Homing Experiments in the Tree-climbing Crab Sesarma leptosoma (Decapoda, Grapsidae)

Ethology 103, 935-944 (1997) 0 1997 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179-1613

Dipartimento di Biologia Animale e Genetica 'Leo Pardi' dell'Universitd degli Studi di Finnre, and Keya Marine and Fisheries Research Institute, Mombasa, and Muse0 Zoologico 'La Specola '

dell'Universita degli Studi di Firenxe

Homing Experiments in the Tree-climbing Crab Sesama leptosoma (Decapoda, Grapsidae)

STEFAN0 CANNICCI, RENISON K. RUWA & M A R C 0 V A " I N 1

CANNICCI, S., RWA, R. K. & VANNINI, M. 1997: Homing experiments in the tree-climbing crab Sesuma leposoma (Decapoda, Grapsidae). Ethology 103, 935-944.

Abstract Sesumu Iepfosomu an East African mangrove-dwelling crab, migrates twice a day from a system of known

dens among the roots to well-defined feeding areas in the branches of trees, reaching 15 m high. Field experiments were performed to test whether chemical or visual cues are involved in the orientation and homing of this species to reach their feeding areas. Manipulation of the substratum at branch junctions, in order to alter possible chemical cues, did not affect homing ability in S. leptosomu. Moreover, crabs trajned to cross an asymmetrical artificial wooden fork could s t i l l follow their preferred directions after (1) the fork branches had been switched, (2) the whole fork had been rotated around the trunk, resulting in a right-left inversion, and (3) the inversion of two wide black and white screens hiding most of the canopy from view of the climbing crabs. These results suggest that S. Leptosomu may not rely on reference systems such as chemical trail-following and chemical or visual cues from the substratum, but probably depend on complex visual information from the surroundings trunks and/or from the sun's position integrated with junction sequence memory.

Corresponding author: Stefano CANNICCI, Dipartimento di Biologia h a l e e Genetica 'Leo Par& deU'Universit9 degh Studi di Firenze; via Romana 17,I-50125 Firenze, Italy

Introduction

Benthic, semiterrestrial and terrestrial crustacean decapods are able to solve a huge number of orientation problems during their excursions. Although their orientation and homing ability has often been observed in the field, very few in-depth analyses on the navigational mechanisms involved have been performed. Furthermore, recent reviews by HERRNJSIND (1983), REBACH (1983) WEHNER (1992) and VANNINI & CANNICCI (1995) have pointed out that a broad variety of sensorial cues are employed in decapod orientation.

Most probably, chemical cues are involved in the homing ability and periodic mass migrations of the Caribbean spiny lobsters of the genus Pandims (HERRNKIND & MCLEAN 1971; HERRNKIND et al. 1975; HERRNKIND 1980). The lobster Homams umericanus has been shown to know its immediate environment well (IQRNoFsKY et al. 1989) and, at least to some extent, relies on chemical stimuli from conspecifics (ATEMA 1986). The terrestrial hermit crabs (genus Coenobita) are able to orient themselves towards food

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936 S. CANNICCI, R. K. RUWA & M. VANNINI

(RITTSCHOF & SUTHERLAND 1986) and, at least in the laboratory, water sources (VAN- NINI & FERRETTI 1997).

None of the above species appear to produce chemical trails, such as those involved in the trail-following behaviour of intertidal molluscs (CHELAZZI 1992) and ants (HOLLDOBLER & WILSON 1990; WEHNER 1992), on their excursions. Rather, they are thought to arrange the chemical stimuli coming from the surrounding environment into a type of mental map of their f d a r area PAPI 1992; W E H N E R 1992; VANNINI & CANNICCI 1995).

Conversely, many intertidal and terrestrial decapods rely mainly on visual cues to find their goals. After passive displacement of 25m Eqbbiu smith (VANNINI & GHERARDI 1988) and Tbukzmitu crenutu (CANNICCI et al. 1995) can only relocate their dens if they are allowed to see the landscape. Terrestrial ghost crabs OGypode cerutophtbuLmas (HUGHES 1966) and 0. surutun (LINSENMAIR 1967) were found to orient themselves on the beach using the prominent visual landmarks they build near their shelters.

This study was designed to investigate the orientation mechanisms that gulde the tree crab Sesumu leptosomu on its feeding migrations in search of suitable feeding areas in an unusual environment - the canopy of tall mangrove trees.

