Aquatic Fauna in Peril > Resource Management of Freshwater Crustaceans in the Southeastern United States

Resource Management of Freshwater Crustaceans in the Southeastern United States

By Guenter A. Schuster

It has been estimated that only about 10 percent of all extant species of crustaceans occur in freshwater (Bowman and Abele, 1982; Covich and Thorp, 1991). However, the freshwater crustaceans consist of a variety of taxonomic groups (Table 1) that may be found in almost any type of freshwater habitat. Freshwater crustaceans can be found in coldwater springs, small to large order streams, in underground caverns and burrows, in a variety of lentic habitats (from swamps and marshes to ponds and lakes), and even in temporary aquatic habitats such as autumnal and vernal pools.

The southeastern region of North America has played an important role in the evolution of several crustacean groups (Holsinger, 1969, amphipods; Steeves, 1969, isopods; Hobbs, 1969, crayfish; Hart and Hart, 1969, ostracods). It has been proposed that at least one group, the crayfish genus Cambarus, has had the Cumberland Plateau as the center of its radiation (Hobbs, 1969). In addition, several crayfish genera (Hobbs, 1989) and many other crustacean species (e.g., Hobbs, 1942, 1981, 1989; Bouchard, 1972) are endemic to specific regions in the Southeast. Distribution maps in Hobbs (1989) clearly show that the crayfish genera Barbicambarus (found in Kentucky and Tennessee), Bouchardina (found in Arkansas), Distocambarus (found in Georgia and South Carolina), Hobbseus (found in Alabama and Mississippi), and Troglocambarus (found in Florida) are all endemic to the Southeast. Crustacean groups other than crayfishes also have southeastern endemic species (e.g., Frey, 1986, cladocera; Holsinger, 1986, amphipoda). In fact, most of the southeastern species that are considered candidates for listing as endangered or threatened (C2) (U.S. Fish and Wildlife Service [USFWS], 1991) show a very high degree of endemism (i.e., one or two known localities), and that may, at least in part, be the reason for their C2 status.

Hobbs (1992) categorized freshwater crustaceans ecologically into cave dwellers (isopods, amphipods, and decapods), surface dwellers (most groups have representatives of these), and burrowers (mostly crayfishes). Hobbs (1992) further categorized cave dwellers according to how much of their life cycle is actually spent in the cave (i.e., troglobites, trogloxenes, and troglophiles). The surface water dwellers are restricted typically either to lotic or lentic habitats, and may be part of the benthic (e.g., isopods, amphipods, and crayfishes), nektonic (e.g., shrimp), or planktonic (e.g., copepods and cladocerans) communities in those habitats (Covich and Thorp, 1991). Freshwater crustaceans are considered ecologically important members of the communities to which they belong, and may fulfill a variety of functions within their communities. Some are predators, some are herbivores, some are parasites, while many, such as crayfishes, are omnivores, and may comprise a large part of the biomass of a community (Covich and Thorp, 1991).

Because of the long-term degradation of fresh waters, it should not come as a surprise that some freshwater crustacean species are having problems surviving. Currently, the USFWS (1993) lists 13 species of freshwater crustaceans as either endangered or threatened, seven (54 percent) of which are from the Southeast (Table 2). In addition, a number of southeastern crustaceans have C2 status (i.e., no conclusive data on biological vulnerability and threat are currently available and further study is necessary; USFWS, 1991), for which listing as either endangered or threatened is possibly appropriate (Table 3).

A number of taxonomic groups (Classes Branchiopoda, Branchiura, Copepoda; Order Mysidaecea) have no species currently listed or being considered for possible listing (USFWS, 1991, 1993). The primary reason is not that these species are currently doing well, but that, in fact, we do not have enough information about their distributions and population dynamics to make a judgment about their conservation status.

Impediments To Crustacean Resource Management

Cultural and Scientific Bias

Crustaceans, like most other animal groups, suffer because of the positive bias associated with the so called "charismatic megafauna" (i.e., large and easily identifiable species such as the grizzly bear or the bald eagle). It is clear that this bias exists both in our culture and in science. Culturally, the field of crustacean resource management completely revolves around the economically important crustaceans for which a fisheries industry has been developed. This of course includes a variety of freshwater species of shrimp and crayfish which are raised as bait and human food.

