Influence of parasitism in controlling the health, reproduction and PAH body burden of petroleum seep mussels

Deep-Sea Research I 46 (1999) 2053}2078

In#uence of parasitism in controlling the health, reproduction and PAH body burden of petroleum seep mussels Eric N. Powell *, Robert D. Barber , Mahlon C. Kennicutt II
, Susan E. Ford Haskin Shellxsh Research Laboratory, Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, 6959 Miller Ave., Port Norris, NJ 08349-9736, USA
Geochemical and Environmental Research Group Texas A & M University, College Station, TX 77843-3146 USA Received 14 April 1998; received in revised form 16 October 1998; accepted 15 December 1998

Abstract Petroleum seep mussels are often exposed to high hydrocarbon concentrations in their natural habitat and, thus, o!er the opportunity to examine the relationship between parasitism, disease and contaminant exposure under natural conditions. This is the "rst report on the histopathology of cold-seep mussels. Seep mussels were collected by submersible from four primary sites in the Gulf of Mexico, lease blocks Green Canyon (GC) 184, GC-234, GC-233, and Garden Banks 425 in 550}650 m water depth. Five types of parasites were identi"ed in section: (1) gill `rosettesa of unknown a$nity associated with the gill bacteriocytes, (2) gill `inclusionsa similar to chlamydia/rickettsia inclusions, (3) extracellular gill ciliates, (4) body `inclusionsa that also resemble chlamydial/rickettsial inclusions, and (5) Bucephalus-like trematodes. Comparison to shallow-water mytilids demonstrates that: (1) both have similar parasite faunas; (2) seep mytilids are relatively heavily parasitized; and (3) infection intensities are extremely high in comparison to shallow-water mytilids for Bucephalus and chlamydia/ rickettsia. In this study, the lowest prevalence for chlamydia/rickettsia was 67%. Prevalences of 100% were recorded from three populations. Bucephalus prevalence was *70% in three of 10 populations. The parasite fauna was highly variable between populations. Some important parasites were not observed in some primary sites. Even within primary sites, some important parasites were not observed in some populations. Bucephalus may exert a signi"cant in#uence on seep mussel population dynamics. Forty percent of the populations in this study are severely reproductively compromised by Bucephalus infection. Only a fraction of petroleum seep mussel

* Corresponding author. Fax: #1-856-785-1544. E-mail address: [email protected] (E.N. Powell) 0967-0637/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 9 ) 0 0 0 3 5 - 7

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E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

populations are maintaining the entire beta-level population structure of this species. Variation in two parasites, gill ciliates and Bucephalus, explained most of the variation in PAH body burden between mussel populations. PAHs are known to be sequestered preferentially in gametic tissue. Bucephalus would be expected to reduce overall body burden, at high infection intensities, by replacing gametic tissue. PAH concentrations exceeded 1 ppm in 4 of 9 populations, a ratio signi"cantly higher than the 8 of 30 mussel locales in the NOAA Mussel Watch Program. Only "ve Mussel Watch locales exceeded the highest value for a petroleum seep population. Digestive gland and gill tissue atrophy were not signi"cantly correlated with PAH body burden, even though some populations were characterized by body burdens exceeding 1 ppm, suggesting that seep mussels may not be as sensitive to PAH exposure as are some shallow-water mytilid populations.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Gulf of Mexico petroleum seeps are cold seeps in which a major source of labile organic carbon is from petroleum migration from deep-seated reservoirs to the near surface (Kennicutt II et al., 1988). As with other cold seeps (e.g., Paull et al., 1984; Olu et al., 1996; Callender and Powell, 1999), petroleum seeps are inhabited by a number of large organisms possessing chemoautotrophic symbiotic bacteria that utilize methane or sul"de oxidation (Fisher, 1990). Among the important taxa are mussels which, in the most common species, possess methane-oxidizing symbionts (Nix et al., 1995). Mussels are obligately associated with petroleum and brine seeps (MacDonald et al., 1989,1990a,b) so that polynuclear aromatic hydrocarbon (PAH) body burdens are elevated in most populations (Wade et al., 1989). The importance of parasitism and disease in seep mussel populations has not been studied. Mussels commonly harbor parasites and diseases of a variety of types (Gee and Davey, 1986; Kent et al., 1989; Kim et al., 1998), and, in some cases, these parasites and diseases can signi"cantly a!ect health and fecundity (Bierbaum and Ferson, 1986; Coustau et al., 1991; PeH rez et al., 1997). The concept that reduced health, brought on by pollutants, limitations in food availability, and other stressors, results in increased susceptibility to a range of parasites and diseases in molluscs was initially propounded by Laird (1961) and has received support from a wide array of subsequent studies. Recent work has focused on immune suppression by certain pollutants (Cheng, 1988; Anderson et al., 1992), the relationship of pollutant exposure to common parasiteinduced diseases (Sindermann, 1983; Winstead and Couch, 1988; Wilson-Ormond et al., 1992), and the in#uence of environmental changes in initiating and terminating disease processes. Modeling e!orts (e.g., Hofmann et al., 1995; Powell et al., 1996) provide a theoretical underpinning for how small changes in environment can produce large changes in the prevalence and intensity of parasitism and disease. WilsonOrmond et al. (1999) provide an example from the Gulf of Mexico where parasite infection intensity was signi"cantly a!ected by nearness to oil and gas production activity, and analysis of NOAA National Status and Trends (Mussel Watch) data for the Gulf of Mexico has identi"ed the in#uence of long-term changes in climate on organism health (Kim and Powell, 1998) and related parasite abundances to

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2055

contaminant body burden in a number of parasite/contaminant pairs (Kim et al., 1999). Because of the close association of petroleum seep mussels with hydrocarbon seepage, parasites, diseases, and tissue pathologies may play an important role in population dynamics. The prevalence and intensity of parasites, diseases, and tissue pathologies may provide a valuable tool for comparing the health of populations with variations in site chemistry and may provide a useful early warning signal of longterm changes in the health of seep communities that may eventually result in local extinction. Evidence indicates that site chemistry varies on time scales as short as weekly to at least as long as decadal (Callender and Powell, 1999). Long-term changes eventually produce signi"cant changes in the biota, including a complete demise of the seep community. Stratigraphic studies show that seep communities disappear (and appear) relatively rapidly on a geological time scale, perhaps over periods of a few decades. However, these changes are relatively slow on a human time scale. Stratigraphic studies also indicate that time periods of hundreds of years are required to reestablish a seep community once it has disappeared, although it may reappear relatively quickly once the process begins. Finally, stratigraphic studies indicate that populations wax and wane in abundance and health (as measured by adult size) during a period of relative stability of the seep community. The waxing and waning of populations is likely tied to the availability of sul"de and methane. Discriminating populations that are in decline from those that are healthy and correlating the state of these populations with the availability of reduced molecules is, therefore, a key component to understanding the processes controlling the structure and persistence of seep communities. Petroleum seep mussels are continuously exposed to relatively high hydrocarbon concentrations in their natural habitat and, thus, o!er the opportunity to examine the relationship between parasitism, disease and contaminant exposure under natural conditions. The purpose of this study was to (1) document the parasite body burdens in petroleum seep mussels, (2) determine whether parasite body burdens varied spatially between nearby populations and between populations separated on larger scales, (3) evaluate the degree to which parasites might impact population dynamics and the persistence of seep communities and (4) examine the relationship between parasite and PAH body burdens.