Methods

The Species

S. leptosomu Hdgendorf, 1869, is a small Sesarminae species confined strictly to East African coast (GUINOT 1967; HARTNOLL 1975), Mauritius @. A. JONES, pers. comm.) and Madagascar (quoted as Paraseramu sp., CROSNIER 1962). Together with the American Arufuspisonii (WARNER 1967; BEEVER et al. 1979) and the West African A. &guns (GREEN 1986), it is one of the three species known to feed among the branches of mangrove trees. Although S. lepfosoma is the best adapted of the Indo-Pacific species to climb mangrove trees (SIVASOTHI et al. 1997; VANNINI et al. 1997, it spends the entire night and part of the day among the aerial roots of Rhizophoru mumnufu and migrates twice daily towards the canopy to feed on fresh leaves (VANNINI & RUWA 1994). AU the crabs living on the same tree begin migrating at dawn (VANNINI et al. 1995) and return to the roots later in the morning. A second synchronous migration takes place mid-afternoon and ends at dusk.

Recent field observations revealed that S. leptosomu is faithful to a particular home tree (CANNICCI et al. 1996a) and is a typical central place forager (WEHNER 1992), i.e. individuals participating in the upward migrations follow well-defined paths which allow them to reach specific feeding areas. These areas provide suitable leaves to eat as well as leaf-buds which supply them with fresh water (CANNICCI et al. 199613).

Study Area and Experimental Techniques

The study was carried out in the mangrove swamp of Dabaso, a locality within Mida Creek, 20 km South of Malindi on the Kenyan coast, from Jul. to Aug. 1994 and Nov. to Dec. 1995. The RizDphoru belt of this swamp, about 60-80 m wide, is composed of mature trees up to 15 m high and is regularly flooded twice per day.

‘Gauze’ Experiment

For the first experiment (in 1994), a 15 m high R. mumnutu was chosen and 156 resident crabs were captured during their downward morning migration, individually marked with white Tippex, and immediately released among the roots of the same tree. On the same day, four branch junctions (branch diameter between 10 and 15 cm), all between 10 and 12 m above ground level, were covered with plastic film and gauze strips (from about 25 crn above to 25 below the junction itself). The plasuc film was used to prevent the migrating crabs from coming into tactile or sensorial contact with any chemical trail previously left on the branches. The medical gauze covering the hlm helped the crabs climb over the covered junctions and at the same time could easily absorb

Orientation in a Tree Crab 937

chemical cues they may have deposited. Two observation stations were then set up among the branches of the experimental tree 12 and 14m above ground level and z 5 m from the treated branches.

The marked crabs were monitored for the following 17 d from the canopy observation points with the aid of binoculars. The directions each crab took at the covered junctions on 12 morning upward migrations were recorded.

After this observation period, the plastic films and gauze from two of the four forks were replaced pig. 1) with film and gauze which had covered the branches of a tree 30 m away over the same period. This tree was therefore unfamiliar to the marked crabs (see CANNICCI et al. 1996a). At the same time, the control gauze strips and plastic films from the other two branches were removed and put back in the same place. Only the directions each crab took the very first time it reached the two control and the two newly replaced junctions were recorded.

‘Artificial Fork’ Experiments

A second series of experiments was performed on a different tree, in 1994 and 1995, where 102 and 160 crabs were individually marked, in each year, respectively. The direction each crab took at a natural fork of the trunk (Fig. 2.) about 4 m above ground level (about 65 cm in diameter), was recorded during several morning and evening migrations, from an observation point 8 m away. The fork was covered with PVC sheeting (from 1 m above to 1 m below the junction) to stop the crabs passing through (Fig. 2b). Three wooden boards were then placed over the PVC to build a new fork, with the two lateral boards equally inclined, which the crabs could use to cross the obstacle represented by the plastic sheeting. In the 1994 experiments, the newly built fork was asymmetrical because the lateral boards differed in width, at 10 cm and 4 cm, respectively (Fig. Zc), while they were asymmetrical both in width and colour patterns (yellow stripes and red spots) in the 1995 experiments (Fig. 2d).

X hpfosomu rapidly solved the problem of crossing the PVC sheeting: after only about 30min of their instalment, all the marked as well as unmarked crabs used the wooden boards as a bridge. The directional

Fig. 7: Schematic diagram of two of the four forks in the branches in the ‘gauze’ experiment. A. Situation during the 17-d observation period; B. experimental junctions where plastic and gauze strips were exchanged


- 50 cm

F&. 2: Semischematic diagram of the ‘artificial fork‘ experiment. A. Natural fork formed by the tree trunk; B. covering the fork with PVC sheets to prevent crab migration; C. artificial wooden fork constructed to allow the

crabs to migrate again; D. fork with coloured side-branches

preferences of each crab on 1&15 consecutive days were noted from the day after the device had been installed. During all this observation period, the number of marked crabs observed to cross the junction fluctuated between a minimum of 15 and a maximum 40 specimens (for a general discussion of the meaning of these fluctuations see VANNINI & RUWA 1994).

