edwards, 1837), Paragrapsus laevis (Dana, 1852), Paragrapsus quadridentatus (Milne Edwards, 1837), Brachynotus spinosus (Milne Edwards, 1853), and Cyclograpsus granulosus (Milne Edwards, 1853)) and 1 species of portunid crab (Nectocarcinus integrifrons (Linnaeus, 1766)) from intertidal zones in southern temperate waters of Australia. Cystacanths of Corynosoma stanleyi Smales, 1986 (Polymorphidae) were also recovered from P. gaimardii, P. quadridentatus, and C. granulosus. Polymorphus (P.) sphaerocephalus was the most prevalent (100%) in C. granulosus at Flinders I. and C. stanleyi was most prevalent (59.1%) in C. granulosus at Dunally Channel, Tasmania.

Infestation of an introduced host, the European green crab, Carcinus maenas, by a symbiotic nemertean egg predator, Carcinonemertes epialti

The recent introduction of the European green crab, Carcinus maenas, to the west coast of the U.S. has provided an opportunity for host transfer of the symbiotic nemertean egg predator, Carcinonemertes epialti, from its native shore crab host, Hemigrapsus oregonensis to the exotic C. maenas. Two surveys of C. maenas in Bodega Harbor, California, revealed that, in March 1995 prevalence of C. epialti on C. maenas was significantly lower than on H. oregonensis (11% versus 74%), but in November 1995 there was no significant difference between the 2 species (79% versus 98%). Only juvenile C. epialti were recovered from C. maenas in March 1995. However, in November 1995, ovigerous C. maenas were harboring actively feeding adult worms. Prevalence in both crab species significantly differed from March to November. Laboratory studies revealed that C. epialti fed and reproduced on eggs of C. maenas. The feeding rate of C. epialti on C. maenas eggs (2.5 eggs/trial) was not significantly different from that on H. oregonensis eggs (3.6 eggs/trial). Our findings suggest that this nemertean may have less host specificity than was previously thought. If C. epialti causes brood mortality of C. maenas in nature, it could potentially impact populations of this exotic crab.

Predation and control of laboratory populations of the snail Biomphalaria glabrata by the freshwater prawn Macrobrachium rosenbergii

Laboratory experiments were conducted on predation by the giant freshwater prawn, Macrobrachium rosenbergii, on Biomphalaria glabrata, a snail intermediate host of human schistosomiasis. Prawns greater than 22 mm carapace length could consume snails of any size. Smaller prawns exhibited a size-specific upper limit on the size of snail they could kill. Below the maximum size of prey consumed, little selectivity for prey size was demonstrated. All prawns heavier than 2.5 g killed the largest size of snail offered (16 mm diameter). Consumption, in terms of the numbers of snails killed, and the snails' wet weight and percentage of prawn body weight eaten per day, increased with prawn size. Large prawns consumed snails at a high rate (39% of body weight per day). Population experiments demonstrated that a prawn of 25 mm carapace length could eliminate 95% of a population of 80 snails in a 20-1 aquarium within 20 days and all snails by day 40. A prawn of 15 mm carapace length could not eliminate all snails. Large snails (24% of the initial population) had some protection from predation because of their size. Although the snails bred continuously, no snails were able to recruit to these populations in the presence of small prawns. In contrast, snail populations in control aquaria without prawns expanded to a mean of 919 snails by the end of the experiment (day 70). Since M. rosenbergii are voracious predators on B. glabrata, exhibit considerable habitat overlap with the snail prey, and are likely to treat snails as highly preferred food, further experimentation on these prawns in the context of biological control of schistosomiasis is warranted. The ready availability of prawns through established prawn hatcheries and the synergistic use of these prawns in aquaculture may ensure the reliability of stocking procedures and meet the standards of availability and cost-effectiveness required of a biological control agent. Polyculture of Macrobrachium with fin fish (Tilapia) indicates that these prawns may be a particularly useful control agent in aquacultural environments.

Carcinonemertes regicides n.sp. (Nemertea), a symbiotic egg predator from the red king crab, Paralithodes camtschatica (Decapoda: Anomura), in Alaska

A symbiotic nemertean worm, Carcinonemertes regicides n.sp., found on red king crabs, Paralithodes camtschatica, is described and compared with other members of the genus Carcinonemertes. The family Carcinonemertidae Humes, 1942 is emended to include this species and to include known but as yet undescribed members. The following familial characters are considered diagnostic: (i) symbiotic relationship with a decapod crustacean, (ii) short proboscis, (iii) absence of cerebral organs, (iv) presence of Takakura's duct, and (v) a hoplonemertean larva. The new species of Carcinonemertes has a relatively large basis and stylet, a large anterior proboscis chamber, and a newly described flame cell pattern of 2[(1 + 1 + 1 + 1)]. In addition, at maturity it is the smallest member of the genus (typical adult size is 2.0 mm in length), and female worms bear ovarian pores before oviposition. The above characters distinguish this species from others in the genus.

