Towards establishing a modern baseline for paleopathology: trace-producing parasites in a bivalve host.

ABSTRACT One hundred one individuals of Protothaca staminea were live-collected from Argyle Lagoon (sand/mud substrate) and Argyle Creek (gravel/sand substrate), San Juan Island, Washington and examined for trace-producing parasite infestation. Eighty-six percent of individuals contained at least one parasite-induced trace. Trematode-induced pits and blisters were identified on 62% of individual clams. Spionid-induced mudblisters and u-shaped borings were identified on 50% and 29% of individuals, respectively. Trematode and spionid parasites were not selective between the left and right valve when infesting the host. Epifaunal clams from Argyle Creek were significantly smaller than their infaunal counterparts from Argyle Lagoon. This size discrepancy between environments may be related to the reduction of growth rates triggered by environmental stress or parasitism, increased susceptibility to durophagous predators, differences in hydrodynamics, or the comparison of different cohorts. Spionid mudblister-infested clams from Argyle Creek are significantly smaller than noninfested clams from the same environment. This suggests that substrate-induced epifaunality and parasite-induced shell weakening reduced the bivalves' defenses against durophagous predators. These results suggest that parasites may negatively affect the survival of infested bivalves. The frequent occurrence of trematode and spionid trace-producing parasites in modern bivalve populations suggests that these traces are common in the fossil record, making the systems amenable to study in deep time.

KEY WORDS: parasitism, trematodes, spionid polychaetes, Protothaca staminea, parasite-host interactions, paleopathology 

INTRODUCTION

Parasitism and predation are important agents of natural selection, and have likely had an important impact on the history of life. Studies of parasite-host and predator-prey interactions have been instrumental in leading to hypotheses such as Red Queen (Van Valen 1973) and Escalation (Vermeij 1987), which have sought to explain various ecological and evolutionary trends (e.g., increasing armament of gastropods and infaunalization of some echinoids) and trends in biodiversity through deep time.

Predator-prey interactions and their evolutionary consequences have been documented extensively in the fossil record (Dietl & Kelley 2001, Kelley et al. 2003, Kowalewski & Kelley 2002, Leighton 1999). The most reliable (and therefore the most extensively-studied) predator-prey interaction preserved in the fossil record is that of predators (most commonly gastropods) that drill holes in their bivalve, gastropod, brachiopod, or echinoderm prey (Kelley & Hansen 2003, Kowalewski et al. 1998, Vermeij 1987). Parasites greatly outnumber predators in terms of number of individuals and diversity. The reporting of parasitism in the fossil record is much less common than the reporting of predation, and studies of long-term trends of parasite-host interactions in the fossil record are rare indeed (Boucot 1990, Brett 1978, Cameron 1967, Clarke 1921, Conway Morris 1981, 1990, Feldman & Brett 1998, Feldmann 1998, Fry & Moore 1969, Gahn & Baumiller 2003, Moodie 1923, Ruiz & Lindberg 1989, Savazzi 1995).

Is the reporting of parasites in the fossil record rare because they are infrequently preserved, or because they are seldom looked for? Perhaps parasite traces are quite common in the fossil record. The purpose of this study is to investigate the occurrence of trace-producing parasites in two populations of modern bivalves. What proportions of the populations are infested with parasites? Are the parasites valve-selective? Is there a relationship between body size and infestation? The answers to these questions will be an important first step toward developing a modern baseline against which to compare infestation of metazoans in the fossil record.

Trace-Producing Parasites of Modern Bivalve Molluscs

Various parasites infest modern bivalve molluscs, but at least two types of parasites leave traces in their bivalve hosts thus making them ideal for study in the fossil record: trematodes (phylum platyhelminthes, class trematoda, order digenea) and spionid polychaetes (family spionidae, genera polydora, boccardia, and pseudopolyclora) (Blake & Evans 1973, Ruiz & Lindberg 1989). Given the ubiquity of these parasites today, traces of their activity should also be common in the fossil record.