2. Methods Seep mussels, often referred to as Bathymodiolus sp. or Seep Mytilid Ia, were collected by the Johnson-Sea-Link in August, 1997, from four disjunct sites in the Gulf of Mexico south of the Barataria Basin of Louisiana, lease blocks Green Canyon (GC) 184 (Bush Hill } MacDonald et al., 1989) (27346.9N 91330.4W, 550}580 m), GC-234

 Possibly, some mussels from GB-425 are Seep Mytilid II (Fisher, pers. comm.). Seep Mytilid II has sul"de-oxidizing symbionts rather than methane-oxidizing ones.,

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Table 1 Description of sampling sites Location

Position

Depth (m)

Comments

27343.4245 91316.7616 27343.4392 91316.7638

649 648 649

N side of pool at inner edge of mussels N side of pool towards outer edge of mussels Directly in from M2 at center of mussel bed

27347.0195 91330.4940

Northern end of Bush Hill Directly west of M1

27346.9461 91330.4961 27346.9994 91330.4605

552 546 540 543

GC-234 M1 M2

27344.7692 91313.3022 27344.7673 91313.2921

538 535

GB-425 M1

27333.1887 92332.4449

567

Brine pool (GC-233) M1 M2 M3 Bush hill (GC-184) M1 M2 M4 M5

(tubeworm site } Fisher et al., 1997) (27344.1N 91315.3W, 525}560 m), GC-233 brine pool (MacDonald et al., 1990c) (27343.4N 91316.8W, 640 m), and Garden Banks (GB) 425 (27333.2N 92332.4W, 600 m). Each of these sites has been extensively studied. More details can be found in Callender and Powell (1997), Carney (1994), and MacDonald et al. (1990a,b). Details of speci"c locations sampled are given in Table 1. Collected mussels were "xed in Davidson's "xative immediately after surfacing following the NOAA Status and Trends protocols (Ellis et al., 1998a). Dissection, embedding and staining procedures followed the same protocol. All assessments were based on quantitative measures or semiquantitative scales so that reproductive stage and parasite/pathology infection intensity could be rigorously evaluated statistically. Direct appraisal of this approach during the GOOMEX program (Wilson-Ormond et al., 1999) showed the power of using quantitative scales of infection intensity, rather than just prevalence. Nearly all statistically signi"cant relationships were observed using quanti"ed measures. Earlier work in the NS&T program had indicated that this approach would be advantageous (Wilson et al., 1990,1992). Friedman et al. (1997) provide a recent comparison example for abalone. Reproductive stage and certain pathologies like digestive gland atrophy were assigned semiquantitative scales describing stage of development (for the former) or severity of the e!ect (for the latter) (Tables 2 and 3). Most parasites were quanti"ed by tallying the number of specimens in consistently obtained tissue cross-sections (e.g., Ellis et al., 1998b; Sericano et al., 1993). The exception was wide-spread, ramifying, or invasive parasites such as certain trematode infections, which were also quanti"ed using a semiquantitative infection scale (Table 4).

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Table 2 Semiquantitative scale used for gonadal stage (Ellis et al., 1998a) Reproductive stage

Description

Resting/spent gonad Stage 0

Inactive or undi!erentiated

Developing gonad Stage 1 Stage 2 Stage 3 Stage 4 Ripe gonad Stage 5

Spawning gonad Stage 4 Stage 3 Stage 2 Stage 1

Gametogenesis has begun; no ripe gametes visible Ripe gametes present; gonad developed to about one-third of its "nal size Gonad increased in mass to about half the fully ripe condition; each follicle contains, in area, about equal proportions of ripe and developing gametes. Gametogenesis still progressing, follicles contain mainly ripe gametes

Gonad fully ripe, early stages of gametogenesis rare; follicles distended with ripe gametes; ova compacted into polygonal con"gurations; sperm with visible tails

Active emission has begin; sperm density reduced; ova rounded o! as pressure within follicles is reduced Gonad about half empty Gonadal area reduced; follicles about one-third full of ripe gametes Only residual gametes remain; some may be undergoing cytolysis

Table 3 Semiquantitative scale used for gill and digestive gland atrophy. This scale should be distinguished from the Status and Trends protocol for digestive gland atrophy (Ellis et al., 1998b), which focuses on the thickness of digestive tubule walls Intensity scale

Description

0 1

No atrophy; all tissues appear healthy Minor tissue degeneration, usually localized and restricted to epithelial or connective tissues Moderate tissue degeneration; widespread or localized, a!ecting epithelial/or connective tissue Severe, widespread tissue degeneration, with most tissues shrunken and heavily necrotic

2 3

Mussels used for PAH analysis were frozen immediately after collection. PAHs were analyzed following the NOAA Status and Trends `Mussel Watcha protocol (Wade et al., 1993). In order to permit easy comparison to Mussel Watch data, a series of 18 PAHs were summed and reported as total PAHs (Wade et al., 1988; Jackson et al., 1994). These PAHs were naphthalene, 2-methylnaphthalene, 1-methylnaphthalene, biphenyl, 2,6-dimethylnaphthalene, acenaphthalene, #uorene, phenanthrene, anthracene, 1-methylphenanthrene, #uoranthene, pyrene, benz[a]anthracene, chrysene, benzo[e]pyrene, benzo[a]pyrene, perylene, and dibenz[a,h]anthracene.

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Table 4 Semiquantitative scale used for Bucephalus infection (Ellis et al., 1998b). Score

Description

0 1 2

Uninfected Present in the gonads only (some gametic tissue still present) Completely "lling the gonads (no gametic tissue present); may be present in digestive gland or gills in very limited amounts Completely "lling the gonads; extensive invasion of the digestive gland and/or the gills Completely "lling the gonad; substantially "lling the digestive gland or gill; individuals appear to be a sac of sporocysts

3 4

3. Results 3.1. Description Five types of parasites were identi"ed in section: gill `rosettesa, gill `inclusionsa, gill ciliates, body `inclusionsa, and Bucephalus-like trematodes. Gill `rosettesa of unknown a$nity are associated with the bacteriocytes in the gill. The cells show no internal structure at the light microscope level (Fig. 1a}c). Each rosette is characterized by a group of 12}28 variable-sized (2}5 lm) rounded cells within or breaking through the gill. The vacuoles containing the cells vary in diameter from 11 to 47 lm. Rosette vacuoles appear to originate at the basement membrane of the gill "laments, migrate to the surface and frequently move through the gill surface, coming to lie outside of the tissue between the gill "laments. With the exception of the location where the rosette breaks through the gill surface, no obvious tissue pathology was observed. Gill `rosettesa resemble the free inclusion bodies observed by Johnson and Pennec (1995) (their Figs. 1}4) in Loripes lucinalis. Gill `inclusionsa are similar to chlamydia/rickettsia inclusions (e.g., Buchanan, 1978; Meyers, 1979; Fries et al., 1991), but have a more visible structure, and often appear as a group of tightly compacted strands usually in a swirled con"guration. For simplicity, we refer to these as rickettsia hereafter. These rickettsial-like bodies were

Fig. 1. Gill `rosettesa } 1a,b: 1000x; 1c: 25x.

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Fig. 2. Rickettsial-like bodies in the gill } 2a: 1000x; 2b: 25x.