Three sets of experiments were then performed. A. Board switGb&g; at the end of the 10d preliminary observations, the two lateral boards were swapped over to jumble any possible chemical, tactile and short-range visual cues; the directions taken by each marked crab were again recorded. B. hj-ngbt inversion; after 14 d, the whole artificial fork was removed and placed on the opposite side of the trunk, thus giving conflicting visual information and information from the crabs memory about the last side chosen, and the crabs’ behaviour was observed after the inversion. C. canopy alteration: two 4 x 8 m lengths of material (black and white) were placed 2 m above the junction pig. 3), thereby hiding about 80% of the canopy, for crabs climbing towards or crossing the wooden fork, while leaving about 50% of the branches and foliage surrounding the trunk visible at the junction level. After 15 d of preliminary observations the two coloured screens were switched over and the crabs behaviour observed after the inversion.

In all these cases only the crabs observed to cross the wooden fork on the first day after the manipulations were analysed. This experimental procedure, while reducing the data for the statistical analysis, was used to avoid recording crabs already experienced in the manipulation features.

All the manipulations, such as setting and/or removing the wooden fork or placing and switching the


F2. 3: Semischematic diagram of the canopy alteration experiment. The position of the white and black tissues is shown

screens under the canopy, were performed at 04.00 hours or around midday, i.e. 2 4 h before the onset of upwards crab migrations (VANNINI & RUWA 1994).

Results

‘Gauze’ Experiment

In the 12 observation sessions, in 1994, 34 marked individuals were tracked more than five times while they climbed over the covered forks. Of these, 26 (76.5%) always followed the same (‘preferred’) direction, and the eight crabs that did not remain faithful to a single branch, changed direction only once during the whole observation period. When the crabs first came into contact with the gauze, they often stopped and tried to pick it up with their claws before deciding which direction to take. From the second time onwards they totally ignored the gauze. The maximum time employed to cross the covered area of the branches was 15 s.

In the gauze exchange session it was possible to track 24 crabs out of the 26 which always followed one direction during the observation sessions. It was observed that 84.6% of the controls (crabs which crossed the same gauzes as before) and 90.9% of the experimentals (those obliged to cross the switched gauzes) kept to their preferred directions (Table 1). There was no significant difference in the rate of direction change between the controls and experimentals (G=O.188; df= 1; p=ns; G-test). The time the crabs took to cross the covered sections of branches did not exceed the 15 s recorded in the previous observation sessions.

‘Artificial Fork’ Experiment, Board Switching

In 1994, 29 crabs were observed to cross the experimental fork more than once during six consecutive observation sessions. During the first session after the setting in place of the artificial fork 14 marked crabs crossed it and 11 of them chose the ‘preferred’ direction recorded in the previous observations (Table 2).


Tubh 1: ‘Gauze’ experiment. Directional choice of crabs after manipulation of control gauze strips and after changing the plastic film and gauze covering the branch junctions; n = number of crabs; p = probability

level determined by binomial test (two-tailed)

Chosen direction (YD) Preferred Other n P

Control junctions 84.6 15.4 13 0.0224 Experimental junctions 90.9 9.1 11 0.0118

Table 2: ‘Artificial fork‘ experiment, board switching. Directional choice of crabs aker placing the wooden artificial fork over the PVC sheets (a) and after inverting the two lateral boards @, c); n =

number of crabs; p = probability level determined by binomial test (two-tailed)

Chosen direction (Yo) Preferred Other n P

(a) After placing the wooden fork over the PVC sheets 78.6 21.4 14 0.0574

After switching the lateral boards: (b) Direction always chosen 89.5 10.5 19 <0.001 (c) Last direction chosen 88.9 11.1 27 <0.001

Observations following construction of the new fork over the PVC revealed that not all the crabs exhibited a strong preference for one particular direction. Some changed from one to the other, but then remained faithful to the last direction chosen.

When the asymmetrical boards were switched over, 89.5% of the crabs (n = 19) that had constantly remained faithful to one direction during the observation period still chose the same direction. Moreover, 88.9% of the crabs that had changed from one direction to the other (n = 27) remained faithful to the last direction taken (Table 2).

‘Artificial Fork’ Experiment, Left-right Inversion

When the whole artificial fork (whose lateral boards differed in both size and colour pattern) was removed and attached to the opposite side of the tnmk, 90.1% of the crabs (n= 11) that had constantly chosen one direction during the observation period kept to the same direction (but not to the same board) while 94.7% of the crabs that had switched from one direction to the other (n = 19) remained faithful to the last direction taken (Table 3).

‘Artificial Fork’ Experiment, Canopy Alteration

After switching over the two cloths, 100% of the crabs (n = 10) that had constantly chosen one direction during the observation period kept to the same direction (under an


Table 3: 'Artificial fork' experiment, right-left inversion. Directional choice of crabs after 180° rotation of the wooden fork around the trunk. n = number of crabs; p = probability level determined by binomial test (two-

tailed)

Chosen direction (YO) Preferred Other n P

~~ ~

(a) Direction always chosen 90.1 9.9 11 0.0117 @) Last direction chosen 94.7 5.3 19 <0.001

inverted canopy pattern) while 95% of the crabs that had changed direction (n = 20) remained faithful to the last direction taken (Table 4).