Trematodes are a class of parasitic worms some of which spend part of their complex life cycle in bivalve molluscs. Trematode infestation results in damage to or total destruction of the bivalve's gonads, thus resulting in either impeded ability or inability to reproduce. Infestation can influence growth rates of the host in various ways, though the effects are far from straight forward. Parasite-induced castration can lead to increased growth rates and gigantism by either: (1) diverting energy that would have been used in reproduction into growth or by (2) triggering rapid growth as a host adaptation so as to outlive or sequester the parasite. Trematode infestation can also stunt the growth of its host by diverting energy and resources that would have gone into growth (Ballabeni 1995, Sorensen & Minchella 1998; 2001, Taskinen 1998). Trematode infestation represents a significant agent of natural selection to their bivalve hosts. There is no body fossil record for trematodes; however their presence is indicated as oval pits and blisters within the pallial line on the interior of bivalves (Fig. 1). Thin-section examination reveals that growth bands are deformed around these pits (Ruiz & Lindberg 1989). This deformation indicates that the pit formed while the clam was living, thus reducing the possibility of misidentification of such parasitic traces with postmortem borings (i.e., Cliona).

[FIGURE 1 OMITTED]

Spionid polychaetes bore dwellings into bivalves, gastropods, brachiopods, corals, and other hard substrates through chemical dissolution (Blake & Evans 1973, Thayer 1974, Zottoli & Carriker 1974). The spionids gain a cryptic habitat by boring, and weaken the shell of their host; thereby making it more susceptible to its own predators. By narrower definitions of parasitism spionids might not classify as parasites. Spionids do not gain nutrition from their hosts. In fact they are suspension feeders and facultative deposit feeders (Lindsay & Woodin 1992). However, for the purposes of this study I will adopt a broader definition of parasitism as spionid infestation can undermine the structural integrity of the host's shell.

Spionids produce three types of burrows/borings: surface fouling, mud-blisters, and u-shaped borings (Fig. 1). In surface fouling, spionids produce their burrows in accumulated mud on the surface of the shell, and do not penetrate the shell. Mud-blisters are formed when spionids settle on the growing margin of the host. The host attempts to grow around the spionid, and a blister is formed that is filled with mud by the boring worm. U-shaped borings are produced by chemical dissolution of the host's shell. The simplest morphology is produced by a single bore hole that penetrates the shell, turns 180 degrees, and emerges adjacent to the initial boring, producing paired holes at the surface of the shell. U-shaped borings can be more complex with many undulations of the single borehole (Blake & Evans 1973, Zottoli & Carriker 1974).

Surface fouling has little chance of being preserved in the fossil record because it is merely an accumulation of mud on the surface of the shell, and will not be addressed further in this study. U-shaped borings are readily preserved in the fossil record (Blake & Evans 1973), however the fossil record of u-shaped borings could produce ambiguous information about the biotic interaction between parasite and host, as they can be formed pre or postmortem. Mud-blisters have the most potential for elucidating the history of spionid-bivalve interactions in the fossil record because they can only be produced when the host is alive.

MATERIALS AND METHODS

A total of 101 live specimens of the venerid bivalve Protothaca staminea were collected between August 10-17, 2004 from the intertidal sand/mud substrate of Argyle Lagoon (50 infaunal specimens) and the intertidal gravel/sand substrate of Argyle Creek (51 epifaunal specimens: the bivalves cannot burrow in the gravel), San Juan Island, WA, US (Fig. 2).

[FIGURE 2 OMITTED]

Argyle Lagoon is a shallow body of marine water connected to the open waters of Argyle Bay and North Bay via Argyle Creek. Even though Argyle Lagoon is in the intertidal environment, it is sufficiently deep that the bottom is only exposed at the margins during low tide. Argyle Creek is a shallow high-energy tidal creek through which circulation is conducted between Argyle Lagoon and Argyle Bay.

Protothaca staminea is a very common bivalve in shallow sandy environments in the Pacific Northwest region (Kozloff 1993), and is one of the dominant bivalves in the study area (Lazo 2004, Stempien submitted). It is a typical member of the Family Veneridae, one of the most evolutionary and ecologically successful bivalve clades (Cox et al. 1969).

The bivalves were sacrificed and measured for anterior-posterior length and dorsal-ventral width. The numbers of parasite traces (trematode-induced pits and blisters, spionid u-shaped borings and mud-blisters) were counted for the left and right valves. Pits are considered to be diagnostic of trematode infestation, whereas trematodes and other irritants can produce blisters (Ruiz & Lindberg 1989). The pit and blister count may overestimate trematode infestation, but may be a more accurate estimate of total parasite infestation of a host.