Fig. 3. Gill ciliates between the demibranchs } 3a,b: 1000x.

usually located at or near the distal end of the thickened tissue of the gill "laments that surround the water tubes (Fig. 2a and b). Average size is 9.9 by 15 lm, but they vary greatly in size. No obvious tissue pathology was observed. Extracellular gill ciliates, averaging 21 lm by 11.4 lm, are found between the gill "laments or near the base of the gill demibranchs (Fig. 3a and b). The ciliates appear to be freely mobile on the gill surface. Whether these ciliates are parasitic or adventitious commensals has not been determined. Body `inclusionsa are also chlamydial/rickettsial-like inclusions, but, as with the gill rickettsia, have a more visible structure and often appear as a group of

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Fig. 4. Rickettsial-like bodies in the digestive tubules and diverticula } 4a: 1000x; 4b: 10x.

tightly-compacted short rods that are in multidirectional groups. Occasionally, similar bodies are observed that appear smooth, without internal structure. Johnson and Pennec (1995) observed a very similar organism in Loripes lucinalis (their Figs. 1}3), which they believed to be a Chlamydia. For simplicity, we refer to these as rickettsia hereafter. These rickettsia are found in the digestive diverticula or the digestive tubules (Fig. 4a and b). Size is highly variable, but averages 26.6 lm by 20.9 lm, about twice the size of the rickettsia in the gills. Besides the substantial size di!erence, signi"cant di!erences in distribution pattern among sites strongly suggest that body and gill rickettsia are distinctive organisms. No obvious tissue pathology was observed. Bucephalus-like trematodes (e.g., Hopkins, 1957,1958; Wardle, 1990) were observed invading the gonadal material and, in heavier infections, other body compartments. The sporocysts are very similar to the Bucephalus trematodes found in shallow-water oysters and mussels, except that the mature cercariae are very large (Fig. 5). These will be referred to hereafter as Bucephalus for simplicity, although we caution the reader that the trematodes observed in these mussels have not been unequivocally assigned to that genus. 3.2. Spatial distribution Infection intensity of gill parasites was signi"cantly higher at the brine pool than at the other sites (Tables 5 and 6). Much of this di!erence was due to a signi"cantly

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Fig. 5. Bucephalus in the gonad } 10x.

higher prevalence and infection intensity of gill rickettsia. Gill parasites were not observed at GC-234 or at 3 of 4 Bush Hill sites, but were present in over half of all brine pool specimens. Body rickettsia were prevalent at all sites and in all populations, never less than 67%, but prevalence at GC-234 was signi"cantly higher than at the brine pool (Table 6). Prevalence of Bucephalus was much higher at Bush Hill than at the other sites; Bucephalus was not recorded at the brine pool or GB-425 but reached prevalences as high as 100% at Bush Hill and 80% at GC-234. Some parasites were not present at certain sites. Gill rosettes were not found at Bush Hill or GC-234. Bucephalus was unrecorded at the brine pool or at GB-425. Gill rickettsia were not observed at GC-234. Other parasites were found at all sites commonly (e.g., body rickettsia) or rarely (e.g., gill ciliates). Mussel bed-to-mussel bed variation within a site was high. All gill parasites recorded from Bush Hill were recorded from only one of the four Bush Hill samples. Gill parasites were not present at the other three. Bucephalus prevalence ranged from 18 to 100% at Bush Hill. Body rickettsia infection intensity ranged from 4.9 to 58 at Bush Hill (Table 5). Thus, in general, each mussel bed had a relatively unique parasite fauna and infection intensity. 3.3. Correlations between parasite prevalence and intensity Prevalences and infection intensities of the three gill parasites were normally highly correlated. That is, mussel beds having high prevalences or infection intensities for one gill parasite normally had high prevalences or infection intensities for all gill parasites

N

10 3 8

11 8 10 10

10 10

7

Location

Brine pool M1 M2 M3

Bush hill M1 M2 M4 M5

GC-234 M1 M2

G-B-425 M1

29

0 0

0 0 0 0

30 0 88

0.6

0.0 0.0

0.0 0.0 0.0 0.0

4.3 0.0 22.6

86

0 0

0 13 0 0

30 67 100

3.7

0.0 0.0

0.0 0.1 0.0 0.0

0.9 25.3 51.5

Infection intensity

Prevalence (%)

Prevalence (%)

Infection intensity

Gill chlamydia/rickettsia

Gill rosettes

0

0 0

0 13 0 0

30 0 25

Prevalence (%)

Gill ciliates

0.0

0.0 0.0

0.0 0.1 0.0 0.0

0.5 0.0 0.6

Infection intensity

100

90 100

100 88 90 90

80 67 88

Prevalence (%)

30.3

46.0 24.1

58.0 4.9 46.0 7.7

28.9 19.7 30.9

Infection intensity

0

80 20

18 100 70 40

0 0 0

Prevalence (%)

Body chlamydia/rickettsia Bucephalus sp.

0.0

1.9 0.3

0.5 3.8 2.0 1.2

0.0 0.0 0.0

Infection intensity

Table 5 Prevalences and infection intensities for the "ve types of parasites identi"ed for each of the four primary sites. In cases where sites were sampled more than once, each sample came from a discrete mussel bed. Infection intensities are counts per tissue cross-section, except for Bucephalus, which is rated on a 0-to-4-point scale

2062 E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

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Table 6 Comparison of sites for which greater than one sample was available. Same letters indicate no signi"cant di!erence at a"0.05 using Tukeys Studentized Range Test. Totals do not include Bucephalus Brine pool

Bush hill

GC-234

Gill rosettes Prevalence Infection intensity

A A

A A

A A

Gill chlamydia/rickettsia Prevalence Infection intensity

A A

B B

B B

Gill ciliates Prevalence Infection intensity

A A

A A

A A

Total gill parasites Infection intensity

A

B

B

Body chlamydia/rickettsia Prevalence Infection intensity

A A

AB A

B A

Total parasites Infection intensity

A

A

A

Bucephalus sp. Prevalence Infection intensity

A A

B B

B AB

Gonadal stage Gill tissue atrophy Digestive gland atrophy PAH body burden

A A A A

A A A A

A A A A

(Tables 7 and 8). The only exception was gill ciliates, an extracellular commensal, which was not correlated with gill rickettsia, an internal parasite. Body parasites were essentially uncorrelated with each other and with any gill parasite. Body rickettsia were not correlated with any other parasite, even gill rickettsia that appeared, in light microscope magni"cation, to be similar, suggesting that the two types of inclusions represent two distinct types of rickettsial-like organisms. The other body parasite, Bucephalus, was also weakly correlated with other parasites. 3.4. Correlation with other health indices General tissue health was assessed by digestive gland atrophy, gill tissue atrophy, and gonadal development (Table 9). No signi"cant di!erences were found between

Gill rosette 0.009 Gill chlamydia/ rickettsia Gill ciliates Body chlamydia/ rickettsia Bucephalus sp. Gonadal stage Gill atrophy Digestive gland atrophy

Gill chlamydia/ rickettsia 0.02 }

Gill ciliates

} (0.05) } }

}

Bucephalus sp.

} }

Body chlamydia/ rickettsia

(0.0009)

} }

} 0.05

Gondal stage

} (0.01)

} }

} }

Gill atrophy

} (0.01) 0.0003

} }

} }

Digestive gland atrophy

} } } }

(0.04) 0.03

} }

PAH body burden

Table 7 P values (a)0.05) for Spearman's Rank correlations for parasite prevalence and measures of general health. Parentheses indicate negative correlations

2064 E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

Gill rosette Gill chlamydia/ rickettsia Gill ciliates Total gill parasites Body chlamydia/ rickettsia Total parasites Bucephalus sp. Gonadal stage Gill atrophy Digestive gland atrophy

0.001

Gill chlamydia/ rickettsia

0.002 }

Gill ciliates

0.02

0.0001 0.0001

Total gill parasites

} }

} }

Body chlamydia/ rickettsia

} 0.005 }

0.02 0.008

Total parasites

} } } } (0.002)

}

} }

Gondal stage

} } }

} }

Bucephalus sp.