In a book edited by Holdrich and Lowery (1988), which specifically dealt with the biology and management of crayfish species, not one chapter was dedicated to the conservation of these crustaceans. Hart and Clark (1989) provided approximately 11,000 citations for crayfishes under a variety of subject headings, but did not have a heading entitled "conservation." These are not oversights, but are instead reflections of the lack of scientific work that has been done in the conservation of freshwater crustaceans. If very little has been written about relatively well-known species such as crayfishes, then it is easy to see why next to nothing has been done for relatively obscure groups such as fairy shrimp or water fleas.

Taxonomy and Systematics

One of the main obstacles for the protection of lesser-known species is the current need for taxonomic experts who can identify various species. Many freshwater crustaceans are microscopic, or at least a microscope is necessary for their identification. In some freshwater crustacean groups, the taxonomy has been fairly well resolved (i.e., crayfishes and shrimps), although new species are often found annually. However, in some groups the taxonomy is still in great flux. A common idea expressed in recently published crustacean taxonomic keys is that for many of these taxa much is left to do, and that species and sometimes even generic level identification is often very tenuous (Williams, 1972, isopods; Frey, 1986, cladocerans; Holsinger, 1986, amphipods; Delorme, 1991, ostracodes; Dodson and Frey, 1991, branchiopods). When experts can not easily and reliably identify species, or even genera, then species protection is nearly impossible. This is a common problem in many invertebrate groups, and it will not be resolved easily, since fewer taxonomists are being trained to work with these difficult to identify taxa.

Distribution Information

The delineation of species’ ranges obviously requires the ability of trained taxonomists to identify field collected specimens (Frey, 1986). In addition to the identification problems mentioned above, distribution data sometimes is lacking for taxonomically sound groups simply because of inadequate field work. In some groups, cladocerans for example (Frey, 1986), older distribution records are suspect because of recent reinterpretation of the taxonomy of the group. The only way to resolve these problems is to initiate extensive faunal inventories. Unfortunately, such inventories historically have not been well-supported through adequate funding.

Life History Data

Even for the best-known groups of freshwater crustaceans, the life history of most species has not been studied in detail. For example, of the over 300 species of crayfishes and shrimps in North America north of Mexico only about 20 have had their life histories studied (Hobbs, 1991). Researchers and resource managers often are left to extrapolate about the biology of a species based on the few studies that have been done on other related species. An example is the recovery plan (Biggins, 1989) for the federally listed endangered species, the Nashville crayfish (Orconectes shoupi). Under the heading of "Description, Ecology and Life History," Biggins (1989; page 2) stated: "However, some life history data does exist, and some speculations can be made based on this species’ similarities to other crayfish… Like many crayfish, this species probably feeds on a variety of organic material, both plant and animal." This points out the lack of hard data available for most species, even for species that have already been federally listed.

Destruction, Degradation, and Fragmentation of Habitat

In recent years much has been written about the loss of aquatic habitats. Two primary causes of aquatic habitat destruction and fragmentation include the construction of dams and stream channelization. Both of these activities are prevalent in the Southeast, especially in states such as Alabama, Kentucky, North Carolina, and Tennessee. Almost all of the major stream systems of the Southeast have been impounded, channelized, drained or otherwise manipulated. This has resulted in the changing of these systems from typical lotic to lentic or semilentic systems (Adams and Hackney, 1992). Soballe et al. (1992) listed 144 major reservoirs for the Southeast. Tennessee has the most with 26, followed by Alabama and North Carolina with 19 each, and Kentucky with 17 (Soballe et al., 1992).

The effects of dams and channelization are well-known (Mulholland and Lenat, 1992; Soballe et al., 1992). Dams may create multiple impacts both upstream and downstream. Impoundments and channelization act as agents of habitat destruction and fragmentation, playing important roles in altering temperature regimes, natural water level fluctuations (both in surface and ground waters), physicochemical processes, deposition of fine particulate matter, erosion patterns downstream, and community composition.

