The Blue Crab: Diseases, Parasites and Other Symbionts
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University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Faculty Publications from the Harold W. Manter Laboratory of Parasitology Parasitology, Harold W. Manter Laboratory of 2003 The Blue Crab: Diseases, Parasites and Other Symbionts Jeffrey D. Shields Virginia Institute of Marine Science, [email protected] Robin M. Overstreet Gulf Coast Research Laboratory, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/parasitologyfacpubs Part of the Parasitology Commons Shields, Jeffrey D. and Overstreet, Robin M., "The Blue Crab: Diseases, Parasites and Other Symbionts" (2003). Faculty Publications from the Harold W. Manter Laboratory of Parasitology. 426. https://digitalcommons.unl.edu/parasitologyfacpubs/426 This Article is brought to you for free and open access by the Parasitology, Harold W. Manter Laboratory of at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Faculty Publications from the Harold W. Manter Laboratory of Parasitology by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Chapter 8 Diseases, Parasites, and Other Symbionts J.D. SHIELDS AND R.M. OVERSTREET And this great structural diversity [of marine life] is paralleled by habits and ways of life which are often bizarre to the point of fantasy. Against such a background, it is not surprising that the whole spectrum of animal associations, from the obviously casual to the intimately complex, can be seen. R.V.Gotto (1969) INTRODUCTION Carcinonemertes carcinophila, Octolasmis muelleri) may serve as markers of host biology by indicating We present a critical review of the microbial dis- migration or molting patterns, and one syndrome, eases, parasites, and other symbionts of the blue crab. shell disease, may even serve as a useful indicator of Previous reviews have provided brief synopses of the poor water quality associated with pollution. diseases of the blue crab (Messick and Sinderman Our synthesis is meant to show the gaps in our 1992; Noga et al. 1998), overviews and synthesis of understanding of the primary diseases of the blue crustacean diseases in general (Couch 1983; Johnson crab and guide future work on their ecological 1983; Overstreet 1983; Brock and Lightner 1990; influences and their pathological processes in the Meyers 1990), or aspects of specific parasitic taxa host.Aspects of the immune system of the blue crab (Couch and Martin 1982; Overstreet 1982; Brad- are discussed in relation to selected diseases with the bury 1994). Infectious diseases of blue crabs have caveat that immune functions are poorly understood received far less attention than those of the inten- in the Crustacea in general. sively cultured eastern oyster or the penaeid shrimps, Throughout the chapter the term “symbiont” is primarily because of differences in resource manage- used broadly as an organism in some form of close ment as well as the dramatic detrimental influences or intimate association with its host (Overstreet of protozoal and viral diseases in the latter hosts, 1978). “Symbiosis” means “living together,” and we respectively. Nonetheless, given the appropriate view it as spanning the gamut of disease, parasitic, environmental conditions, several pathogenic agents commensalistic, mutualistic, and phoretic, but not (e.g., viruses, Vibrio spp., Hematodinium perezi, Para- predatory, relationships. A “disease” imparts abnor- moeba perniciosa, Loxothylacus texanus) have the capac- mal function within the host.“Pathogens” cause dis- ity to severely affect blue crab fisheries. Several bac- ease by damaging physiological functions within the teria and a few parasites (e.g., Vibrio spp., microphallid host. “Parasites” may or may not be pathogens that trematodes) represent minor human health con- cause disease, but they have the potential to produce cerns, and a few bacteria (Listeria monocytogenes, a negative effect on the host, especially in heavy Clostridium botulinum) represent safety hazards to the infections. “Hyperparasitism,” a second order para- seafood industry. In addition, several symbionts (e.g., sitism, signifies the condition when one parasite 224 THE BLUE CRAB infects another.A “facultative symbiont” is not phys- late 1960s and early 1970s saw the advent of scien- iologically dependent on a host but can establish a tific interest in diseases of the blue crab, especially as relationship with it when the opportunity occurs. It crab fisheries became more fully exploited.With the contrasts with an “obligate symbiont,” which has a expansion of the softshell industry has come an physiological dependency on its host. The terms increased awareness of the role of diseases in the “infection” and “infestation” refer to internal and