Jeffrey M. Lotz

Parasitology Meets Ecology on Its Own Terms: Margolis et al. Revisited.

We consider 27 population and community terms used frequently by parasitologists when describing the ecology of parasites. We provide suggestions for various terms in an attempt to foster consistent use and to make terms used in parasite ecology easier to interpret for those who study free-living organisms. We suggest strongly that authors, whether they agree or disagree with us, provide complete and unambiguous definitions for all parameters of their studies.

Selective breeding of Pacific white shrimp (Litopenaeus vannamei) for growth and resistance to Taura Syndrome Virus

Abstract From 1995 to 1998, the Oceanic Institute operated a breeding program for Pacific white shrimp, Litopenaeus vannamei, based on a selection index weighted equally for growth and resistance to Taura Syndrome Virus (TSV). In 1998, two separate breeding lines were established. One line was selected 100% for growth (Growth line) and a second line was selected on an index weighted 70% for TSV resistance and 30% for growth (TSV line). After one generation of selection, select shrimp from the Growth line were 21% larger than unselected control shrimp (24.2 vs. 20.0 g). The half-sib heritability (h2) estimate for growth was 0.84±0.43(s.e.) and realized h2 was 1.0±0.12. Females were 12.7% larger than males. Shrimp tails accounted for 65.1% of total body weight and males had a significantly higher percent tail than females (65.7% vs. 64.5%; P

Disease will limit future food supply from the global crustacean fishery and aquaculture sectors

Seafood is a highly traded food commodity. Farmed and captured crustaceans contribute a significant proportion with annual production exceeding 10 M metric tonnes with first sale value of

Viruses, Biosecurity and Specific Pathogen-Free Stocks in Shrimp Aquaculture

40bn. The sector is dominated by farmed tropical marine shrimp, the fastest growing sector of the global aquaculture industry. It is significant in supporting rural livelihoods and alleviating poverty in producing nations within Asia and Latin America while forming an increasing contribution to aquatic food supply in more developed countries. Nations with marine borders often also support important marine fisheries for crustaceans that are regionally traded as live animals and commodity products. A general separation of net producing and net consuming nations for crustacean seafood has created a truly globalised food industry. Projections for increasing global demand for seafood in the face of level or declining fisheries requires continued expansion and intensification of aquaculture while ensuring best utilisation of captured stocks. Furthermore, continued pressure from consuming nations to ensure safe products for human consumption are being augmented by additional legislative requirements for animals (and their products) to be of low disease status. As a consequence, increasing emphasis is being placed on enforcement of regulations and better governance of the sector; currently this is a challenge in light of a fragmented industry and less stringent regulations associated with animal disease within producer nations. Current estimates predict that up to 40% of tropical shrimp production (>

The Role of Selective Breeding and Biosecurity in the Prevention of Disease in Penaeid Shrimp Aquaculture

3bn) is lost annually, mainly due to viral pathogens for which standard preventative measures (e.g. such as vaccination) are not feasible. In light of this problem, new approaches are urgently required to enhance yield by improving broodstock and larval sourcing, promoting best management practices by farmer outreach and supporting cutting-edge research that aims to harness the natural abilities of invertebrates to mitigate assault from pathogens (e.g. the use of RNA interference therapeutics). In terms of fisheries losses associated with disease, key issues are centred on mortality and quality degradation in the post-capture phase, largely due to poor grading and handling by fishers and the industry chain. Occurrence of disease in wild crustaceans is also widely reported, with some indications that climatic changes may be increasing susceptibility to important pathogens (e.g. the parasite Hematodinium). However, despite improvements in field and laboratory diagnostics, defining population-level effects of disease in these fisheries remains elusive. Coordination of disease specialists with fisheries scientists will be required to understand current and future impacts of existing and emergent diseases on wild stocks. Overall, the increasing demand for crustacean seafood in light of these issues signals a clear warning for the future sustainability of this global industry. The linking together of global experts in the culture, capture and trading of crustaceans with pathologists, epidemiologists, ecologists, therapeutics specialists and policy makers in the field of food security will allow these issues to be better identified and addressed.

THE ECOLOGY OF “CROWDING”

The greatest threat to the future of world shrimp aquaculture is disease, in particular the virulent untreatable viruses, infectious hypodermal and haematopoietic necrosis virus (IHHNV), taura syndrome virus (TSV), yellow head virus (YHV), and white spot syndrome virus (WSSV). To overcome these hazards, the industry of the future must be based on: (i) specific pathogen-free and genetically improved shrimp stocks; (ii) biosecure systems including enclosed, reduced water-exchange/increased water-reuse culture systems; (iii) biosecure management practices; and (iv) co-operative industry-wide disease control strategies. Specific pathogen-free shrimp are those that are known to be free of specified pathogens and such stocks will ensure that seed shrimp are not the conduit for introduction of pathogens and that if pathogens are encountered the stocks will not be severely affected. Commercially acceptable biosecure culture systems that are under cover and use recirculated sea water will need to be developed for shrimp production. Adherence to operating protocols that incorporate strict biosecurity practices, including restricted access and disinfection strategies, will need to become standard. Co-operative efforts will include: early warning surveillance; co-ordination of harvest and water exchange schedules of contaminated ponds; processor co-operation to ensure that processing wastes are not threats; quick response to outbreaks.

