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 THE FLORIDA CAVE SHRIMP
    
A Reminiscence and Rumination

Robert B. Cumming [1]

About 55 years ago, on a solo visit to Squirrel Chimney, a solution sinkhole and cave in the Crystal River Formation limestone in Alachua County Florida near Gainesville, I discovered and collected a specimen of a cave adapted shrimp. I was a senior biology student at the time at the nearby University of Florida, and when I first saw the shrimp I knew that it was a very important observation. I had been to Squirrel Chimney many times in the two years before that brilliant July day, and I knew the fauna of the Squirrel Chimney site and that of a number of other Florida aquatic caves very well, but I had never before seen a shrimp in any Florida cave. I also knew the literature of the biota of the Florida karst areas well, and again, no mention of a shrimp.

    Looking back I now realize that to travel alone to a remote site and descend with a rope down a 50 foot shaft to swim under water in a cave was ill advised, even reckless. But familiarity may lead to carelessness and perhaps overconfidence. I was young (24) and healthy and Squirrel Chimney was for me at the time a very special and familiar place. This was not a voyage of discovery – it was more like checking up on things, looking in on an old friend to make sure that everything was all right.

What I Remember

    I traveled that day to Squirrel Chimney in my tattered Morris Minor with a minimum of equipment: a climbing rope, face mask and fins, an assortment of waterproof lights, a couple of 6" tea strainers, a few small jars with tight lids, and a small back pack. I descended the shaft by rope, donned my mask and fins, ducked under the clear water, and swam the short distance to the large underground chamber with air space above an approximately 20 foot long surface. As I dove through the clear water I saw specimens of the cave adapted crayfish, Procambarus pallidus, and the remarkable and rare spider cave crayfish, Troglocambarus maclanei, both species often seen in Squirrel Chimney in that era, but I didn’t bother to collect them. I also saw a cave adapted amphipod, probably Crangonyx hobbsi, but again did not take specimens. As I approached the end of my visit I saw the shrimp swimming near the surface of the pool, down the center of the chamber, and was able to capture it with a tea strainer and place it alive in a
small jar. I looked extensively for additional shrimp that day, but saw none, and returning to my dorm room, I placed the shrimp specimen in a small aquarium where it seemed to thrive.

    I returned to Squirrel Chimney with several friends four days after the initial discovery to look for additional examples of the shrimp. A thorough examination of the underwater regions of the cave, strangely, revealed no animals of any kind, but as I prepared to leave, having dressed and stowed my gear, ready to climb up the shaft and leave the cave, I saw a single specimen of the shrimp swimming in open water in the center of the entrance pool about two inches below the surface. By the time I got my mask and strainer out of the pack and prepared to dive in, jeans and all, the shrimp dove straight down out of sight and was seen no more. On subsequent visits to Squirrel Chimney that summer of 1953 I saw and clearly identified five more specimens of the shrimp but was unable to capture any of them.

    A week or so after my first encounter with the shrimp I started corresponding with Horton Hobbs Jr., the world authority on fresh water Crustacea of Florida [2], who was at that time on the faculty at the University of Virginia. Dr. Hobbs was friendly enough but he clearly did not believe that I had captured a cave adapted shrimp in Squirrel Chimney, and for good reason. He had studied the fauna of this cave over a period of years with many visits to the site. He had discovered several species new to science at this locality and knew it well in all of its moods. But he had never seen a shrimp at Squirrel Chimney, and from his tone I thought he believed that I had confused the slender spider cave crayfish, T. maclanei, with a shrimp. So he asked me to preserve the specimen I had been studying in the aquarium for several weeks in 80% alcohol and mail it to him along with some of my notes. I did this, and his tone quickly changed. He could not understand how he had missed the shrimp in his intensive study of Squirrel Chimney for such a long time. He quickly shipped the specimen on to Dr. Fenner A. Chace of the US National Museum, and I promptly got a letter from Dr. Chace expressing the same surprise and excitement. The following year the shrimp was described as a new species and named Palaemonetes cummingi [3]. In the paper describing the species Dr. Chace said, “It is a pleasure to name this species for the collector, Robert B. Cumming. Our knowledge of invertebrate animals can be greatly enhanced by observers who display the interest and determination shown by Mr. Cumming.” Perhaps a bit excessive, but . . .

