When I was growing up, I would keep a freshwater crayfish (also called crawfish) in an aquarium. I found it fascinating to see the animal pile up pebbles, beautifully and meticulously, to make or guard a rock shelter. It seemed to be building a stone wall around a home. I wondered, Does the crayfish do this in nature? If so, How long does the crayfish keep its home? I first conducted my study in 1977, and revised the resulting paper since then.
A few years after my experimental study, the invasive virile crayfish, Faxonius virilis, began to accelerate its competition with the native crayfish, Cambarus bartonii, the subject of this study, especially downstream of my study site. Much has been reported in the literature about how the invasive has managed to spread so widely, and much remains un-answered. And in recent years, popular media report that the other widely invasive species, red swamp crayfish, Procambarus clarkii, and marbled crayfish, Procambarus virginalis, pose a threat to endemic crayfish and other organisms in ecosystems from the U.S to Europe, but may also prove useful as human food. A relevant question, perhaps, about the current study, is whether shelter occupancy, or crayfish travel, can relate to crayfish competition.
A recent environmental project reported from South Dakota, Forest Reserve District of DuPage County, puts crayfish burrows in a positive light for conservation. A team of ecologists, along with the Lincoln Park Zoo, released the Great Plains Mudbug crayfish into streams to help the Hines Emerald Dragonfly. The federally endangered dragonfly needs crayfish burrows for reproduction.
My study below, never published before, is hardly peer reviewed, and I still research crayfish, consult with researchers, and learn new discoveries, understandings, and scientific conventions, revising the draft manuscript accordingly. One peer reviewer wondered whether more than one species of crayfish could have been present in the sample (it is likely that all the crayfishes I studied in the smaller of two tubs were the same species), and he would have preferred an experimental design that avoided disturbing the crayfish when tallying their chosen shelters. One reviewer would prefer a larger sample size. I am considering initiating a new study with different methodology.
I have discovered from feedback that the biological sciences have changed swiftly and significantly from when I was a student. For example, now researchers, mindful of the risk of contaminating streams with non-native organisms, often obtain a permit before capturing or transporting crayfish.
(The experience of peer review also reminded me of evaluating students' writing attempts -- but with me as the recipient of the critique -- and thus was a useful experience for this composition instructor.)
The Length of Time that the Freshwater Crayfish
Cambarus bartonii Keeps its Shelters After Hibernation
[Draft]
Caleb M. Kriesberg
first revisited and revised February 22, 2004
Abstract
The extent to which post-winter shelters occupied by the freshwater crayfish Cambarus bartonii are temporary is studied. Crayfish were put into two large tubs with rocks, and the crevice each crayfish occupied was recorded daily. Data analysis with the chi-square test showed that for most of the crayfish, the difference between their shelter choice pattern and that expected for random choosing was very highly significant. The majority of the crayfish had two or three shelters in which they were found most of the days, but most did not stay in any one shelter many days in succession. It is believed that this deliberate but temporary choosing of shelters is natural behavior, and speculation is made that it evolved to combine reliable protection with sufficient food exploitation.
Introduction
Freshwater crayfish, when not in hibernation, generally hunt for food most of the night and during most of the day stay in a shelter. The shelters of some are tunnels or burrows (Tarr, 1884), while those of others are a protected spot under a rock (Crocker, 1952). Crayfish scavenge for decaying matter, plants, and small live animals (Crocker and Barr, 1968). Their antennae and antennules are distance chemoreceptors and the mouth and first two walking legs may be also, which allows them to detect distant food, especially in swift water currents (Ameyaw-Akufi, 1977).
The length of time a crayfish keeps a shelter in nature may be influenced by such factors as competition from other animals, availability of food, presence of predators (Bergman, et al, 2003), suitability and availability of shelters, (Goessmann, et al, 2000), and age or health of the crayfish. Though there are studies of crayfish and shelters, no study has been found in literature examining the length of time that C. bartonii keeps a shelter, or examining crayfish shelter occupancy for more than a week’s duration. From studying the pattern of rock shelter occupancy of captive C. bartonii for two weeks, along with field observations and facts about their ecology, this paper attempts to deduce a term of shelter occupancy of C. bartonii in their natural habitat (Swecker, et al, 2019) and to rationalize this hypothesized length of occupancy in terms of its survival value.
