Pathology of Hematodinium Infections in Snow Crabs (Chionoecetes Opilio) from Newfoundland, Canada
Total Page：16
File Type：pdf, Size：1020Kb
Journal of INVERTEBRATE PATHOLOGY Journal of Invertebrate Pathology 95 (2007) 93–100 www.elsevier.com/locate/yjipa Pathology of Hematodinium infections in snow crabs (Chionoecetes opilio) from Newfoundland, Canada Kersten Wheeler a, Jeffrey D. Shields a,*, David M. Taylor b a Virginia Institute of Marine Science, The College of William and Mary, School of Marine Science, 1208 Greate Rd., Gloucester Point, VA 23062, USA b Department of Fisheries and Oceans, St. John’s, Newfoundland, Canada A1C 5X1 Received 6 September 2006; accepted 8 January 2007 Available online 26 January 2007 Abstract Bitter crab disease (BCD) of snow crabs, Chionoecetes opilio, is caused by a parasitic dinoflagellate, Hematodinium sp. The disease has shown an alarming increase in prevalence in the commercial fishery in eastern and northeastern areas of Newfoundland and Labrador since it was first recorded there in the early 1990s. We documented histopathological alterations to the tissues in snow crabs with heavy infections of Hematodinium sp. and during sporulation of the parasite. Pressure necrosis was evident in the spongy connective tissues of the hepatopancreas and the blood vessels in most organs. In heavy infections, little remained of the spongy connective tissues around the hepatopancreas. Damage to the gills varied; in some cases it was severe, particularly during sporulation, involving apparent thinning of the cuticle, loss of epithelial cells, and fusion of the membranous layers of adjacent gill lamellae. Affected lamellae exhibited varying degrees of distention with a loss of trabecular cells, hemocyte infiltrations, and swelling or ‘‘clubbing’’ along the distal margins. Large numbers of zoospores were located along the distal margins of affected lamellae suggesting that sporulation may cause a lysis or bursting of the thin lamellar cuticle, releasing spores. Pressure necrosis, due to the build up of high densities of parasites, was the primary histo- pathological alteration in most tissues. Hematodinium infections in the snow crab are chronic, long-term infections that end in host death, during sporulation of the parasite. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Bitter crab disease; Bitter crab syndrome; Sporulation; Gills; Hepatopancreas; Fishery; Disease; Crab 1. Introduction in a predomination of recently molted, low-yield male crabs, consequently resulting in premature closing of the Snow crabs, Chionoecetes opilio, are highly prized for fishery. In the last decade, bitter crab disease (BCD) has their delicately flavored meat, and have become one of proliferated in the northern bays of Newfoundland, caus- Newfoundland’s largest commercial fisheries. In 2005, the ing at least three documented outbreaks (Shields et al., fishery landed 44,000 metric tonnes (mt) of snow crab, val- 2005; Shields et al., in review) and may have contributed ued at CAN $140 million (DFO, 2006). In comparison, the to the decline in landings. snow crab fishery in 2003 had landings in excess of In snow and tanner crabs (C. opilio and Chionoecetes 58,000 mt worth nearly CAN $264 million (DFO, 2006). bairdi), BCD is caused by Hematodinium sp., an endopara- Changes in value of the fishery are largely due to market sitic dinoflagellate (Meyers et al., 1987). Bitter crab disease fluctuations, such as demand and price, and biotic forces was first found in snow crabs from Newfoundland in 1990, such as highly synchronous molting events which result at low levels (0.037% prevalence) (Taylor and Khan, 1995). Since then the prevalence of BCD has rapidly increased. In 2000, it was found in nearly 25% of females sampled from * Corresponding author. Fax: +1 804 684 7186. Conception Bay, Newfoundland, and an epizootic in 2005 E-mail address: jeff@vims.edu (J.D. Shields). affected approximately 25% of legal-sized males surveyed 0022-2011/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jip.2007.01.002 94 K. Wheeler et al. / Journal of Invertebrate Pathology 95 (2007) 93–100 (Shields et al., in review). The disease is most prevalent in to the gills during sporulation of Hematodinium sp. in the the northern bays of Newfoundland (Taylor and Khan, snow crab. The intensity of infection in relation to the life 1995; Pestal et al., 2003; Shields et al., 2005), although it history of the parasite was also examined. also occurs at low prevalence offshore (Dawe, 2002). The disease may have a profound effect on the snow crab indus- 2. Materials & methods try, including the unmarketability of infected legal-sized crabs, and the mortality imposed on pre-recruits to the fish- 2.1. Collection of animals ery (Shields et al., 2005). Snow crabs obtain infections in the spring, during molt- Crabs were collected from Conception Bay on the ing, and infections take approximately 10–15 months to northeast coast of Newfoundland (see Fig. 1 in Shields develop (Shields et al., 2005). Macroscopically diagnostic et al., 2005). The bay is partially enclosed and has a range infections appear to develop over the course of 2–4 months of bottom types; sand