A Troubled Trail
    
    Recently I was asked about the shrimp again and realized that I had not thought about this animal for several decades. For me the shrimp was always something of a biological enigma — it represented a number of interesting biological questions. In researching the subject I found that Palaemonetes cummingi has had an eventful and not altogether happy history since I last looked in on it. Part of the intrigue was its very rarity, its known occurrence only in a single locality, and the unpredictability of its presence even there. I already knew that this shrimp had attained some notoriety when, 10 years after its original description, a photograph of a living female specimen appeared in The National Geographic Magazine in an article on exploring American caves written by Charles E. Mohr [4]. Mr. Mohr is also credited with taking that photograph (the only photo from life I know of that exists for this species.) A few specimens were captured in the 1960s including the one photographed for The National Geographic. In 1968 after nearly a year of fruitless searching in Squirrel Chimney, David Lee, on the instigation of Sheldon Dobkin, captured a single ovigerous female specimen of P. cummingi. This specimen produced young which were reared in the laboratory and became the basis of Dobkin’s 1971 description [5] of the larvae of this species. Again information about this species seemed to depend on infrequent and fortuitous encounters in a limited and stressed environment.

    In 1990, twenty-seven  years after its discovery, this species was listed by the US Fish and Wildlife Service (USFWS) on the federal List of Threatened and Endangered Species and classified as “Threatened” [6]. This listing has lead to the species’ inclusion on many lists of threatened and endangered species and to a number of attempts to clarify the status of this mysterious and elegant creature. In 2001 Terry J. Doonan of the Florida Fish and Wildlife Conservation Commission published a comprehensive report [7] which attempted to collect and summarize data needed to decide whether this species should be retained on the list as a “Threatened” species. Bill Brooks of the US Fish and Wildlife Service also produced a report in 2001 summarizing the status of P. cummingi [8] and recommending that the Squirrel Chimney site be protected with restricted assess, a buffer zone around it, and monitoring on a regular basis. There have been several attempts to have the species removed from the “Threatened and Endangered Species” list as probably extinct, the most recent on a petition by the Florida Game and Fresh Water Fish Commission which was denied in 1998 [9,10]. The motivation for the State of Florida in having P. cummingi declared officially extinct seems clear enough. As long as there is a threatened species known from a single locality, development around that site may be limited (see discussion by Hollingsworth [11]), and more options would open up if the issue would just go away. The status of the species was again reviewed in 2007 by the Fish and Wildlife Service, but again no action was ruled to be warranted and so P. Cummingi remains listed as “Threatened” [12] by the USFWS.  Clearly, many things have changed in north-central Florida in the last half century. Probably much has been lost.

    In the past two decades attention to Palaemonetes cummingi has been sporadic and episodic; this has always been a rare and elusive animal. But it is a well established species and not a ghost in spite of its ghostly appearance. It is very distinct from other species of the genus; the adults of both sexes and the larval forms have been well described. It was infrequently but regularly encountered at Squirrel Chimney through the 1950s and 1960s, and it is the only species of the widespread genus, Palaemonetes, known to occur or to have recently occurred in the state of Florida5. This is the reason that Dobkin targeted this species for study of the larval stages [5]. To my knowledge the species was last seen alive in 1973 – twenty years after its original discovery. There are seven specimens in the collections of the U. S. National Museum, and this is the extent of museum collections of this species known to me. There may be other verified records and there certainly are other observations of the species, but probably not many, and as far as the scientific record goes the species is still known only from its type locality, Squirrel Chimney. The effort to clarify the status of this species continues, but irregularly, and it’s on nobody’s list of high priorities. Still this little animal remains a key species for understanding some crucial issues of biodiversity and conservation in the central Florida aquatic cave ecosystem.

The Allee Effect

    In the spring of 1950, three years before the discovery of the cave shrimp at Squirrel Chimney, I met and got to know one of the most memorable men I have ever encountered. This was before my collecting sojourns to Panama (1950-51) and the Dominican Republic (1952), but long after I had become deeply committed to a career in biological science. Dr. W. C. Allee [13] arrived that spring from the University of Chicago to assume the chairmanship of the Department of Biology at the University of Florida and to dispel the deep malaise that had  settled in on that department. He was, we already knew, one of the most important ecologists of the 20th century, but he turned out to be more than that to me. I’ll never forget my first impression of the man – slouched in his wheel chair in a floppy hat and challenging the world with his sparkling eyes. Unknown to me or to anyone else at the time was the profound effect his work would have many years later on my thinking about the Florida Cave Shrimp.