As noted above, it has become increasingly clear that the southeastern region has played an important role in the evolution of several crustacean groups (Holsinger, 1969, amphipods; Steeves, 1969, isopods; Hobbs, 1969, crayfish; Hart and Hart, 1969, ostracods). Therefore, it stands to reason that the widespread habitat destruction caused by damming and channelization has and will have major impacts on these groups and will play an important role in the future management plans for these species. Unfortunately, changes in biogeography caused by habitat destruction make it difficult or impossible to interpret the historical ranges of impacted species, and thus knowledge concerning our natural heritage can become forever lost and unavailable for restoration efforts.

Habitat degradation may be the result of either point- (such as effluent from a pipe) or nonpoint-sources (such as agricultural runoff) of pollution. Habitat degradation is often difficult to document and its impact on species may be insidious and long-term. Habitat degradation may also play an important role in future management of freshwater crustaceans in that its effects may need to be mitigated in order to protect and/or restore some crustacean species.

Introduced Species

For the purposes of this discussion, introduced or non-indigenous species have been divided into two main categories. The first category includes other crustaceans that may be closely related to some native species. The second category includes non-crustacean species that may have an impact on native freshwater crustaceans. Through competition for resources, the first category often has greater impacts on the native fauna than does the second category of introduced species (Capelli, 1975).

Probably the best examples of introduced crustaceans come from the crayfishes. Crayfishes have been used both for food and bait for many years and, therefore, have been widely introduced. For example, Procambarus clarkii has been introduced on all continents except Australia and Antarctica (Huner, 1988). The effects of some such introductions have been widely documented. In Wisconsin Orconectes rusticus has had a serious impact on the native O. virilis and O. propinquus populations (Capelli, 1975). Orconectes limosus was introduced in Europe, and it has since been reported that it has all but eliminated many of the native species (Laurent, 1988). The introduction of Pacifasticus leniusculus and Orconectes virilis into California streams inhabited by P. fortis, the Shasta crayfish, has resulted in a serious decline of P. fortis (Hogger, 1988). In the Southeast, Biggins (1989) pointed out that Orconectes shoupi, a federally listed endangered species, may be at risk because of an impending invasion of O. placidus, a much more successful crayfish, from adjacent watersheds.

Even though there are many other examples of problems associated with the introduction of non-indigenous crayfishes in the Southeast, fisheries research continues to pursue species that may be exploited for culturing. A recent study on the red claw crayfish (Cherax quadricarinatus) from Australia is an example of such on-going fisheries research (Webster et al., 1994).

Non-crustacean introductions that have impacted native freshwater crustaceans range from fishes, which are potential predators, to zebra mussels (Dreissena polymorpha). Zebra mussels have been observed to attach to the exoskeleton of crayfish, and in doing so may potentially cause all sorts of biological problems (O’Neill and MacNeill, 1989). Leitheuser and Holsinger (1983) and Leitheuser (1988) reported that introduced rainbow trout (Oncorhynchus mykiss) eat Palaemonias ganteri, the Mammoth Cave shrimp, which is a federally listed endangered species known only from the Mammoth Cave system in Kentucky. Even though this predatory behavior was first reported over a decade ago (Leitheuser and Holsinger, 1983), trout are still being introduced into the Green River in the vicinity of Mammoth Cave National Park in order to maintain a put-and-take trout fishery (J. Axon, Kentucky Department of Fish and Wildlife Resources, pers. comm.). This situation has obvious management and recovery implications for the shrimp, and the solution here seems quite clear. The established trout should be eradicated from waters confluent with the cave and the reintroduction of rainbow trout to the cave should be halted.