short-term culture of the blue crab and their nega- external invasion of the host by endo- and ecto- tive effects on the fisheries. Blue crabs are now symbionts, respectively. “Commensalism” and known to be infected by a large, disparate fauna “phoresy” are relationships where the symbiont comprised of viruses, bacteria, fungi, protozoa, derives benefit, but the host is not affected by the helminths, and other crustaceans. Most of the para- association. Commensalism results when the sym- sites and diseases are relatively benign and cause little biont shares nutritional resources or a living space pathological alteration in the crab host. Several, with the host. Phoresy results when the symbiont however, cause considerable alteration and occasion- uses the host for transportation. An “epibiont” is an ally fulminate into epizootics, or outbreaks, resulting organism that lives on the external surface of the in crab mortalities. Unlike dead fish that float, dead host. In “mutualism,” both the symbiont and the crabs generally sink, hence large mortalities often go host benefit from the association. These definitions unnoticed or underreported. The true influence of represent a continuum that exists among symbiotic several diseases, therefore, may be difficult to assess associations. without intensive sampling in enzootic locations. Standard parasitological terms were defined by Margolis et al. (1982) and updated by Bush et al. VIRAL INFECTIONS (1997). Briefly, “prevalence” is the number of infected hosts divided by the total number of hosts Other than those from some penaeid shrimps, examined, usually expressed as a percentage.“Inten- viral infections in the blue crab are some of the bet- sity” is the number of parasites infecting a host, with ter known from a marine invertebrate host. These “mean intensity” representing the mean number of blue crab infections are known primarily from parasites per infected host. “Density” refers to the descriptive ultrastructural studies by Johnson (e.g., number of parasites per unit of host or habitat mea- 1986a). There are seven or eight reported viruses sured in area, volume, or weight (e.g., parasites per infective to C. sapidus along the Atlantic Coast, with ml hemolymph), and is often used with bacterial at least two occurring in the Gulf of Mexico.Three and protozoal agents. An “epizootic,” or “epidemic” species are lethal and two are found concurrently if related to people, is an outbreak of a disease, usu- with other viruses, with strong experimental evi- ally expressed as a large increase in prevalence or dence indicating a synergistic pathogenic effect.The intensity of infection in the host population. When identification and characterization of the viruses occurring over a wide geographic area, such an epi- infecting crabs are less well established than those zootic is referred to as a “panzootic.”An “enzootic,” infecting shrimps, perhaps because there is less eco- or native, disease is one caused by factors consis- nomic incentive to culture crabs than to rear tently present in the affected host population, envi- penaeid shrimps. However, when one of the shrimp ronment, or region. viruses is introduced to the blue crab, it can produce The first reported symbionts of the blue crab an infection and induce mortality. In addition to were the rhizocephalan barnacle Loxothylacus texanus hosting viruses that are apparently specific to the by Boschma (1933), the fungus Lagenidinium call- blue crab and its relatives, the blue crab can also inectes by Couch (1942), and the nemertean worm accumulate human enteric viruses. Carcinonemertes carcinophila by Humes (1942). The A B C Figure 1. Bi-facies virus (BFV). (A) Infected hemocyte with large viral induced inclusions in both cytoplasm and nucleus. From Johnson (1988). (B) Type-A mature particle showing electron-dense core bound by electron-dense sphere, which in turn is surrounded by an inner and outer membrane. From Overstreet (1978). (C) Type-B mature particle showing single envelope. Note mass of rods (perhaps core of undeveloped BFV) in cytoplasmic material asso- ciated with fixed phagocyte. Note small rhabdo-like virus in same cell. From Johnson (1988). 225 226 THE BLUE CRAB DNA Viruses bly causes the death of infected crabs, but the condi- tion is not necessarily a stress-related disease. When Bi-facies Virus (BFV) hemolymph from a moribund crab was injected Biology into healthy crabs or infected tissue was fed to such a Bi-facies virus, formerly referred to as herpes- crab, death resulted 30 to 40 d later (Johnson 1978), like virus (HLV), is a dsDNA, enveloped virus that is much sooner than that for naturally infected indi- extracellular or in the nucleus or cytoplasm of viduals. Naturally infected crabs may survive for at