d. Thermal Bioh,ffy Vol. 4. pp. 165 to 166 0306-4565/79/0104 0165502.00/0 © Pergamon Press Lid 1979. Prinled in Greal Britain

T H E R M O R E G U L A T O R Y B E H A V I O U R O F T H E

P R I M I T I V E A R T H R O P O D L I M U L U S POLYPHEMUS I N A N E L E C T R O N I C S H U T T L E B O X

WILLIAM W. REYNOLDS and MARTHA E. CASTERLIN Department of Biology, The Pennsylvania State University, Wilkes-Barre, PA 18708, U.S.A.

(Received 6 June 1978; accepted 10 June 1978)

Abstract--1. Ten Limulus polyphemus were tested individually for 3-day periods in an electronic shuttle- box (Ichthyotron) to characterize their thermoregulatory performance.

2. With access to a potential temperature range of 0-50°C, the Limulus voluntarily occupied a range of 15-40°C without ill effect.

3. The mean thermal preferendum of these animals was 29.4°C (S.E.M. = 0.19°C; S.D. = 5.27°C; median 29°C; midpoint 27.5°C; mode 27°C, Pearson's skewness coefficient = +0.46).

4. Limulus is capable of thermoregulatory behaviour, but is less precise in terms of voluntarily occu- pied range (a 25°C span) than aquatic ecothermic animals previously tested; it is quite eurythermal in tolerance, and can endure 40°C for at least 8 h.

INTRODUCTION

Limulus polyphemus, the horseshoe "crab", is a primitive chelicerate arthropod found in coastal and estuarine waters along the east coast of North Amer- ica from Yucatan to Nova Scotia (Robertson, 1970). It is tolerant of diverse environmental conditions, in- cluding hypoxic water conditions (Johansen & Petersen, 1975) and temperatures as high as 40°C (Fraenkel, 1960). Studies have been conducted on the orientation of Limulus to light (Lall & Chapman, 1973) and to water movement (Rudloe & Herrnkind, 1976), but little or nothing is known of its behavioural responses to temperature. The purpose of the present investigation was to determine the ability of Limulus to thermoregulate in an electronic shuttlebox (Ich- thyotron) which permits an aquatic animal to control water temperatures, and thereby its own body tem- perature, by its movements (for the historical develop- ment of electronic shuttleboxes for studying thermo- regulatory behaviour and locomotor activity patterns in aquatic ectotherms, see Frank 1971; Neill et al., 1972; Reynolds et al., 1976; McCauley, 1977; Reynolds, 1977).

We have previously tested numerous species of aquatic ectotherms in the Ichthyotron, including ver- tebrates ranging from lamprey ammocoete larvae (Reynolds & Casterlin, 1978c) to sharks (Reynolds & Casterlin, 1978a), bony fishes (Reynolds & Caster- l i~ 1976, 1977, 1978b; Reynolds et al., 1976), and anuran amphibian larvae (Casterlin & Reynolds, 1977, 1978). All these vertebrates exhibited a range of voluntarily occupied (preferred or non-avoided) temperatures covering a span of less than 10°C, in- dicative of fairly precise thermoregulatory behaviour (the potential range of available temperatures spans 0-50oc).

METHODS AND MATERIALS Ten Limulus polyphemus shipped from the Panacea,

Florida area were tested individually in the electronic

165

shuttlebox described previously (Reynolds et al., 1976; Reynolds, 1977), in which the animals control water temperatures by shuttling between chambers differing slightly in temperature; in doing so, they interrupt light beams monitored by photocells, and associated circuitry activates heating or cooling equipment depending on the direction of movement (toward higher or lower temperatures). Temperatures and movement are recorded automatically. Each animal was allowed an initial 24-h period to habituate and acclimate (cf. Richards et al., 1977), after which it was tested for a 3-day period. Data from all 10 individuals was pooled to generate a relative frequency polygon of voluntarily occupied temperatures, showing the amount of time spent at each occupied temperature (Fig. 1). Tests were conducted at 27 ppt salinity, using artificial sea salts.

~ - i t ' "' J I - 9,- ~ -- 8 ! -

15 2 0 25 3 0 ,

°C

Fig. I. Relative frequency polygon of preferred tempera- tures from pooled day and night data for 10 Limulus poly- phemus tested for 3 days each in the Ichthyotron. Above is a modified Dice-Leraas diagram showing the range. (horizontal line), mean (vertical line), + 2 S.E.M. (dark

shaded bar), and __. I S.D. (open bar).

166 WILLIAM W. REYNOLDS and MARTHA E. CASTERLIN

RESULTS

The range of voluntarily occupied temperatures spanned 25°C, from 15 to 40°C (Fig. 1). There was no evident difference between night and day, so data for the entire 24-h period are pooled together. The mean thermal preferendum was 29.4°C, with a stan- dard error of 0.19°C and a standard deviation of 5.27°C. The modal preferendum was 27°C, while the midpoint was 27.5°C and the median was 29°C. The skewness (Pearson's coefficient; cf. Reynolds & Cas- terlin, 1976) was + 0.46. One individual was observed to tolerate 40°C for 8 h without apparent ill effect.

DISCUSSION

The 25°C range of voluntarily occupied tempera- tures exhibited by Limulus indicates that it is much less precise in its thermoregulatory behaviour than are vertebrates similarly tested, which occupy volun- tarily a range spanning less than 10°C (see references cited in introduction). Limulus also exhibited the high- est voluntarily occupied temperature (40°C) of any animal we have so far tested in the Ichthyotron. That Limulus can tolerate this temperature for at least 8 h is indicative of its considerable eurythermality (cf. also Fraenkei, 1960). (It is also worthy of note that, follow- ing the tests, the air was accidentally shut off in the holding tank for more than 24 h, the water becoming quite anoxic--smell ing of H2S--with no resultant mortality; apparently Limulus is often exposed to hypoxia by burrowing into muddy substrates, and it exhibits unusual respiratory adaptations--cf. Johansen & Petersen, 1975). In spite of the impreci- sion and wide range of tolerance, it is apparent that Liraulus is nevertheless capable of thermoregulatory behaviour to the extent of avoiding lethal tempera- tures, for it had access to a potential range from 0°C to more than 50°C in the Ichthyotron, and occupied voluntarily a more limited range of 15-40°C, which it obviously can tolerate.

Acknowledgement--This study was aided by PSU grant No. SAF-78-050.

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Key Word lndex--Thermoregulation: temperature preference: Limulus polyphemus; Ichthyotron: behaviour, thermal tolerance.