TRANSFER OF NEMERTEAN EGG PREDATORS DURING HOST MOLTING AND COPULATION

Juvenile nemertean egg predators were able to efficiently transfer from the premolt cuticle to the postmolt cuticle of male and female crabs when the host molted. These worms also efficiently transferred from male to female hosts at copulation. The synchronized responses of the nemertean worms to host physiology and behavior dramatically concentrate the nemertean population on the sole food source required for worm reproduction: crab eggs. The efficient location of reproductive crabs by juvenile worms increased the likelihood that these worms can have significant effects on crab fisheries when worm population density is high.

Echinostome cercarial penetration and metacercarial encystment as mortality factors for a second intermediate host, Biomphalaria glabrata

Cercarial penetration and metacercarial encystment of Echinostoma liei in Biomphalaria glabrata caused high mortality of juvenile snails (3-8 mm in diameter) after 4 to 6 days of continuous exposure to about 150 cercariae per snail per day. Larger snails (10-13 mm) withstood cercarial penetration longer, significant mortality appearing 16 days after exposure. Life-table analysis showed that the snail mortality rate per age interval increased with increasing exposure to echinostome penetration. Single exposures of 3- and 6-mm snails to 500 cercariae caused 97% mortality for the former and 84% for the latter within 2 days. Snails surviving the first 2 days did not suffer further mortality when infected with as many as 488 metacercariae. Thus, mortality seems to result from penetration and early stages of encystment, rather than the presence of large numbers of metacercariae in the pericardial region. Growth of 3-mm snails exposed to 10 or 100 cercariae and of 6-mm snails to 500 cercariae was significantly less than the growth of unexposed controls after 20 days. Cercariae rapidly located the snails, 90% penetrating within 1 hr when individual, 6-mm snails were exposed to 100 cercariae in 5 ml of water. The high rate of cercarial penetration needed to cause snail mortality suggests that echinostome penetration and encystment will not cause much mortality of snail intermediate hosts in nature unless the density of echinostome cercariae is high.

LIFE CYCLE, DISTRIBUTION AND ABUNDANCE OF CARCINONEMERTES EPIALTI, A NEMERTEAN EGG PREDATOR OF THE SHORE CRAB, HEMIGRAPSUS OREGONENSIS, IN RELATION TO HOST SIZE, REPRODUCTION AND MOLT CYCLE

1. The geographic range of Carcinonemertes epialti has been greatly extended. The worms are found from Bahia San Quintin, Baja California, Mexico, to Page's Lagoon, Vancouver Island, British Columbia, Canada. 2. New host records for C. epialti include H. oregonensis, and H. nudus. It is rare on its type host Pugettia producta. Specimens of Carcinonemertes of uncertain affinities are also found on Cancer antennarius, C. anthonyi and C. productus. 3. Carcinonemertes epialti adults are egg predators on ovigerous hosts. Growth, demography and abundance are described in relation to the embryogenic stage of the host brood at Bodega Harbor, California. 4. Nonfeeding juveniles are ensheathed on individuals of both host sexes over 8.0 mm carapace width. 5. Transmission experiments show that contact transfer of juvenile nemerteans from males to other hosts may occur. 6. The percentage of infestation and mean density peak in autumn on H. oregonensis at Bodega Harbor. 7. Ovigerous female hosts are more frequently infested with C. epialti, particularly at small host sizes, than are male or nonovigerous female hosts at Bodega Harbor. However, average worm density on ovigerous females is low. 8. Mean density of C. epialti rises through late postmolt, declines during intermolt and rebuilds to a high level in late premolt H. oregonensisfrom Bodega Harbor. 9. Large crabs have a higher percentage of infestations and mean densities per infection than do small crabs. Nemerteans are more frequently found in the sternal-abdominal furrow and less frequently in the limb axillae on large crabs. 10. A model of C. epialti transmission and site occupancy is proposed, incorporating the influence of host size, sex, reproductive state, embryogenesis, molt cycle stage and molt cycle duration of H. oregonensis at Bodega Harbor. Site availability increases with host size. At higher densities the juvenile nemerteans increasingly occupy less preferred sites. Transferral of juvenile nemerteans occurs and is considered responsible for the high frequency of low infestation levels. Ovigerous females are more likely to be infested but with low density infestations.

Effect of limb regeneration on size increase at molt of the shore crabs Hemigrapsus oregonensis and Pachygrapsus crassipes

Size increase at molt is reduced following multiple limb regeneration in the shore crabs, Hemigrapsus oregonensis and Pachygrapsus crassipes. Limb loss per se does not influence postmolt size. Effect of increasing number of regenerating limbs is additive. Postmolt size is programmed early in the premolt period of the preceding instar and is probably not readily influenced by water uptake mechanics at ecdysis. A simple model for growth, molting, and regeneration in heavily calcified Crustacea is developed from the viewpoint of adaptive strategies and energetic considerations.