} } (0.03) (0.01) 0.03 (0.01) 0.0003

(0.04) } (0.01)

} }

Digestive gland atrophy

} } (0.02)

} }

Gill atrophy

} } } } }

} } }

} }

PAH body burden

Table 8 P values (a)0.05) for Spearman's Rank correlations for parasite infection intensity and measures of general health. Parentheses indicate negative correlations

E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078 2065

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Table 9 Health indices and PAH body burden (ppb) for mussels taken from each of the four primary sites. In cases where sites were sampled more than once, each sample came from a discrete mussel bed Location

Gonadal stage

Gill tissue atrophy

Digestive gland atrophy

PAH body burden

Brine pool M1 M2 M3

3.5 3.0 4.4

0.7 0.7 0.5

0.5 0.7 0.1

565 } 214

Bush Hill M1 M2 M4 M5

3.1 0.0 0.5 0.0

0.0 2.1 0.7 2.7

0.3 2.4 1.1 2.4

1346 283 697 1135

GC-234 M1 M2

0.8 3.8

1.1 0.5

0.5 0.5

614 1065

GB-425 M1

4.0

0.6

1.0

1160

primary sites for any health index (Table 6). Signi"cant mussel bed-to-mussel bed variability did exist within primary sites, however. For example, gonadal stage varied from 0 (no gonad present) to 3.1 on a 0-to-5-point scale, and gill tissue atrophy varied from 0 (no atrophy) to 2.7 on a 0-to-3-point scale at Bush Hill. Gonadal stage was negatively correlated with gill tissue atrophy and digestive gland atrophy (Table 7). The latter two were tightly correlated. Thus, healthy mussels were reproductively active; unhealthy mussels were not. Unlike many shallow water molluscs, tissue atrophy appeared to be unambiguously related to poor health in these animals. In no case were sites signi"cantly di!erent from one another in overall health (Table 6), however bed-to-bed variation within sites was often high, as it was with parasite prevalence and infection intensity. Gill parasite prevalence and infection intensity were unrelated to gonadal stage or tissue health. Body parasites had signi"cant e!ects on both, however. Bucephalus is known to sterilize its host (Hopkins, 1957), and, indeed, gonadal development was strongly negatively correlated with Bucephalus infection. Infection intensity, but not prevalence, of body rickettsia was strongly negatively correlated with gill and digestive gland tissue atrophy. That is, high numbers of body rickettsia were associated with relatively healthy mussels (low atrophy scores). The opposite was true for Bucephalus, where higher infection intensities were associated with higher levels of digestive gland atrophy. Accordingly, with the exception of Bucephalus, high infection intensities were not associated with reduced tissue health. Put another way, Bucephalus seemed to be the overwhelming determinant of mussel health. Regression analysis con"rmed that Bucephalus infection intensity was the single important determinant of gonadal stage (Table 10). The combination of Bucephalus

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Table 10 Best two-variable model (highest R) yielding an R'0.70 R

Variables

Ginadal stage Digestive gland atrophy

0.73 0.75

Gill tissue atrophy

0.72

Total body parasites Total gill parasites

0.78 }

Bucephalus infection intensity Body chlamydia/rickettsia infection intensity, Bucephalus infection intensity Body chlamydia/rickettsia infection intensity, Bucephalus infection intensity Gill tissue atrophy, gonadal stage None

PAH body burden PAH body burden PAH body burden

0.81 0.86 0.87

Gill ciliate prevalence, Bucephalus prevalence Gill ciliate infection intensity, Bucephalus infection intensity Total gill parasites infection intensity, Bucephalus infection intensity

infection intensity and the number of body rickettsia explained most of the variation in gill and digestive gland tissue atrophy. 3.5. Correlation with PAH body burden For ease of comparison to Mussel Watch data, total PAHs are reported as the sum of the 18 PAHs listed in the Methods section and hereafter referred to simply as PAHs. The PAH source for all mussel populations sampled is leakage of subsurface oil to the nearsurface and expulsion into the water column, typically associated with the ebullition of gas (mostly methane). Low molecular weight compounds, being more water soluble, should contribute most to tissue body burden in this system, and, indeed, most PAH was contributed by the naphthalenes and #uorenes; however, a signi"cant presence of phenanthrenes was also evident. No signi"cant di!erences in body burden existed between primary sites; however total PAH body burden di!ered considerably between populations from di!erent mussel beds within a site (Table 9). PAH body burden was not correlated with any measure of health (Table 7). Body burden was negatively correlated with gill ciliate prevalence and positively correlated with body rickettsia prevalence (Table 7), but was not correlated with any measure of infection intensity (Table 8). Nevertheless, the best two-variable models consistently explained 80% or more of the variation in PAH body burden using Bucephalus prevalence or infection intensity and gill ciliate prevalence or infection intensity (Table 10). No correlation exists in prevalence or infection intensity between these two parasite types. 4. Discussion 4.1. Comparison to shallow-water mussels The NOAA Status and Trends `Mussel Watcha Program sampled about 70 mytilid mussel populations on the eastern and western coasts of the United States yearly for

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the past 13 years (Lauenstein et al., 1993). In 1997, as an example, Bucephalus spp. were observed in Mytilus edulis at seven east coast locales, but never at an infection intensity exceeding 0.4 on the 0-to-4-point infection scale. [We use the term locale to refer to the site groupings de"ned in Kim et al. (1998). A locale typically represents all of the sites in a single watershed, such as a single estuary, or a group of nearby sites on an open coast line.] Gill ciliates were also commonly observed in Mytilus edulis and Mytilus californianus at nearly every locale on both coasts. Highest infection intensity was 7.5 observations per tissue cross-section at one locale (Boston Harbor); however, no other locale exceeded 3. Values above 0.5 were common, however. Nematodes were also frequently observed in east coast Mytilus edulis. Rickettsia were extremely rare, and the gill `rosettesa observed in petroleum seep mussels were not observed (Kim et al., 1998). Infection intensities of all parasites, excluding Bucephalus, reached 9 in M. edulis and M. californianus, but were typically around 2. Overall, in comparison to shallow-water mussel populations sampled by the Mussel Watch program, parasite infection intensities were much higher, by a factor of 10, in seep mussels. Two of the three important parasites in seep mussels, Bucephalus and gill ciliates, were also among the common parasites in shallow water mussels. Infection intensity of gill ciliates was relatively low in seep mussels in comparison to many shallow-water locales. The opposite was true for Bucephalus, where infection intensities were much higher in seep mussels than observed in any mytilid or oyster species. Seep mussels were characterized by high rickettsial numbers; no nematodes were observed. The opposite was true for the shallow-water mytilids. Comparison to shallow-water mytilids throughout the east and west coasts of the US, then, demonstrates that: (1) the parasite fauna of the seep and shallow-water mytilids is similar in many respects; (2) seep mytilids are relatively heavily parasitized; and (3) infection intensities are extremely high in comparison to shallow-water mytilids for Bucephalus and rickettsia. Higher prevalence can be due to higher susceptibility, greater exposure per time period, or just simply age. Older animals are exposed for a longer time and, thus, may accumulate more parasite types. Seep mussels are likely signi"cantly older than shallow-water mytilids (Lutz et al., 1985; Powell and Cummins, 1985; Hessler et al., 1988; Heller, 1990) and, thus, age alone may explain the higher prevalences observed. Size is not a factor. Seep mussels are often larger than M. edulis, but normally not nearly as large as M. californianus, and the latter two have about the same parasite body burdens in West coast Mussel Watch locales (Kim et al., 1998). 4.2. Infection intensity The high infection intensities observed in seep mussels are predominantly caused by high prevalences and infection intensities of rickettsia and Bucephalus. These high prevalences and infection intensities are extremely unusual. Chlamydia/rickettsia are common parasites of bivalve molluscs. However, reported prevalences are normally very low, rarely above 20% (Table 11). In this study, the lowest prevalence for rickettsia was 67% (Table 6). Prevalences of 100% were recorded from three populations. Interestingly, Johnson and Pennec (1995) report similar high prevalences from