To determine if parasites were valve selective within the individuals they infest, the number of valves with pits/blisters, u-shaped borings and mud-blisters were tallied for left and right valves, respectively, from both locations. Each individual was coded with a value of zero (0) if the left valve had more parasitic traces than the right or coded with a value of one (1) if the right valve had more parasitic traces than the left. The data were analyzed using a binomial test with an assumed probability of 0.5.

Each individual was classified for the presence or absence of any parasite trace, trematode pit/blister, spionid mud-blister, spionid u-shaped boring, and by its environment (lagoon or creek). Mann-Whitney U-tests were performed using PAST 1.37 (Hammer et al. 2001) to investigate differences in median size between bivalves from the two environments and between parasitized and nonparasitized bivalves within each environment. An [alpha] = 0.05 was assumed for all statistical analyses. The results of statistical tests for length and width were identical, therefore only length is reported here.

RESULTS

Live-collected specimens of P. staminea from Friday Harbor exhibited very high parasite infestation frequencies. Eighty-six percent of individuals examined in this survey contained at least one parasite-induced trace on its shell (Fig. 3). Seventy-four percent of bivalves from Argyle Lagoon and 98% of bivalves from Argyle Creek contained at least one parasite-induced trace. Sixty-eight percent of Argyle Lagoon bivalves and 57% of Argyle Creek bivalves possess traces attributable to trematode parasites. Spionid mudblisters were located in 24% of Argyle Lagoon bivalves and 75% of Argyle Creek bivalves. Spionid u-shaped borings were found only in Argyle Creek bivalves (57%).

[FIGURE 3 OMITTED]

Trematode and spionid parasites were not valve selective when infesting P. staminea (Table 1). The binomial tests examining valve selectivity of all 3-trace types (trematode pits and blisters, spionid mudblisters, and spionid u-shaped borings) were insignificant when comparing bivalves from each environment (Argyle Creek and Argyle Lagoon) separately.

Protothaca staminea from Argyle Lagoon (M = 45.7 mm) were significantly larger (P < 0.001) than those from Argyle Creek (M = 36.5 mm; Fig. 4). Therefore median sizes of infested versus noninfested bivalves need to be evaluated separately for each environment. There were no significant differences in anterior-posterior length between infested and noninfested bivalves with regards to any parasite trace from either environment with one exception. Protothaca staminea infested with spionid mudblisters (M = 36.0 mm) from Argyle Creek were significantly smaller (P = 0.02) than noninfested individuals (M = 39.3 mm; Fig. 4).

[FIGURE 4 OMITTED]

DISCUSSION

Parasite infestation frequencies of P. staminea reported in this study are very high, and are by and large much larger than values of drilling predation intensity of bivalves, gastropods, brachiopods, and echinoderms reported in the paleontological literature (Kelley & Hansen 2003, Kowalewski et al. 1998, Kowalewski et al. 2005). The traces of these parasites do not indicate the death of the bivalve (as most drill holes on bivalves likely do); however they likely indicate a significant agent of selection. Trematode parasites consume the gonads of their bivalve host, resulting in the loss or limitation of the host's fecundity. The boring and blister-forming behavior of spionid parasites reduces the strength of the bivalve's shell. Weakened shells make bivalves more vulnerable to their durophagous predators (Vermeij 1983, 1987, 2004).

Given the high infestation frequencies of bivalves it is likely that parasite traces on molluscan hosts are common in the fossil record and suffer a lack of attention by invertebrate workers. Such a fossil record of parasite-host interactions would be as useful as the well-studied drilling predator-mollusc/brachiopod/echinoderm prey systems in elucidating organismal interactions in deep time. It is time that workers begin to devote more attention to, not only identifying, but also quantifying, the relationship between invertebrate hosts and their trace-producing parasites.

Spionid infestation frequencies were typically higher in the epifaunal clams from Argyle Creek than in the infaunal clams of Argyle Lagoon. There appears to be little difference in trematode infestation frequencies between infaunal and epifaunal clams. These results are to be expected as spionids are suspension-feeding polychaetes. Feeding is much easier for a suspension-feeder that makes its home on an epifaunal clam rather than on a clam buried in the mud. Trematodes, however, are endoparasites that feed on bivalve gonads; therefore it is not surprising that there is little difference in infestation frequencies between infaunal and epifaunal hosts.