Materials and Methods
The Cambarus bartonii were found in spring 1977 among rocks in a stream, Minnehaha Creek, along Goldsboro Road in Bethesda, Maryland, near Washington, DC, at an average depth of 30.6 cm in two areas of still water, one about 6.1 m by 1.5 m and the other about 3.0 m by 1.2 m. They were collected by wading into the water, turning over rocks, and catching the crayfish with the hands or in Styrofoam cups. Five females and four males were used for this 1977 experiment. [Note: new information suggests that the two largest crayfish in the study, kept apart from the others, may have been not the native Cambarus bartonii, but the invasive Faxonius virilis.]
Figure 1: The creek that was visited in 1977 to acquire crayfish for laboratory observation
(Latitude 77 N 07' 30" Longitude 38 W 57' 30")
Two metal tubs, one 1.80 m by 76 cm and 62 cm high and the other 1.60 m by 57 cm and 59 cm high, were used. Each had a pile of rocks along one length and the bottom covered with sand from the stream where the crayfish were collected and roadside gravel (see Figure 1). The water was 16 cm deep and ranged from 4 – 11 degrees Celsius, about the temperature of the stream. Starting on the sixth day of experimentation, the water was aerated; a tube from an air pump went into each tub, but it was difficult to have bubbles going through each tube, so usually only one tub was aerated on any one day. Goose-necked lamps, one clamped to each tub, went on at about 6:30 am and off at about 7:30 pm each day.
Seven crayfish with an average carapace length of 20.8 mm were put in the larger tub, and two females, one 49.3 mm and the other 39.6 mm, were put in the smaller tub. This separation of crayfish by size was designed to keep the two much larger crayfish from eating the others. The crayfish were given food such as bread, worms, and sowbugs about every other day. Each day for 13 days with the larger tub and 14 days with the smaller, at an average time of 11:30 am, the tubs were checked and the location of each crayfish recorded by matching its site with numbers printed abound the inside of the tubs above the water line. The numbers were later grouped by letters that denoted individual shelters. Until the eighth day from when recording began, the crayfish’s locations were found by lifting rocks, but this disturbed the crayfish, so subsequently, as often as possible, they were found by probing with the eraser end of a pencil under rocks and seeing if the crayfish revealed themselves. The crayfish were identified by oil paint marks on their carapaces or by distinctive physical traits.
Statistical analyses were conducted comparing the number of days the crayfish were found in shelter with the number of days expected for random choosing of shelters, using the chi-square test as explained by Glase (1977).
Figure 2. Arrangement of rocks and location of shelters in two tubs for experiment on length of time crayfish stay in shelters.
Results
The pattern of shelter choosing was different from a chance pattern to a very highly significant extent (α = .005) for all crayfish in the larger tub and for 9) but not 8) in the smaller tub (see Table 1). When the tubs were divided into three regions with approximately equal number of shelters in each, the chi-square test showed that for the number of shelters available in each region, crayfish 6) and 9) chose shelters from the lightest region to a very highly significant extent, and crayfish 4) chose the darkest, stillest region to a highly significant extent (α = .05). There was no statistical significance to the regions favored by the other crayfish, nor any consistency among them.
By looking across the rows of Table 1 at the pattern of shelter choosing for each crayfish in both tubs, one can see that many crayfish occupied the same shelter a few days in
Table 1. Rock shelters at which crayfish in two tubs were found when checked once a day. Each letter denotes a crevice or uninterrupted space under a rock that a crayfish considered suitable for a shelter. The locations of these shelters are shown in Figure 1.
succession, or else returned to a shelter repeatedly during the experiment, and it was this behavior that made the pattern of shelter choosing differ greatly from what would be expected by chance for all but one crayfish. In neither tub did a crayfish choose one shelter for the majority of the days. In the larger tub, all the crayfish but 3) and 5) chose two or three shelters the majority of the days, and in the smaller tub both crayfish did, though this was not statistically significant for 8). Several times, after the experimenter disturbed a crayfish, it left a shelter it had been occupying for a few days, and it seems certain that in such cases it would have stayed longer had there been no disruption; other times, a crayfish left a shelter it had been occupying for a few days though the experimenter had touched neither it nor its shelter; the three instances of this were on day 12 with 2), day 10 with 8), and day 14 with 9) (see Table 1).