Discussion

Results show that the tree crab S. ltptosomu, does not rely on direct cues from the substrate when migrating along the trunks to reach its familiar feeding grounds. When faced with a junction, S. leptosomu was able to maintain its preferred direction, even when any possible chemical cues coming from substratum had been removed (Table 1) or when chemical and visual cues related to the artificial boards were inverted (Table 2). Moreover, it should also be noted that the crabs were well oriented the very first time they were forced to cross the boards placed over the PVC in order to reach their feeding areas (Table

Although volatile odour tracking is now known in terrestrial decapods (RITTSCHOF & SUTHERLAND 1986; WELLINS et al. 1989; VANNINI & FERRETTI 1997, it does not appear that this species employs this, or at least not for orientation purposes.

Alternatively, S. ltptosomu could rely on the memorization of the correct left-right choice sequence in its orientation. On average, a crab comes across five junctions on its migration from roots to canopy, on its way to its preferred feeding ground (CANNICCI et al. 199613). Inversion of the left-right direction by rotating the whole fork by 180° did not affect the crabs' choice, even when the difference between the two boards was emphasized with different colours and patterns. In other words, after the boards were switched, the crabs immediately ceased to use the board they used before (the right hand one with red

2).

Table 4: 'Artificial fork' experiment, Canopy alteration. Directional choice of crabs after inverting the black and white screens; n = number of crabs;

p = probability level determined by binomial test (two-tailed)

Chosen direction (YO) Preferred Other n P

(a) Direction always chosen 100 0 10 0.002 @) Last direction chosen 95.0 5.0 20 <0.001


spots or the left hand one with yellow stripes) and walked along the other one to reach their usual branch in the tree.

The crabs probably do use visual cues from the canopy or surrounding trunks and foliage: if this is so, their visual evaluation must be highly refined if the partial manipulation of the canopy we attempted, by spreading white and black lengths of material above their heads, failed to affect their orienting capability. It is in any case worth noting that immediately after the screens were set in place, many of the crabs @erhaps up to 75”’) abandoned the tree. It should also be borne in mind that in other experiments, almost all crabs which were caught, marked and removed from their home trees, returned home within 2-3 d (CANNICCI et al. 1996a). In the case of the screen experiments, manipulation could not have caused the crabs to vacate the tree but rather visual modification of the canopy.

There is currently no evidence that intertidal and terrestrial decapods rely on any type of egocentric frame of reference (sensu WEHNER 1992), as in the path-integration mechanism which well known in ants and bees (WEHNER & WEHNER 1986; WEHNER 1992). Conversely, these crustaceans most likely use a geocentric orientation system based on the storage of visual landmarks, similar to that employed by ants and bees once they are in proximity of their goal (WEHNER & RABER 1979; CARTWRIGHT & COLLET 1982).

The horizon is not visible from within the depths of the mangrove forest but, nevertheless, crabs can perceive celestial cues such as the position of the sun or polarized light. However, it is difficult to conceive how this could assist crabs in their junction sequence, especially when the junctions are perpendicular to the sun, unless the position of the sun is coupled to sequence memorization. For example, they could choose the right arm in the morning when the sun is in front of it, or the left one if the sun comes from behind; the opposite would be true in the evening.

Furthermore, there are no experimental data available on the spatial behaviour of arthropods which feed in an intricate environment except for forest-living ants. Most of these species rely on trail-following mechanisms based on pheromones (BREED & HAR-

RISON 1987), but at least two species, PultotLyreus tursutus (HOLLDOBLER 1980) and Paruponera ckzvutu (HARRISON et al. 1989), can learn to reach their feeding areas using distant landmark panorama. P. tarsutus, in particular, orients itself by vision of the pattern of the surrounding canopy, while P. cluvutu changes from a trail-following mechanism to a visual frame of reference with experience.

For the time being, the most plausible hypothesis is that S. leptosomu finds its way through the canopy by using visual cues from surrounding trunks and foliage and/or the sun position coupled with junction memorization, i.e. the only reference systems not manipulated in our experiments.

Acknowledgements

Thanks are due to S. RITOSSA, L. MORINO and C. BARELLI for their help during the field work. We also thank Dr E. OKEMwA, Director of KMFRI, for his help in the mangrove research programme. Climbing and spelaeological apparatus was kindly supplied by Kong Ltd, Montemarenzo, Bergamo, Italy. The research was funded by Centro di Studio per la Faunistica ed Ecologia Tropicali of the Italian CNR and from MURST grants.

Orientation in a Tree Crab

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Homing Experiments in the Tree-climbing Crab Sesarma leptosoma (Decapoda, Grapsidae)