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Table 11 Example literature reports of chlamydia/rickettsia prevalence and infection intensity Species

Prevalence

Infection intensity

Source

Crassostrea virginica Crassostrea virginica Crassostrea virginica Loripes lucinalis Mercenaria mercenaria Mercenaria mercenaria Mya arenaria Mytilus edulis Mytilus galloprovincialis Pecten maximus Siliqua patula Tellina tenuis

0-60% 0}0.5% 5.6% *85% 8}47% 12.3% 20.2% 0}20% 5.3% 100% 30% )75%

0}9.1 } } } 1}270 } } 0}0.8 } 40 }

1997 NOAA NS&T Couch (1985) Otto et al. (1977) Johnson and Pennec (1995) Meyers (1979) Otto et al. (1977) Otto et al. (1977) 1997 NOAA NS&T Cajaraville and Angulo (1991) Le Gall et al. (1988) Elston and Peacock (1984) Buchanan (1978)

a symbiont-bearing lucinid, Loripes lucinalis, and Le Gall et al. (1988) found 100% prevalence in a population of scallops Pecten maximus. Whether rickettsia are routinely high in symbiont-bearing bivalves in comparison to other bivalve taxa is not known, but two of three reports of high prevalences are from symbiont-containing bivalves. Bucephalus prevalences are nearly always low. Wardle (1988,1990) records some prevalences over 50%, interestingly enough for molluscan species in the Gulf of Mexico o! Texas. Allen (1979) records one putative Bucephalus epizootic in Ostrea lutaria, which apparently was responsible for considerable mortality. However, prevalences typically do not exceed 20% (Table 12). NOAA's Status and Trends program did not record a prevalence over 20% in 67 mussel and oyster locales sampled in 1997 (Kim et al., 1998). In this study, Bucephalus prevalence was *70% in three of 10 populations. Thus, seep mussels have unusually high prevalences and infection intensities of these common parasites. 4.3. Spatial distribution Signi"cant variability was observed in the parasite fauna within and between primary seep sites. Some important parasites were not observed in some primary sites (e.g., Bucephalus). The data caution against the use of transplant experiments in hypothesis-testing in these animals. Even within primary sites, some important parasites were not observed in some populations; gill ciliates and Bucephalus being particularly good examples. Thus, the parasite fauna was highly variable between populations and, potentially, population health varied signi"cantly from one mussel bed to another. Whether these di!erences are environmentally driven or due to chance recruitment events remains unclear. Bucephalus requires a second intermediate host and a de"nitive host (Tennet, 1906; Hopkins, 1954; Cheng, 1969; Calvo-Ugarteburu

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Table 12 Example literature reports of Bucephalus spp. prevalence and infection intensity Species

Prevalence

Infection intensity

Source

Anadara brasiliana Crassostrea madrasensis Crassostrea virginica Crassostrea virginica Crassostrea virginica Crassostrea virginica Crassostrea virginica Donax variabilis Ischadium recurvum Mytilus edulis Pecten alba Periploma margaritaceum Perna perna Perna perna Perumytilus purpuratus Pleuromeris armilla Rangia cuneata Semimytilus algosus

48% 1.7}3.5% 0}20% 5}25% 0.5% (5% 6.5% 2% 1.3% 0}20% 31% 1.5% 2}12% 49% (1% 67% 3.6}11.6% 20}32%

} } 0.0}0.15 } } } } } } 0.0}0.3 } } } } } } } }

Wardle (1990) Joseph (1978) 1997 NOAA NS&T Tennet (1906) Turner (1985) Menzel and Hopkins (1955) Mackin et al. (1950) Hopkins (1958) Wardle (1990) 1997 NOAA NS&T Sanders and Lester (1981) Wardle (1990) Lasiak (1993) Calvo-Ugarteburu and McQuaid (1998) Lasiak (1991) Wardle (1988) Wardle (1990) Lasiak (1991)

and McQuaid, 1998). The distribution of these, as yet unknown, hosts may also be important. Parasite prevalences and infection intensities are often highly variable between populations (e.g., Goater and Weber, 1996; Caceres-Martinez et al., 1996). Examination of NOAA Status and Trends `Mussel Watcha data demonstrates highly variable infection intensities in both East and West coast mussel populations, but prevalences show signi"cant regional continuity in many common parasites. Nevertheless, the absence of important parasites in some populations is a frequent enough occurrence (Kim et al., 1998). Unlike many oyster parasites, such as Nematopsis and Perkinsus marinus, which are present in nearly every population over wide regions (Craig et al., 1989; Wilson et al., 1990; Kim et al., 1998), the distribution of even the most common mussel parasites might be characterized as disjunct, with some populations and some locales free of certain of the important parasite fauna. In this light, the ubiquity of rickettsia inclusions in the body tissue of petroleum seep mussels is highly unusual. Thus, the primary site to primary site di!erences observed in seep mussels do not appear to be particularly unusual in comparison to their shallow-water counterparts, with the one exception, the ubiquity of chlamydia/rickettsia in the body tissue. 4.4. Implications for reproduction Bucephalus infections are initiated in the gonadal tissue of a mollusc (infection intensity 1). As the sporocysts grow and the infection spreads, the gonadal tissue is completely consumed and the parasite begins to invade the body tissue (infection

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intensity 2). In heavy infections (infection intensities 3,4), the bulk of the host tissues have been displaced by the trematode. Although mortalities have rarely been recorded (see Allen, 1979 for an example), the loss of reproductive potential in infected animals is well known (Hopkins, 1957). Loss of reproductive potential by destruction of gonadal tissue or the prevention of formation of gonadal tissue is well described in marine parasitology and a normal result of infection by a diversity of parasitic species (e.g., trematodes } Wardle, 1979; Cheng et al., 1983; Coustau et al., 1991; isopods } Beck, 1980; nematodes } Hagen, 1996). However, these parasites very rarely occur in prevalences high enough to signi"cantly impact host population dynamics. Jensen and Mouritsen (1992) provide an exception. The evidence indicates that Bucephalus exerts a signi"cant in#uence on seep mussel population dynamics. Entire populations are no longer reproductively active. Seep mussel populations were often found to have prevalences exceeding 70% and infection intensities as high as 3.8 on the 0-to-4-point scale. Four of ten populations, including populations at two primary sites, had mean infection intensities exceeding 1. In these populations, reproductive potential had been nearly or completely lost. This is an extraordinary percentage rarely equaled by other marine host-multicellular parasite associations. (Cases of protozoan and bacterial epizootics are, of course, well-described.) Of even more interest is the fact that 50}75% of populations at two primary sites, Bush Hill (GC-184) and GC-234, are heavily impacted by this parasite. Thus, the populations of seep mussels at certain primary sites may be characterized by insu$cient fecundity to maintain themselves. Very likely, only a fraction of petroleum seep mussel populations are maintaining the entire beta-level population structure of this species. [Whether animals ever recover from Bucephalus infections is unknown, but it is generally viewed as unlikely (Tennet, 1906; Lasiak, 1991,1993; Calvo-Ugarteburu and McQuaid, 1998); however, seasonal cycles in cercariae production in some species suggest that a complex temporal pattern in infection may exist (Joseph, 1978; Sanders and Lester, 1981).] Finally, a great many studies have been made of seep mussel physiology and growth. In three of ten populations, mussels were su$ciently heavily infected with Bucephalus that a considerable portion of the tissue providing the experimental medium in physiological experiments would have been trematode tissue. These populations are at the most well-studied sites (GC-184, GC-234). The data argue for a reevaluation of mussel physiological and growth results to evaluate the in#uence of this parasite in our present understanding of seep mussel physiology, growth and population dynamics. 4.5. Measures of tissue atrophy Measures of tissue atrophy were inversely correlated with gonadal stage, which was determined, in large measure, by Bucephalus infection. Digestive gland atrophy has received considerable attention in bivalves. Correlations have been drawn with contaminant body burdens, disease, condition and nutritional state (e.g., Widdows et al., 1982; Axiak et al., 1988; MarigoH mez et al., 1990; Winstead, 1995). Most recent data link digestive gland atrophy to nutritional state and thus suggest a normal