and gravel at the mouth, and thick (Fall season) with mortalities occurring 3–6 months later, mud in the deep inner basin. Collections were made during few infected hosts survive more than a year (Shields et al, the Department of Fisheries and Oceans (DFO) annual 2005). BCD is generally a disease of juvenile males and stock assessment surveys with the CCGS Shamook. Crabs immature females, as they molt more frequently, and of were sampled as in Pestal et al. (2003) and Shields et al. males that have recently molted to sexual maturity (Shields (2005). Snow crabs examined in this study were taken from et al., 2005; Shields et al., in review). Epizootics of Hema- DFO surveys in Conception Bay during October 1998 todinium have been linked to synchronized molting events (n = 28), February 2005 (n = 200), and October 2005 of snow crabs (Shields et al., in review). Cannibalism (n = 25) research cruises. Crabs were examined for sex, car- may be a mode of transmission of the disease (Meyers apace width (CW), shell condition, maturity status, color et al., 1987; Dawe, 2002), but there is at present little evi- change of carapace, and other macroscopic indications of dence for this mode of transmission in snow crabs (Shields BCD. Shell condition was categorized as in Taylor et al. et al., 2005). (1989) to determine the molt status of the animal. BCD In crab and Norway lobster (Nephrops norvegicus) hosts, infections were diagnosed by a pronounced color change the rapid proliferation of the parasite in the hemolymph, of the carapace giving the crabs a pink, cooked appearance and its high metabolic requirements during growth, (see Fig. 1 in Pestal et al., 2003). Heavily infected crabs also decrease the protein and carbohydrate reserves of the host exhibit other gross signs of BCD such as lethargic behav- leading to host morbidity and eventually mortality (Stenti- ior, the presence of a solid white ventrum, milky or chalky ford et al., 2000, 2001; Shields et al., 2003; Stentiford and colored hemolymph, and cream-colored hearts and gills Shields, 2005). The depletion of energy reserves coupled (Shields et al., 2005). with changes in the oxygen-binding characteristics of host hemocyanin, results in lethargy of heavily infected Norway 2.2. Histopathological examination lobsters (Taylor et al., 1996). Additionally, tissues of infect- ed animals likely experience hypoxia, with resultant ische- After physical parameters were recorded, the hepato- mia, because of tissue destruction caused by numerous pancreas, heart, and gills were removed and preserved in parasites (Stentiford and Shields, 2005). Death probably either Bouin’s solution or Safe-Fix (Fisher). After 48 to results from massive tissue disruption, caused by sporula- 168 h, tissues were transferred into 70% EtOH and tion of the parasite, or through depletion of the host’s met- shipped to the Virginia Institute of Marine Science abolic reserves. (VIMS). Gill tissues were decalcified using the formic Several studies have described aspects of the histopathol- acid–sodium citrate method (Luna, 1968). All tissues were ogy of Hematodinium infections in various crustacean hosts. processed using paraffin histological techniques and stained The pathology includes occlusion of hemal spaces by the with Mayer’s hematoxylin and eosin (Luna, 1968). Pre- parasite (Meyers et al., 1987; Field et al., 1992; Hudson pared sections were examined for the presence of Hemato- and Shields, 1994; Messick, 1994; Field and Appleton, dinium sp. with an Olympus BX51 compound microscope 1995; Stentiford et al., 2002; Sheppard et al., 2003), effects and photographs were taken using a Nikon DXM1200 dig- on respiratory function and gill structure (Field et al., ital camera with aid of the ACT-1 computer program 1992; Hudson and Shields, 1994; Field and Appleton, (Nikon). 1995; Taylor et al., 1996; Sheppard et al., 2003), and dam- age to muscle fibers (Meyers et al., 1987; Field et al., 2.3. Histological examination 1992; Hudson and Shields, 1994; Messick, 1994; Stentiford et al., 2002; Sheppard et al., 2003). However, the effects of Sections of gill, heart, and hepatopancreas were exam- Hematodinium infections on the tissues of the snow crab ined for the presence of Hematodinium. When present, and pathological changes during sporulation events have the life history stage and the relative intensity of the para- not been specifically addressed. site were noted. The life history stages of the parasites in In this study, we documented the histopathological the tissues were categorized as single cells, filamentous alterations to the tissues in heavy infections and alterations trophonts, clump colonies, and plasmodial sheets (cf. K. Wheeler et al. / Journal of Invertebrate Pathology 95 (2007) 93–100 95 arachnoid sporonts, see Appleton and Vickerman, 1998). possible synchronization of the BCD infections with sea- The intensity of infection was rated as follows: no parasite son. In addition, most of the infected tissues had the plas- cells present, light infection (1–5 parasites per microscopic modial sheet form of the parasite, not single cell or clump field at 200·), moderate infection (5–20 parasites per field), colony forms.