    Though I was a lowly undergraduate at that time, I got to know Dr. Allee quickly and fairly well. I was already scheming to travel to Panama in the summer of that year and to spend a good part of my time there on Barro Colorado Island, The Canal Zone Biological Area. I knew that Allee had traveled to Barro Colorado in 1924 when the biological station was in its early developmental stages, and with his wife he had written a book about it [14]. He was on a first name basis with many of the giants of early 20th century field biology who had been drawn to that striking and unique place. I quickly sought Dr. Allee out, and he was happy to talk about his experiences in the tropics and to offer advice. I knew him until his death in 1955, and enjoyed that relationship inside and outside of the classroom. He was a gentle Quaker and a towering intellect –  one of the most driven and determined individuals I have ever met.

    W. C. Allee had developed a concept that has come to be known as the “Allee Effect,”and  now, more than 50 years after his death, it is much more important in our understanding the fragility of life on earth than it was during his lifetime. Allee worked in an era in which Darwinian evolution was thought of by most as an intense struggle between individuals – the strongest and fittest individuals survive to confer their genetic heritage on their progeny – but Allee thought in terms of populations. The title of his last book, Cooperation Among Animals With Human Implications [13], perhaps gives some indication about how he was framing the issue. The “Allee Effect” posits that at some population levels there is decreased individual fitness (probability of survival) at decreased population density. Just as there can be overpopulation (too many individuals for the environment to support) there can be underpopulation (too few individuals to successfully confront environmental challenges or to form effective social groups.) A number of factors may contribute to Allee Effects [15] such as predator/parasite avoidance, overcoming hosts or prey, resource defense or acquisition, amelioration of the environment, reproduction, demographic stochasticity, and genetic factors. Several of these factors may have been involved in the viability, or lack of it, of the Florida Cave Shrimp populations of the mid 20th century.

    Allee Effects have been extensively modeled mathematically [15], and the dynamics of risks to species at suboptimal population density are now better understood than when I first saw the startling cave shrimp at Squirrel Chimney. Populations at low density, in the region of the curve where the Allee Effect is operating are unstable, and can quickly go to extinction. Of species which are known to have gone extinct since 1600 and for which a cause of their demise can be assigned, overexploitation accounts for about 25%, introduction of non-native species accounts for about 40%, and habitat destruction is blamed for most of the rest[16]. The overwhelming majority of these species go through a phase where the Allee Effect is operative, but the population density where a species is in danger, of course, varies with the population dynamics of the particular species involved. There is speculation that an Allee Effect could have been a factor in the dramatic extinction of the North American passenger pigeon (Ectopistes migratorius) [17] as optimal population densities for this species appear to have been very high because of the particulars of its mating system18. This species, once the most abundant bird in North America, became extinct in the early 20th century. Clearly the Allee Effect has been  a major factor in many of the species extinctions in recent decades.  The many complex biological factors that are involved in the Allee Effect are beyond the scope of this essay, but it is clear that this principle is very important in our thinking about the stressed and limited population of the Florida Cave Shrimp in the 1950s and 1960s.

A Common Story

    It is becoming apparent that there is a general, almost subliminal, pressure on environments throughout the world. This is a common story, but one that is usually just beneath our notice. The damage is usually invisible, little items that add up, and the tapestry of life on earth is so complex and interdependent that it is impossible to predict the long term damage of these subtle insults to the biosphere, and even to human life.

    The north Florida karst ecosystem [19,20], like many other ecosystems, is under considerable stress, and often that stress is not apparent to the casual observer. The water level in the Alachua County aquatic caves is by most accounts down about eight feet from the levels I observed when the Squirrel Chimney Cave Shrimp was first encountered, and this is not a matter of random fluctuations. In the early 1950s the water level in the north Florida karst aquifer was fairly stable, but it was already under considerable pressure from pumping for irrigation purposes and domestic uses. The extensive use of fertilizers for agriculture has lead to leaching of nutrients into the groundwater. Reports from divers who have visited caves and springs of the area over many years describe a slow increase in turbidity over time, a gathering haze [21]. One of my early memories from the late 1940s was spending a summer working on a north Florida farm. The farmer had placed a very large diesel powered pump near a sinkhole on his property and each day was pumping many thousands of gallons of water for sprinkler irrigation of his large acreage. The extraction of water has greatly increased since those years, far beyond the natural capacity to recharge the aquifers to their historic levels. But these waters are out of sight, and largely out of mind.
    
    At the time the shrimp was first encountered I had been visiting the caves for several years and noted little variation in water level. In his survey [7] of the status of P. cummingi in these same caves in the mid 1990s, Terry Doonan noted that the water level was again stable over the several years of the study. But a study of the diagrams of Squirrel Chimney in his report [7] reveal a difference of 8-10 feet in water level from what I experienced over 40 years earlier. In that report he notes: “In at least 3 of the caves surveyed, evidence indicates that water levels historically had been higher than levels recorded here. Franz [20] reported that the water level in Squirrel Chimney had dropped 2.5 meters since the 1960s. Doonan [7] discounted changes in water level as a factor in the failure to find P. cummingi at this site in the mid 1990s since he notes that aside from water level “there was no indication of any significant change in the physical environment at Squirrel Chimney.” I’m not so sure. I think that significant loss of water from an aquifer from its historic levels is an indication of potential environmental stress whether we know how to measure it or not.