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physiological change rather than a pathology (Winstead, 1995); however, these studies have focused on the thickness of the digestive tubule walls, whereas we have used a more general assessment of tissue integrity in this study. The in#uence of nutrition on tissue structure has not been demonstrated in seep mussels. Gill tissue has not previously been rated in this way, to our knowledge. The in#uence of Bucephalus on overall tissue structure has not previously been quanti"ed, and a relationship was not obvious for mussels or oysters in the NS&T `Mussel Watcha data (our unpubl. observations); however, prevalences and infection intensities were low (Kim et al., 1998). The relationship between tissue structure, parasitism, nutrition and environment in seep mussels will require a more in-depth study to relate cause and e!ect. However, if nothing else, the observed relationships certainly indicate that tissue atrophy in seep mussels is a considerably di!erent variable than in shallow-water bivalves. Healthy mussels, those with low atrophy scores, were reproductively active, averaging a 3 or 4 on the 0-to-5-point scale for gonadal stage. Unhealthy mussels with high atrophy scores were rarely advanced in reproductive condition. Furthermore, gill tissue atrophy was also negatively correlated with gonadal stage, suggesting that tissue atrophy, in seep mussels, as we measured it, is a pathological condition. Body rickettsia were most common in animals with low atrophy scores, possibly indicating a requirement of healthy tissue for infection intensi"cation in this parasite. Generally, rickettsia/chlamydia are not thought to be harmful in molluscs (Otto et al., 1977; Elston and Peacock, 1984; Cajaraville and Angulo, 1991); however, mortalities are frequently seen associated with infections in crustaceans (Johnson, 1984; Sparks et al., 1985; Bower et al., 1996), and occasional reports of pathologies associated with high prevalences are seen in bivalves (Le Gall et al., 1988). 4.6. PAH body burden We summed a series of 18 PAHs to provide a ready comparison to NOAA Status and Trends `Mussel Watcha data. Petroleum seeps are characterized by a single source of contamination for all PAHs, whereas multiple sources are probably the norm in Mussel Watch locales, so comparisons must be interpreted in that light. PAH body burden was relatively high in petroleum seep mussels in comparison to mussel populations sampled by the NOAA Mussel Watch Program. PAH concentrations exceeded 1 ppm in 4 of 9 seep populations, a ratio signi"cantly higher than the 8 of 30 mussel locales in the NOAA Mussel Watch Program. Only 5 Mussel Watch locales exceeded the highest value for a petroleum seep population. Thus, although PAH body burden in seep mussels was not unusually high in comparison to their shallowwater counterparts, on the average, these populations had relatively high PAH body burdens. Lowe and Pipe (1987) and Moore et al. (1989) observed gonadal resorption at PAH concentrations in the range observed in seep mussels. Recognizing that individual PAHs have a range of toxicities, so that comparisons of total PAH body burden between animals exposed to varying mixtures of PAHs must be made cautiously, the fact that we observed no such e!ect on gonadal stage in our analyses suggests that seep mussels may not be as sensitive to PAH exposure as are shallow-water mytilids.

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Variation in two parasites, gill ciliates and Bucephalus, explained most of the variation in PAH body burden between mussel populations. Although the data are admittedly few, so that relationships of this sort must be considered with caution, the amount of variation explained is extremely high by NS&T standards, where the infection intensity of any three parasites rarely explains more than 40% of the PAH body burden (Kim et al., 1998). The higher average body burdens in seep mussels may be responsible for this greater predictability, as many Mussel Watch locales were characterized by low PAH body burdens ((400 ppb). The partial regression for gill ciliates on PAH body burden is negative. The data do not permit a determination of causality, but the size and infection intensity of gill ciliates certainly suggest that ciliates are a surrogate for some other physiological variable. On the other hand, Bucephalus at infection intensities of 1 or higher replaces a substantial fraction of the host tissue, including all of the gametic tissue. PAHs are known to be sequestered in lipid-rich gametic tissue at a much higher concentration than somatic tissue (Friocourt et al., 1985; Ellis et al., 1993) in some bivalves, due to the increased lipid concentration of the gametes. The relationship holds for both eggs and sperm (Ellis et al., 1993). Bucephalus would be expected to reduce overall body burden, at high infection intensities, by replacing gametic tissue. The partial regression of PAH with Bucephalus is negative, as would be anticipated. The three highest PAH body burdens came from populations without Bucephalus and with few gill parasites (the exception was sample M1 at Bush Hill, which had a low PAH body burden and also low parasite levels). The simplest explanation for the observed relationship between PAH and parasites is that PAH body burden was low in some mussels due to the absence of gametic tissue and PAH body burden was low in other mussels in which overall health was low, accounting for the frequency of gill ciliates, which would not have been found in healthier animals. This results in the unanticipated result that higher body burdens are found more frequently in healthy petroleum seep mussels and again suggests that seep mussels are not as sensitive to PAH exposure as are shallow-water mytilids. Acknowledgements This study was funded by a contract from the Minerals Management Service (MMS) through the Gulf of Mexico Regional OCS O$ce (1435-01-96-CT-30813). Submersible support was received from MMS and the NOAA-National Undersea Research Program (Johnson-Sea-Link). We would like to thank the crews of the Johnson-Sea-Link and its support vessel and the entire "eld crew for the MMSChemosynthesis Program for their assistance in sample collection. References Anderson, R.S., Oliver, L.M., Jacobs, D., 1992. Immunotoxicity of cadmium for the eastern oyster (Crassostrea virginica [Gmelin, 1791]): e!ects of hemocyte chemiluminescence. Journal of Shell"sh Research 11, 31}35.