Trace-producing parasites were shown not to be valve-selective in our samples. This result suggests that there is no advantage for a parasite to live on one valve or the other. This is to be expected because the bivalve hosts are equi-valved. The nonselectivity of parasites between host valves maximizes the likelihood of detecting this phenomenon in the fossil record.

Infaunal P. staminea from Argyle Lagoon were significantly larger than epifaunal individuals from Argyle Creek. A number of scenarios (perhaps working in combination with one another) might explain this phenomenon and are discussed later. (1) The epifaunal clams from Argyle Creek may be experiencing greater stress (e.g., occasional subaerial exposure and the inability to burrow) than their lagoonal counterparts thus reducing their growth rates. To calculate growth rates, one must know the age and size of the clam. The ages of the bivalves are not known, and the estimation of age through the number of major growth bands in P. staminea is problematic (major growth bands are difficult to identify) and inaccurate (Berta 1976). (2) Higher incidence of parasitism in Argyle Creek may reduce growth rates. Trematode infestation is known to effect host growth rates in multiple ways (Sorensen & Minchella 2001, Taskinen 1998), however regression analyses (the results of which are not shown here) do not suggest a relationship between the extent of infestation of a bivalve and its length. (3) Higher incidence of parasitism combined with forced epifaunality may make clams from Argyle Creek more susceptible to durophagous predators such as crabs, birds, and raccoons. Spionid-infestation is much more common in Argyle Creek than in Argyle Lagoon, and the characteristic mudblisters and u-shaped borings likely weaken the shell. (4) Differences in hydrodynamics between the two environments may also affect shell size and morphology (Vermeij 1973). (5) The two populations may simply represent different cohorts. Future investigations of bivalve age (through stable isotope sclerochronology) and predator-prey interactions in these environments can elucidate the cause of the disparity in size between these two populations.

Clams infested with spionid mudblisters were significantly smaller than those that were not infested from Argyle Creek. There was no discernable difference in size between mudblister-infested and noninfested clams from Argyle Lagoon. Again, it is problematic to determine the cause of this size discrepancy. Given that both groups lived in the creek differing environmental stress and hydrodynamic conditions can be ruled out as likely causes. The small difference in size (3.3 mm), though significant, seems to rule out the possibility that the infested and noninfested clams are from different cohorts. Spionid infestation is not known to affect growth rates. Perhaps spionid-induced mudblisters weakened the shells of the Creek bivalves and made them more vulnerable to their predators thus reducing their likelihood of surviving to a larger size. Once again further work needs to be done to determine not only the

source of this size discrepancy but also the stability of this discrepancy over longer time scales (seasons to years).

ACKNOWLEDGMENTS

This research stemmed from a course taught by Michal Kowalewski (VPI&SU) and Lindsey Leighton (SDSU) at the Friday Harbor Laboratories, University of Washington. Their instruction, suggestions, insights and support greatly benefited the author and this project. The author also benefited greatly from the ideas and camaraderie of the TA's, Rich Krause, and Jen Stempien, and other students in the course: Carlos Cintra-Buenrostro, Kate Bulinski, Una Farrell, Karen Koy, Sabrina Rodrigues, Emily Stafford, and Sebastian Willman. This research was supported by a Stephen Jay Gould Grant-in-Aid from the Paleontological Society, the Geological Society of America, and Friday Harbor Laboratories. This manuscript was greatly improved by the constructive comments of Greg Dietl and Carl Brett. This is Center for Forensic Malacology publication number 1.

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TABLE 1.

Results of bionomial test comparing distribution of parasitic
traces on left and right valves of Protothaca staminea. Ratio
represents the number of right valves having more parasitic
traces than left valves to the total number of comparisons.

                            Argyle Creek       Argyle Lagoon

Trematode pits/blisters        15:28               15:29
                               p=0.71             p=0.65
Spionid mud-blisters           19:32               4:11
                               p=0.89             p=0.27
Spionid u-shaped borings       10:26        No u-shaped borings
                              p = 0.16         from lagoon.

Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, Virginia 24061