Looking down the columns for both tubs at the location of the different crayfish on certain days, one can see that sometimes two crayfish were occupying the same place. Twice, with two crayfish pairs, a crayfish was observed unsuccessfully opposing the movement of another crayfish into its shelter and being driven out, but most of the time (observing in the daytime), the crayfish seemed not to be territorial. Though the crayfish were all sexually mature, there was no consistency in the sexes of the crayfish found staying in the same shelters.
Discussion
Three main initial questions one may ask of this experiment are
(1) whether the results reflect natural behavior, and if they do, (2) why the crayfish leave a shelter after occupying it for a few days, and (3) why they do not choose each of their shelters randomly. Doubts about the results are produced because the experiment may have been conducted for too short a time and because the crayfish were disturbed. It is difficult to decide whether the crayfish moved frequently only because they were not sufficiently acclimated to their surroundings or sensed that their shelters had not protected them, or, in part, because this is the way they normally behave.
It is believed that keeping shelters temporarily is natural behavior. At times toward the end of the experiment, when the crayfish might have been adjusted and when they had not been disturbed by the experimenter, they still left apparently suitable shelters after occupying them for at least two consecutive days. Encounters with other crayfish may have caused this, and this same occurrence may make shelters in the natural habitat temporary. Other investigators, videotaping two crayfish species in lakes, found them competing with each other and leaving shelters (Bergman, et al, 2003).
A study of garter snakes, Thamnophis sirtalis sirtalis, found that in captivity, when provided with shelters, groups of snakes tend to form, each group generally returning to the same shelter, repeatedly. "Snakes eventually re-formed their preferred social environment" (Skinner and Miller, 2020). So, in comparison to the crayfish in this study, each group of garter snakes behaved analogously to one crayfish.
After observing how mobile the crayfish were in the tubs, it is believed that most members of C. bartonii in nature rarely keep a shelter for as long as a month. The low degree of territoriality may fit the view that this species keeps short-term shelters. Observations from this experiment seemed to indicate that the older crayfish might keep shelters longer than the younger ones. Perhaps interestingly, the popular natural history and history of science, The Secret Life of Lobsters, about the American lobster, Homarus americanus, reports that young lobsters may molt more frequently than older ones, requiring the younger lobsters to find new and larger shelters more frequently, for protection (Corson, 2004) -- though no crayfish molted during this study. Investigators studying other crayfish species in nature hypothesize that “A shelter’s protective value may outweigh the value of the food sources when the threat of predation is especially high” (Bergman, et al, 2003). Another study, observing groups of the crayfish Astacus astacus in aquariums for a week, concluded that a dominance hierarchy becomes established among the crayfish, leading to more stable shelter occupancy over time; this was hypothesized to be typical of crayfish behavior in isolated rock pools (Goessmann, et al, 2000). More studying must be done, perhaps testing variables, before a view on the length of time C. bartoniii occupies its shelters can be expressed confidently.
This tendency for C. bartonii to leave its shelters, if real, may be favored by natural selection. Excrement and parasites are probably not factors. The crayfish do not excrete only in their shelters, and in the tubs other crayfish shelters they moved to should be as repellent to them as their own. The main parasites of C. bartonii do not live well in the darkness of the host’s shelters (Bishop, 1968), so would not provide pressure for the crayfish to leave them. But C. bartonii usually lives in calm water, and in such a medium, the ability of crayfish chemoreceptors to detect distant food is markedly limited (Ameyaw-Akumfi, 1977). In this experiment, it was observed that crayfish that had had very little food seemed to find food accidentally, while feeling (see also Corson, 2004) .In their natural habitat, perhaps those individuals that move from a shelter in one area to one in another are more likely to locate stationary food such as plants or decaying material than crayfish that do not. This interpretation may be supported by the data showing that the majority of the crayfish did not stay in any one of three regions of the tubs a significant number of days in their movement from on shelter to another.
The data do indicate unequivocally that C. bartonii in its usual habitat probably does stay in the same shelters for at least a few days, and that its choosing is not random. This can be explained by the dependability of protection provided. Crayfish that choose suitable shelters and return to them probably use less energy and are also more likely to survive the next day than those that search each night and may not find adequate concealment.
Literature Cited
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https://dnr.maryland.gov/streams/Documents/KeytotheCrayfishesofMD_8_18_10.pdf. Accessed 27 October 2020.
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