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E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

Allen, R.L., 1979. A yield model for the Foveaux Strait oyster (Ostrea lutaria) "shery. Rapports et Proces-Verbaux des ReH unions Conseil International pour l'Exploration de la Mer 175, 70}79. Axiak, V., George, J.J., Moore, M.N., 1988. Petroleum hydrocarbons in the marine bivalve
E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

2075

Elston, R.A., Peacock, M.G., 1984. A Rickettsiales-like infection in the Paci"c razor clam, Siliqua patula. Journal of Invertebrate Pathology 44, 84}96. Fisher, C.S., 1990. Chemoautotrophic and methanotrophic symbioses in marine invertebrates. Reviews in Aquatic Sciences 2, 399}436. Fisher, C.S., Urcuyo, I., Simpkins, M.A., Nix, E., 1997. Life in the slow lane: growth and longevity of cold-seep vestimentiferans. Pubblicazioni della Stazione Zoologica di Napoli I: Marine Ecology 18, 83}94. Friedman, C.S., Thomson, M., Chun, C., Haaker, P.L., Hedrick, R.P., 1997. Withering syndrome of the black abalone, Haliotis cracherodii (Leach): water temperature, food availability, and parasites as possible causes. Journal of Shell"sh Research 16, 403}411. Fries, C.R., Grau, S.B., Tripp, M.R., 1991. Rickettsiae in the cytoplasm of gill epithelial cells of the soft-shelled clam, Mya arenaria. Journal of Invertebrate Pathology 57, 443}445. Friocourt, M.P., Bodennec, G., Berthou, F., 1985. Determination of polyaromatic hydrocarbons in scallops (Pecten maximus) by UV #uorescence and HPLC combined with UV and #uorescence detectors. Bulletin of Environmental Contamination and Toxicology 34, 228}238. Gee, J.M., Davey, J.T., 1986. Stages in the life history of Mytilicola intestinalis Steuer, a copepod parasite of Mytilus edulis (L.), and the e!ect of temperature on their rates of development. Journal du Conseil International pour l'Exploration de la Mer 42, 254}264. Goater, C.P., Weber, A.E., 1996. Factors a!ecting the distribution and abundance of Mytilicola orientalis (Copepoda) in the mussel, Mytilus trossulus, in Barkley Sound. B.C. Journal of Shell"sh Research 15, 681}684. Hagen, N.T., 1996. Parasitic castration of the green echinoid Strongylocentrotus droebachiensis by the nematode endoparasite Echinomermella matsi: reduced reproductive potential and reproductive death. Diseases of Aquatic Organisms 24, 215}226. Heller, J., 1990. Longevity in Mollusca. Malacologia 31, 259}295. Hessler, R.R., Smithey, M., Boudrias, M.A., Keller, C.H., Lutz, R.A., Childress, J.J., 1988. Temporal change in megafauna at the Rose Garden hydrothermal vent (Galapagos Rift; eastern tropical Paci"c). Deep-Sea Research 35, 1681}1709. Hofmann, E.E., Powell, E.N., Klinck, J.M., Saunders, G., 1995. Modeling diseased oyster populations I. Modelling Perkinsus marinus infections in oysters. Journal of Shell"sh Research 14, 121}151. Hopkins, S.H., 1954. The American species of trematode confused with Bucephalus (Bucephalopsis) haimeanus. Parasitology 44, 353}370. Hopkins, S.H., 1957. Our present knowledge of the oyster parasite `Bucephalusa. Proceedings of the National Shell"sheries Association 47, 58}61. Hopkins, S.H., 1958. Trematode parasites of Donax variabilis at Mustang Island, Texas. Publications of the Institute of Marine Sciences of the University of Texas 5, 301}311. Jackson, T.J., Wade, T.L., McDonald, T.J., Wilkinson, D.L., Brooks, J.M., 1994. Polynuclear aromatic hydrocarbon contaminants in oysters from the Gulf of Mexico (1986-1990). Environmental Pollution 83, 291}298. Jensen, K.T., Mouritsen, K.N., 1992. Mass mortality in two common soft-bottom invertebrates, Hydrobia ulvae and Corophium volutator } the possible role of trematodes. HelgolaK nder Meeresuntersuchungen 46, 329}339. Johnson, M.A., Le Pennec, M., 1995. Association between the mollusc bivalve Loripes lucinalis and a Chlamydia-like organism, with comments on its pathogenic impact, life cycle and possible mode of transmission. Marine Biology (Berlin) 123, 523}530. Johnson, P.T., 1984. A rickettsia of the blue king crab, Paralithodes platypus. Journal of Invertebrate Pathology 44, 112}113. Joseph, M.M., 1978. Observations on the larval trematode Bucephalus sp. parasitic in the oyster Crassostrea madrasensis. Journal of Invertebrate Pathology 32, 381}383. Kennicutt II, M.C., Brooks, J.M., Denoux, G.J., 1988. Leakage of deep, reservoired petroleum to the nearsurface on the Gulf of Mexico continental slope. Marine Chemistry 24, 39}59. Kent, M.L., Elston, R.A., Wilkinson, M.T., Drum, A.S., 1989. Impaired defense mechanisms in bay mussels, Mytilus edulis, with hemic neoplasia. Journal of Invertebrate Pathology 53, 378}386.

2076

E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

Kim, Y., Powell, E.N., 1998. In#uence of climate change on interannual variation in population attributes of Gulf of Mexico oysters. Journal of Shell"sh Research 17, 265}274. Kim, Y., Powell, E.N., Wade, T.L., Presley, B.J., Sericano, J., 1998. Parasites of sentinel bivalves in the NOAA Status and Trends Program: Distribution and relationship to contaminant body burden. Marine Pollution Bulletin 37, 45}55. Laird, M., 1961. Microecological factors in oyster epizootics. Canadian Journal of Zoology 39, 449}485. Lasiak, T., 1991. Bucephalid trematode infections in mytilid bivalves from the rocky intertidal of southern Chile. Journal of Molluscan Studies 58, 29}36. Lasiak, T.A., 1993. Bucephalid trematode infections in the brown mussel Perna perna (Bivalvia: Mytilidae). South African Journal of Marine Science 13, 127}134. Lauenstein, G.G., Harmon, M.R., Gottholm, B.W., 1993. National Status and Trends Program: monitoring site descriptions (1984-1990) for the National Mussel Watch and Benthic Surveillance Projects. National Oceanic and Atmospheric Administration Technical Memorandum. NOS-ORCA-70, pp. 1}358. Le Gall, G., Chagot, D., Mialhe, E., Grizel, H., 1988. Branchial Rickettsiales-like infection associated with a mass mortality of sea scallop Pecten maximus. Diseases of Aquatic Organisms 4, 229}232. Lowe, D.M., Pipe, R.K., 1987. Mortality and quantitative aspects of storage cell utilization in mussels Mytilus edulis, following exposure to diesel oil hydrocarbons. Marine Environmental Research 22, 243}251. Lutz, R.A., Fritz, L.W., Rhoads, D.C., 1985. Molluscan growth at deep-sea hydrothermal vents. Bulletin of the Biological Society of Washington 6, 199}210. MacDonald, I.R., Boland, G.S., Baker, J.S., Brooks, J.M., Kennicutt, M.C., Bidigare, R.R., 1989. Gulf of Mexico hydrocarbon seep communities. II. Spatial distribution of seep organisms and hydrocarbons at Bush Hill. Marine Biology (Berlin) 101, 235}247. MacDonald, I.R., Callender, W.R., Burke Jr., R.A., McDonald, S.J., Carney, R.D., 1990a. Finescale distribution of methanotrophic mussels at a Louisiana cold seep. Progress in Oceanography 24, 15}24. MacDonald, I.R., Guinasso Jr., N.L., Reilly, J.F., Brooks, J.M., Callender, W.R., Gabrielle, S.G., 1990b. Gulf of Mexico hydrocarbon seep communities. VI. Patterns in community structure and habitat. GeoMarine Letters 10, 244}252. MacDonald, I.R., Reilly, J.F., Guinasso Jr., N.L., Brooks, J.M., Carney, R.S., Bryant, W.A., Bright, T.J., 1990c. Chemosynthetic mussels at a brine-"lled pockmark in the northern Gulf of Mexico. Science (Washington D.C.) 248, 1096}1099. Mackin, J.G., Welch, B., Kent, C., 1950. A study of mortality of oysters of the Buras area of Louisiana. Texas A&M University Research Foundation Project 9, Final Report, 41 pp. MarigoH mez, J.A., SaH ez, V., Cajaraville, M.P., Angulo, E., 1990. A planimetric study of the mean epithelial thickness (MET) of the molluscan digestive gland over the tidal cycle and under environmental stress conditions. HelgolaK nder Meeresuntersuchungen 44, 81}94. Menzel, R.W., Hopkins, S.H., 1955. E!ects of two parasites on the growth of oysters. Proceedings of the National Shell"sheries Association 46, 184}186. Meyers, T.R., 1979. Preliminary studies on a chlamydial agent in the digestive diverticular epithelium of hard clams Mercenaria mercenaria (L.) from Great South Bay, New York. Journal of Fish Diseases 2, 179}189. Moore, M.N., Livingstone, D.R., Widdows, J., 1989. Hydrocarbons in marine mollusks: biological e!ects and ecological consequences. In: Varanasi, U. (Ed.), Metabolism of Polycyclic Aromatic Hydrocarbons in the Aquatic Environment. CRC Press, Boca Raton, Florida, pp. 291}329. Nix, E.R., Fisher, C.R., Vodenichar, J., Scott, K.M., 1995. Physiological ecology of a mussel with methanotrophic endosymbionts at three hydrocarbon seep sites in the Gulf of Mexico. Marine Biology (Berlin) 122, 605}617. Olu, K., Sibuet, M., Harmegnies, F., Foucher, J.-P., Fiala-MeH dioni, A., 1996. Spatial distribution of diverse cold seep communities living on various diapiric structures of the southern Barbados prism. Progress in Oceanography 38, 347}376. Otto, S.V., Harshbarger, J.C., Chang, S.C., 1977. Status of selected unicellular eucaryote pathogens, and prevalence and histopathology of inclusions containing obligate procaryote parasites, in commercial bivalve mollusks from Maryland estuaries. Haliotis 8, 285}295.