Reconciliation of Biology, Politics, and Economics

In the course of managing for an endangered or threatened species, social and political considerations often become crucial stumbling blocks, and they must be considered before a management plan can be implemented. This is often very hard for biologists, who often are not very politically active or prone to thinking about large scale economic issues. The biological and physical needs of species are sometimes relatively more easily defined. However, convincing the general public that a species deserves protection can be very difficult. The first question usually asked is, "What good is it?" Often there is a compelling need to justify the existence of a species according to how useful it is to humans. Along with this, there often must be appeasement of numerous local, state, and federal agencies, and many citizen groups as well as industry, all of whom may have their own agendas which ultimately may harm nature. This set of circumstances of course is not unique to crustacean management, but is something that must be faced with all species that need protection.

Mammoth Cave Shrimp — A Possible Worst Case Scenario

The Mammoth Cave shrimp may represent the best or the worst case scenario depending on your point of view for crustacean management. As mentioned above, this species is restricted in distribution to the caves in and around Kentucky’s Mammoth Cave National Park. Logically, one would assume that this would afford the shrimp a great deal of protection, but ironically it does not.

Biological Profile

The following biological profile (mainly taken from Leitheuser and Holsinger [1983], Lisowski [1983], and Leitheuser [1988]) provides important life history information that has impact on the management of the Mammoth Cave shrimp. Palaemonias ganteri belongs to the family Atyidae. Its closest relative is Palaemonias alabamae, another endangered cave shrimp that is restricted to caves in Alabama (USFWS, 1991). The Mammoth Cave shrimp lives within nine distinct groundwater basins in the Mammoth Cave National Park, Kentucky vicinity, and at least three of these basins are outside of the Park. Within its preferred habitat of deep pools with minimal currents, the shrimps have been observed filter feeding and skimming food (mostly detritus) off the water surface.

The Mammoth Cave shrimp is dioecious. However, the average instantaneous sex ratio, the number of reproductive periods, and the time to sexual maturity are all unknown. These shrimp have been observed carrying 1 to 30 eggs, and it is thought that their longevity is probably 10 to 15 years. Lisowski (1983) indicated that there may have been recent declines in this shrimp population due to local groundwater pollution and hydrological changes in the cave system caused by dams on the Green River, both upstream and downstream of the Park boundaries. Leitheuser (1988) estimated the population size to be between 7,000 and 10,000. Palaemonias ganteri belongs to a community that also includes Amblyopsis spelaea (northern cavefish), Typhlichthys subterraneus (southern cavefish), Orconectes pellucidus (cave crayfish), and the aquatic cave snail Antroselates spiralis.

Like all troglobites (i.e., species restricted to caves), P. ganteri has certain biological characteristics which are associated with species known as K-strategists (Hobbs, 1992). These features include small population size, late age to maturity, low fecundity, large hatching size, and increased longevity. When these characteristics are combined with a very restrictive required habitat, management and protection of such species become very difficult problems.

Mammoth Cave Shrimp Management Problems

Even though most of the P. ganteri populations are known to occur within the Mammoth Cave National Park, this species is not well-protected from anthropogenic changes to its environment. Because of the hydrogeology of the area, activities outside of the Park (i.e., outside the sphere of its legal protection) may have devastating impacts on the fauna that lives within the cave system. Lisowski (1983) indicated that declines in P. ganteri population size were linked to water pollution that originated outside of the Park, and to two dams (lock and dam number 6 and the Green River Dam; respectively downstream and upstream of the Park) that greatly influence hydrological events within the caves.

Leitheuser (1988) described four toxic spills in the area of the Park that could also have had serious impacts on the cave fauna. Three spills were along Interstate 65 adjacent to the Park boundaries: 1980, cyanide salts; 1985, cresol; and 1985, synthetic solvents. Additionally, in 1985 a local train derailment spilled pesticides and methyl alcohol. Leitheuser (1988) also discussed the possible impacts of area petroleum wells that have leaked oil and gas in the past into the cave system. In addition, local agriculture has had impacts on local water quality via pesticides, herbicides, fertilizers, siltation, and livestock runoff (W. Sampson, Kentucky Division of Water, pers. comm.). There has also been a history of poor sewage treatment in the area (Lisowski, 1983; Leitheuser, 1988). This includes inadequate or poor treatment of the sewage of surrounding municipalities and the extensive use of septic systems which, because of the karst topography, drain directly or at least very quickly into the underlying cave system.