    Aside from the drop in water level, the most notable difference in the Squirrel Chimney environment in the 1990s from the conditions that prevailed in the 1970s and before is the consistent presence of the small minnow-like fish known as the redeye chub (Notropis harperi in the literature about these caves, but there are other taxonomic treatments of this fish.)  I never saw fish of any kind in Squirrel Chimney in the 1950s and 60s, but the redeye chub was consistently  present during surveys of Squirrel Chimney and other nearby caves from the early 1990s on. The redeye chub is an opportunistic predator that could prey upon larval P. cummingi [7] and other cave species, and this may be the reason why the shrimp has never been seen when redeye chubs are present. But of course, we have no direct evidence of this, and we are tempted to ask why these fish are present in these subterranean environments where they did not exist a half century ago. Two possible factors readily come to mind: the first is the water level itself, (the topology of these environments changes with changes in water level) and the second is changes in energy flow from pollution into these cave ecosystems.

    The redeye chub (Notropis harperi or Hybopsis harperi subterranea) has a disjunct distribution, perhaps reflected by the subspecific designation, H. h. subterranea, given to some Alachua County populations by Hubbs in 1956 [22]. This fish is often seen in clear water streams and springs and with increasing frequency in caves and sinkholes. Speculation about the distribution patterns of species within subterranean environments is an interesting and important topic, but beyond the scope of this note. It is sufficient here to observe that when the water level in the north central Florida karst region fell, a species that had evolved in surface waters occupied some subterranean environments and could have put pressure on indigenous species there.
 
    The intricate webs of life in these fragile cave ecosystems can help us understand changes that are now occurring in environments of wild places throughout the earth. Where there are communities of living things, the physical parameters of the environment must stay within strict limits or the complex dynamics of those ecosystems will spin irreversibly out of control. Cave ecosystems are particularly sensitive because the species there have evolved in such constant conditions, and they lack the buffering capacity of large populations with the genetic diversity and environmental elasticity often found in surface species. As in other ecosystems the survival of cave populations depends on genetic variability and orderly gene flow through time to be able to respond to unpredictable selection pressures. Maintenance of a viable environment in caves is dependent on stability which requires protection from rapid chemical and physical changes and in the maintenance of an appropriately constant energy flow into and out of these subterranean environments. Since caves are particularly sensitive to disruption, with catastrophic consequences for their troglobitic inhabitants, these creatures are among the indicator species,  “coal mine canaries,” we have to help us understand the consequences of environmental stress in the apparently more robust surface communities. And being unusually vulnerable to significant damage from casual intruders and the predations of recreational vandalism, they highlight the environmental challenges now becoming all too apparent in other ecosystems throughout the world.

References and notes

1.    The author was one of the founders of a small group of cave explorers around 1950 in Gainesville, Florida which evolved into the Florida Speleological Society. He was a student of Biology and Geology, and holds two degrees from the University of Florida. He also has a doctorate in zoology and genetics from the University of Texas, and has been a biologist, engineer, teacher, writer, editor and publisher in the years since.  He still misses some of the people he knew in Florida at the time this reminiscence started, such as his friend and mentor, Archie Carr.

2.    Hobbs, Horton H. Jr, 1942, Crayfishes of Florida, University of Florida Biological Sciences Series 3(2). Univ of Florida Press, Gainesville, FL.

3.    Chace, F. A. Jr., Two New Subterranean Shrimps (Decopoda: Caridea) from Florida and the West Indies, With a Revised Key to the American Species. J. Wash. Acad. Sci. 44(10):318-324, October 1954.

4.    Mohr, Charles E, Exploring America Underground, National Geographic Magazine, Vol.125(6):802-837, June 1964. The photo of P. cummingi is on p. 828 of that article and appears on the first page of this note.

5.    Dobkin, S. 1971, The larval development of Palaemonetes cummingi Chace, 1954 (Decapoda, Palaemonidae), reared in the laboratory, Crustaceana 20(3):285-297.

6.    US Fish and Wildlife Service, 1990, Endangered and threatened wildlife and plants; endangered status for the Lower Keys rabbit and threatened status for the Squirrel Chimney cave shrimp, Federal Register 55 (120):25588–25591.