E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

2077

Paull, C.K., Hecker, B., Commeau, R., Freeman-Lynde, R.P., Neumann, A.C., Corso, W.P., Golbic, S., Hook, J.E., Sikes, E., Curray, J., 1984. Biological communities at the Florida escarpment resemble hydrothermal vent taxa. Science (Washington D.C.) 226, 965}967. PeH rez Camacho, A., Villalba, A., Beiras, R., Labarta, U., 1997. Absorption e$ciency and condition of cultured mussels (Mytilus edulis galloprovincialis Linnaeus) of Galicia (NW Spain) infected by parasites Marteilia refringens Grizel et al. and Mytilicola intestinalis Steuer. Journal of Shell"sh Research 16, 77}82. Powell, E.N., Cummins, H.C., 1985. Are molluscan maximum life spans dominated by long-term cycles in benthic communities? Oecologia (Berlin) 67, 177-182. Powell, E.N., Klinck, J.M., Hofmann, E.E., 1996. Modeling diseased oyster populations. II. Triggering mechanisms for Perkinsus marinus epizootics. Journal of Shell"sh Research 15, 141}165. Sanders, M.J., Lester, R.J.G., 1981. Further observations on a bucephalid trematode infection in scallops (Pecten alba) in Port Phillip Bay, Victoria. Australian Journal of Marine and Freshwater Research 32, 475}478. Sericano, J.L., Wade, T.L., Powell, E.N., Brooks, J.M., 1993. Concurrent chemical and histological analyses: are they compatible? Chemistry and Ecology 8, 41-47. Sindermann, C.J., 1983. An examination of some relationships between pollution and disease. Rapports et Proces-Verbaux des ReH unions Conseil International pour l'Exploration de la Mer 182, 37}43. Sparks, A.K., Morado, J.F., Hawkes, J.W., 1985. A systemic microbial disease in the Dungeness crab, Cancer magister, caused by a Chlamydia-like organism. Journal of Invertebrate Pathology 45, 204}217. Tennet, D.H., 1906. A study of the life-history of Bucephalus haimeanus; a parasite of the oyster. Quarterly Journal of Microscopical Science 49, 635}690. Turner, H.M., 1985. Parasites of Eastern oysters from subtidal reefs in a Louisiana estuary with a note on their use as indicators of water quality. Estuaries 8, 323}325. Wade, T.L., Atlas, E.L., Kennicutt II, M.C., Fox, R.G., Sericano, J., Garcia-Romero, B., DeFreitas, D., 1988. NOAA Gulf of Mexico Status and Trends Program: trace organic contamination distribution in sediments and oysters. Estuaries 11, 171}179. Wade, T.L., Brooks, J.M., Kennicutt II, M.C., McDonald, T.J., Sericano, J.L., Jackson, T.J., 1993. GERG trace organics contaminant analytical techniques. In: Sampling and analytical methods of the National Status and Trends Program National Benthic Surveillance and Mussel Watch Projects 1984}1992. Vol. IV Comprehensive descriptions of trace organic analytical methods, National Oceanic and Atmospheric Administration Technical Memorandum, NOS ORCA, 70, 121}139. Wade, T.L., Kennicutt II, M.C., Brooks, J.M., 1989. Gulf of Mexico hydrocarbon seep communities: Part III. Aromatic hydrocarbon concentrations in organisms, sediments and water. Marine Environmental Research 27, 19}30. Wardle, W.J., 1979. A new marine cercaria (Digenea: Aporocotylidae) from the southern quahog Mercenaria campechiensis. Contributions in Marine Science 22, 53}56. Wardle, W.J., 1988. A bucephalid larva, Cercaria pleuromerae n. sp. (Trematode: Digenea), parasitizing a deepwater bivalve from the Gulf of Mexico. Journal of Parasitology 74, 692}694. Wardle, W.J., 1990. Larval bucephalids (Trematoda: Digenea) parasitizing bivalve molluscs in the Galveston Bay area, Texas. Journal of the Helminthological Society of Washington 57, 5}11. Widdows, J., Bakke, T., Bayne, B.L., Donkin, P., Livingstone, D.R., Lowe, D.M., Moore, M.N., Evans, S.V., Moore, S.L., 1982. Responses of Mytilus edulis on exposure to the water-accommodated fraction of North Sea oil. Marine Biology (Berlin) 67, 15}31. Wilson-Ormond, E.A., Ellis, M.S., Powell, E.N., Kim, Y., Li, S., 1999. E!ects of gas-producing platforms on continental shelf macroepifauna in the northwestern Gulf of Mexico: reproductive status and health. Internationale Revue des gesamten Hydrobiologie. Wilson, E.A., Powell, E.N., Craig, M.A., Wade, T.L., Brooks, J.M., 1990. The distribution of Perkinsus marinus in Gulf coast oysters: its relationship with temperature, reproduction, and pollutant body burden. Internationale Review der gesamten Hydrobiologie 75, 533}550. Wilson, E.A., Powell, E.N., Wade, T.L., Taylor, R.J., Presley, B.J., Brooks, J.M., 1992. Spatial and temporal distributions of contaminant body burden and disease in Gulf of Mexico oyster populations: the role of local and large-scale climatic controls. HelgolaK nder Meeresuntersuchungen 46, 201}235.

2078

E.N. Powell et al. / Deep-Sea Research I 46 (1999) 2053}2078

Winstead, J.T., 1995. Digestive tubule atrophy in Eastern oysters, Crassostrea virginica (Gmelin, 1791), exposed to salinity and starvation stress. Journal of Shell"sh Research 14, 105}111. Winstead, J.T., Couch, J.A., 1988. Enhancement of protozoan pathogen Perkinsus marinus infections in American oysters Crassostrea virginica exposed to the chemical carcinogen n-nitrosodiethylamine (DENA). Diseases of Aquatic Organisms 5, 205}213.