In addition to the above anthropogenic problems, the Park also has been subjected to the introduction of a non-indigenous species. As mentioned above, Leitheuser and Holsinger (1983) and Leitheuser (1988) both discussed the possible impacts of the introduction of the rainbow trout. Leitheuser observed trout eating cave shrimp in Pike Spring, one of the groundwater basins inhabited by P. ganteri. He indicated that the trout population was relatively small, but was well-established. Leitheuser and Holsinger (1983) indicated that trout were released by the Kentucky Department of Fish and Wildlife Resources as an on-going trout fisheries put-and-take program. As of this writing, the trout continue to be released monthly from April to November in the tailwaters of Nolin Lake Dam (Edmonson County), which is just downstream of the Park, and monthly from April to July in Lynn Camp Creek (Hart County), upstream of the Park (J. Axon, Kentucky Department of Fish and Wildlife Resources, pers. comm.). Leitheuser (1988) suggested that a trout survey should be conducted to determine the distribution of trout in the Park, and he indicated that it may become necessary to both stop the introductions and to remove trout from the river.

As in most endangered species cases, political activity can often become intense, and numerous federal, state, and local agencies as well as a variety of citizen groups become involved in the attempt to resolve the issue. In the case of P. ganteri, there are at least five federal agencies, four state agencies, five citizen groups as well as a number of local city and county governments, that are involved (Table 4). It is indeed a challenging proposition to bring such a diversity of interest groups toward a consensus about how best to protect the Mammoth Cave shrimp. In fact, some of these groups and agencies seem to be opposed to the protection of P. ganteri, and the reasons for this opposition appear varied. As Salwasser (1991) pointed out, it is very important for us to recognize that the issues of biodiversity are political issues, and that generally they are very complex with no simple solutions. One of the most challenging issues then is to reconcile the differences of all of these groups and arrive at a workable and acceptable management plan.

Another important management problem, again not unique to crustacean management, is the implementation of the species recovery plans. In the case of the Mammoth Cave shrimp, a recovery plan was prepared in 1988 (Leitheuser, 1988), but to date limited funds have been appropriated to fund the plan (D. Biggins, USFWS, pers. comm.). The plan outlines in detail the recovery tasks as well as a time schedule for their completion. However, without the necessary funding the recovery goals cannot be achieved.

A greater commitment must be made to fund recovery plans, especially for non-charismatic microfauna species. It is clear that the distribution of these funds is often skewed in favor of larger, more noticeable species, as exemplified by funding to assist spotted owls and bald eagles. There must be a more equitable distribution of funds so that lesser publicized species also benefit.

The reason for noting the Mammoth Cave shrimp here as a possible worst case scenario is that virtually all of the problems associated with endangered species management are present. These include habitat alteration and destruction, point- and nonpoint-source water pollution, an introduced species, politics, and inadequate funding of the recovery plan. The biology of the species (i.e., low fecundity, small population size, extremely small range of distribution, etc.) also introduces additional management problems. That is not to say that any of these problems are unique to this one species, but rather P. ganteri embodies many of the problems that are common to all endangered species. If the list of the USFWS (1991, 1993) endangered, threatened, and C2 crustacean species is reviewed, it is apparent that many of the problems associated with P. ganteri are also common to other crustaceans. Six of the seven listed species (see Table 2) are restricted to living in caves, and many of the C2 species (see Table 3) are also cave dwellers.

Future Of Freshwater Crustacean Management

As Soule (1985) observed, conservation biology currently is an exercise in crisis control. Most often species are not placed on the endangered species list until they are virtually on the edge of extinction, which is the ultimate crisis for any species. One of the main problems with crisis management, as pointed out by Meffe and Carroll (1994), is the search for a "quick fix" when actually long-term stewardship of the environment is what is needed. Meffe and Carroll (1994) maintain that there are five basic principles associated with good conservation management:

1. critical ecological processes must be maintained,
2. goals and objectives are the result of understanding the ecology of the system,
3. whatever threats exist must be minimized, and whatever benefits exit must be maximized,
4. evolutionary processes must be maintained, and
5. management must have minimal intrusion into the system.