Excess positive associations in communities of intestinal helminths of bats : a refined null hypothesis and a test of the facilitation hypothesis

About 3.5 million metric tons of farmed shrimp were produced globally in 2009 with an estimated value greater than USD

Recruitment-driven, spatially discontinuous communities: a null model for transferred patterns in target communities of intestinal helminths.

14.6 billion. Despite the economic importance of farmed shrimp, the global shrimp farming industry continues to be plagued by disease. There are a number of strategies a shrimp farmer can employ to mitigate crop loss from disease, including the use of Specific Pathogen Free (SPF), selectively bred shrimp and the adoption of on-farm biosecurity practices. Selective breeding for disease resistance began in the mid 1990s in response to outbreaks of Taura syndrome, caused by Taura syndrome virus (TSV), which devastated populations of farmed shrimp (Litopenaeus vannamei) throughout the Americas. Breeding programs designed to enhance TSV survival have generated valuable information about the quantitative genetics of disease resistance in shrimp and have produced shrimp families which exhibit high survival after TSV exposure. The commercial availability of these selected shrimp has benefitted the shrimp farming industry and TSV is no longer considered a major threat in many shrimp farming regions. Although selective breeding has been valuable in combating TSV, this approach has not been effective for other viral pathogens and selective breeding may not be the most effective strategy for the long-term viability of the industry. Cost-effective, on-farm biosecurity protocols can be more practical and less expensive than breeding programs designed to enhance disease resistance. Of particular importance is the use of SPF shrimp stocked in biosecure environments where physical barriers are in place to mitigate the introduction and spread of virulent pathogens.

A model of Taura syndrome virus (TSV) epidemics in Litopenaeus vannamei.

There are 2 elements that make ‘‘crowding’’ interesting— mechanism (causality) and manifestation (effect). In our companion paper, Larry Roberts addresses mechanisms. Here, we focus on manifestation. As ecologists, it is difficult to get too excited about Clark Read’s paper on the ‘‘crowding effect’’ in cestodes, at least initially. The paper acknowledges clearly that the phenomenon has been observed repeatedly, is mostly a discussion of appropriate techniques, and concludes with a brief overture to causality. Recalling a popular television commercial from several years ago, ‘‘Where’s the beef?’’ The answer lies in the implication of crowding. Simply stated, crowding means too many of something. In the case of worms in the gut of a host, it is obvious that there exists a finite number of individuals that can physically fit into a gut. Crowding is a common phenomenon, certainly not restricted to tapeworms in the small intestine of a rat. It is very, very common in managed systems where the intent is to get the ‘‘biggest bang for the buck.’’ For example, gardeners know that too many plants in a prescribed area will result in a poor crop; so too do aquaculturists raising fishes, crustaceans, or shellfish. Under such artificial conditions, the remedy for alleviating crowding is simple, at least in theory. Either reduce (thin) the target population or artificially enhance the environment. The latter may be accomplished by supplemental feeding, removing toxic wastes, and so forth. Trivial observations perhaps but ‘‘crowding’’ is overwhelmingly common, at least where humans have intervened. But, what of the ecology of crowding in a natural context? We consider crowding as being important ecologically in 2 contexts—first as it relates to predator–prey relationships and second as it relates to the much-maligned idea of competition. In a food web, crowding will always impact most severely on the prey population. If there are too many predators, e.g., the predators are crowded, more prey will be taken simply because there are more things to eat them. Similarly, if there are too many prey, predators will find and, perhaps, capture them more easily. However, our focus here is not on predator–prey relationships, rather it is on crowding as it might relate to parasites in a host. At the time Clark Read’s paper was published, ecology was more qualitative natural history than the quantitative science we know today. If predator–prey interactions could not account for the observed patterns on the distribution and abundance of organisms, then surely the answer must lie in competition. It is perhaps for that reason, that competition was apparently so pervasive that Read ignored manifestation in his paper. Why emphasize what was so readily obvious? Basically, competition takes 2 forms: interference and exploitation. With interference competition, organisms may impact on others in a direct fashion, for example, releasing toxins.

Advances in Research of Necrotizing Hepatopancreatitis Bacterium (NHPB) Affecting Penaeid Shrimp Aquaculture

The null hypothesis that the number of positive pairwise covariances should equal the number of negative pairwise covariances in samples from communities of randomly associated helminth species was reevaluated. The proportion of positive covariances in a sample from a community of independent species depends upon the proportion of rare species (prevalence less than 10%), the proportion of common species (prevalence greater than 90%), and the size of the sample of hosts. If rare species dominate, then there will be an excess of negative associations; if common species dominate there will be an excess of positive associations. Many helminth communities have more rare than common species, therefore samples from communities that show an equal number of positive and negative covariances have a greater number of positive associations than is expected for randomly associated species. Increased sample size will reduce the sampling bias, but at least 100 hosts are necessary and often 500-7,500 hosts are required. The excess of positive covariances between helminth species in 10 populations of bats disappeared after restricting the analyses to hosts in which both members of a species pair were present. This result suggests that excess positive associations between helminth species in bats are due to joint presences and absences in hosts rather than to interspecific facilitation. Interspecific facilitation would be supported by observed positive correlations between the intensities of individuals of the species pairs.