7.    Doonan, T. J., 2001, Survey of Squirrel Chimney and other selected caves to determine the status of Squirrel Chimney cave shrimp (Palaemonetes cummingi): Final Performance Report, Florida Fish and Wildlife Conservation Commission, Tallahassee, Florida, USA.

8.    Brooks, Bill, 2001, Squirrel Chimney Cave Shrimp, Palaemonetes cummingi, US Fish and Wildlife Service internal report.

9.    US Fish and Wildlife Service, 1998, Endangered and threatened wildlife and plants; 90 day finding for a petition to Delist the Squirrel Chimney Cave Shrimp, Federal Register 63 (235):67618–67619.

10.    US Fish and Wildlife Service, December 8, 1998, News Release: US Fish and Wildlife Service finds Petition to Delist the Squirrel Chimney Cave Shrimp Unwarranted, Release # R98-118.

11.    Hollingsworth, Cathrine, Rare cave shrimp stays on the threatened list, Gainesville Sun, Saturday, December 26, 1998.

12.    US Fish and Wildlife Service, 2007, Endangered and threatened wildlife and plants; 5-year review of 22 Southeasters Species, Federal Register 72 (80): 20866–20868.

13.    W. C. (Warder Clyde) Allee, born June 5, 1885 in Bloomingdale, Indiana and died March 18, 1955 in Gainesville, Florida. He joined the faculty of the University of Chicago in 1921 and became a Dean there in 1925. Editor of Physiological Zoology from 1928 to 1954. President of the Ecological Society of America 1929. Major books: Animal Aggregations: A Study in General Sociology (1931); Social Life of Animals (1938); Principles of Animal Ecology (1949, with Alfred E. Emerson, Thomas Park, Orlando Park, & Karl P. Schmidt); Cooperation Among Animals With Human Implications (1949). In 1938 he had surgeries to remove a spinal tumor and became a paraplegic. Later he had a nearly fatal accident at the University of Chicago, but, contrary to predictions, beat the odds to come back as strong as ever. His final position was as Professor and Chairman, Department of Biology, University of Florida, Gainesville, Florida, 1950-1955.  He was elected to The National Academy of Sciences in 1951.

14.    Allee, W. C. and M. H. Allee, Jungle Island, Rand McNally, 1924, 215 pp.

15.    Stephens, Philip A. and William J. Sutherland, 2000, Vertebrate Mating Systems, Allee Effects and Conservation, In Vertebrate Mating Systems, M. Apollonio, M. Festa-Bianchet and D. Manardi, Eds., World Scientific Publishing.

16.    Groombridge, B., .Ed, Global Biodiversity: Status of the Earth’s Living Resources, 1992, London, Chapman and Hall.

17.    Halliday, T. R., Biol. Conserv. 17(1980) 157-162.

18.    The passenger pigeon (Ectopistes migratorius) was once the most common bird in North America. It has been estimated that there were five billion individuals of this species present when Europeans arrived on the North American continent. “Martha,” thought to be the last surviving individual died in Cincinnati, Ohio on September 1, 1914. A number of factors were thought to be involved in this species extinction, including over exploitation (particularly large harvests of birds in their communal breeding areas) and loss of habitat, but toward the end, when population numbers were greatly reduced, an Allee Effect operating through the bird’s mating system is implicated. The passenger pigeon practiced communal roosting and communal breeding. It required the social interaction of large groups in order to breed successfully, so when populations diminished to a certain size, their doom was sealed.

19.    Franz, R., J. Baier, and T. Morris, 1994, Review of the biologically significant caves and their faunas in Florida and south Georgia, Brimleyana, 20:1–109.

20.    Franz, R. 1994, Squirrel Chimney cave shrimp survey, Unpublished report to the Florida Game and Fresh Water Fish Commission, Tallahassee, FL USA.

21.    Pruitt, Buford, (long-term Florida cave diver) Personal communication, June, 2008: “When I began cave diving in the mid-1980s, cave waters were generally clear, with visibilities of 30ft, 50ft and more. Cave limestone walls were white to cream colored, or coated with a layer of the mineral geothite. Today, those same underwater caves have visibilities more like 10ft to 20ft, and the cave walls are coated with an eighth-inch thick layer of scum called a ‘biofilm.’”

22.    Hubbs, Carl L, and Walter R. Crowe, 1956, Preliminary analysis of the American cyprinid fishes, seven new, referred to the genus Hybopsis, subgenus erimystax, Occa Pap Mus Zool Univ of Mich 578:1-8