These five principles are all logical and straightforward. However, their implementation may be extremely difficult. Since the complicated relationships among species and their environments are just being realized, there is still a great deal that needs to be learned about ecosystems before predictable and effective management plans can be set in place.

Meffe and Carroll (1994) pointed out that scale is a very important consideration for the conservation of endangered species. Both biological (population or community) and physical (habitat or landscape) scales need to addressed. In the past the greatest emphasis has always been at the species level. This is the whole philosophy behind the U.S. Endangered Species Act (ESA). It is clear, however, that in most cases this probably is not the most effective level of management. The well-being of the spotted owl, for example, depends on the well-being of the old growth forest in which it lives. Management of the owl cannot take place unless the whole system is managed.

The same principle is true in the management of the Mammoth Cave shrimp, and probably all other imperiled aquatic cave organisms. In order to protect this and other cave species, it is not enough to deal with the immediate habitat. The entire drainage of the cave system must be managed. This of course is a formidable task for a cave system that is more than 777 km (483 miles) long, as such the Mammoth Cave system. The staff of Mammoth Cave National Park is currently taking on the difficult task of trying to understand this complex system (Mammoth Cave National Park, 1994), and ultimately their work may result in an overall management plan that will protect not only the cave shrimp, but all other fauna and flora of the Park. For smaller cave systems the task may not be quite as daunting, but there may be greater land use and land ownership problems in these cases.

Logically, the same rationale should be used for the management of surface dwelling species. Entire drainage systems must be managed rather than individual habitats within the systems. It does very little good to protect a segment of a stream if the water quality upstream of this so-called protected area is subject to development or degradation. Ecosystem conservation is really the only logical way by which we can expect to save individual imperiled species and communities. That is not to say we should do away with the ESA, for the recognition of imperiled species raises the red flag and alerts us that entire systems are not well.

For freshwater crustaceans there are a number of specific problems that stand in the way of the development of adequate management plans. Some of the most serious problems that need to be addressed include the following.

1. There is a tremendous need to train taxonomists to work with these groups of animals. For some of the more "obscure" crustacean groups there are few if any taxonomic experts. Even for better known groups, such as the crayfishes, there are only a few active workers, and death of even an individual (such as the recent passing of Dr. Horton H. Hobbs, Jr.) can produce a huge void.
2. Associated with the above, there is a need to produce reliable contemporaneous species level keys for all groups, as well as a need to educate and train people to use and update these keys as needed. For most crustacean groups, taxon identification is not merely a matter of learning a few terms, but it also usually involves a great deal of microscopy experience to develop the necessary skills.
3. Intensive inventories are needed in the Southeast for freshwater crustaceans in order to better understand the distribution and status of each species. Inventories are not just needed for listed and C2 species, but for all crustacean species. It is amazing that the exact distribution of many, if not most, species is still unknown.
4. Intensive life history studies need to be done on all federally listed and C2 crustacean species.
5. There is a need for money to support conservation work. It does no good to draw up elaborate recovery plans if they will not be adequately funded. There must be a more equitable distribution of research and management funds to benefit all species.

Acknowledgements

I thank Dick Biggins (Asheville, NC), Bob Butler (Jacksonville, FL) and Paul Hartfield (Jackson, MS) (all U.S. Fish and Wildlife Service) for graciously providing me with literature I would otherwise not have seen. I thank James Axon (Kentucky Fish and Wildlife Resources) and Bill Sampson (Kentucky Division of Water) for providing information for this paper. I also thank Donald Batch, Branley Branson, Amy Bruendermann, Eve Kimsey, and Greg Pond (all Eastern Kentucky University) and Ron Cicerello (Kentucky State Nature Preserves Commission) for critically reading various versions of this manuscript. Lastly, I dedicate this paper to Dr. David Etnier (University of Tennessee), my mentor and friend. Over the years his enthusiasm and love for aquatic organisms have touched hundreds of students. I thank him for the influence he has had on my life and career, and I am proud to say I am one of his students.

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