Stefano Cannicci

Homing behaviour and possible cognitive maps in crustacean decapods

Abstract This review on homing in decapods, which has been shown by Herrnkind (1983) and Wehner (1992), indicates how scarce our knowledge is, not only on the physiological mechanisms involved in such behaviour but also on the reality and extent of the behaviour itself. The case studies cited (first part) show that among decapods (only “reptants” are considered here), some species seem to wander at random, some can keep within a more or less well defined home range with no particular “home” while yet others are known to relocate periodically a definite restricted goal. Although burrows and shelters are the primary homing goals, cases of relocation of food resources and mates have also been reported. Some decapods occupy a single shelter, while others tend to visit, on a more or less regular basis, a system of shelters. The second part of this review deals with the mechanisms involved in decapod homing. Apart from idiothetic mechanisms, visual cues certainly play the most important role among terrestrial or semi-terrestrial species. Chemical cues may play a role in benthonic species, although direct evidence for this is still lacking. In certain cases a cognitive map relying on a system of orienting cues can be inferred.

Spatial variations in macrobenthic fauna recolonisation in a tropical mangrove bay

Recolonisation by crab species and sediment-infauna taxa (at class level) in artificially regenerated mangrove stands of Avicennia marina, Rhizophora mucronata and Sonneratia alba (5 yr old) were studied using respective bare sites (open without mangroves or denuded) and natural sites (relatively undisturbed) as controls. The controls were chosen based on site history, physical proximity and tidal inundation class in reference to the particular reforested mangrove stand and samples randomly taken. A number of environmental variables were measured; interstitial water salinity and temperature (measured at low tide) were lower, whereas sediment organic matter content was higher in the areas with mangrove cover, with the natural sites having the highest content. The bare sites were generally sandier, whereas the areas with mangrove cover had higher proportions of clay and silt. Generally, there was a higher crab density in the reforested sites than in the bare sites, whereas crab species diversity (Shannon diversity index) did not vary from one site to another for any of the mangrove species. In terms of crab species composition, the reforested sites were more similar (Sørensen similarity coefficient) to the natural sites and less to the bare controls. For sediment-infauna, the reforested sites had a significantly higher density than the respective bare controls, while the natural sites had the highest density. The number of sediment-infauna taxa in both the reforested and natural sites of all the mangrove species was similar and higher than in the comparable bare sites. The results suggest that the reforested sites are supporting more faunal recolonisation, and therefore becoming more akin to the natural mangrove sites in terms of the investigated functional indicators. The findings seem to support the use of artificial mangrove regeneration (in areas where natural regeneration has been impeded by physical conditions or otherwise) as an effective management tool in the restoration and conservation of the functional integrity of degraded mangrove habitats.Key words: Crabs, Environmental variables, Kenya, Recolonisation, Restored mangroves, Sediment-infauna

Marching to a different drummer: Crabs synchronize reproduction to a 14-month lunar-tidal cycle

Biological rhythms with lunar components are common in nature. In the sea, the moons gravitational pull on earth is the principal cause of the tides, which normally reach maximum amplitudes every new and full moon. Many populations synchronize spawning to this time. Some choose either the new or the full moon, implying that moonlight is important; but one lunar phase usually has higher tides than the other, and many species select the phase with the higher tide to improve the offshore transport of their progeny. However, tidal dominance by one lunar phase is not constant; it switches between new and full moon every seven months. We tested the influence of this 14-month “syzygy inequality cycle” (SIC) on lunar synchrony by sampling 11 populations of intertidal crabs at two locations in East Africa for 21 months. Eight populations synchronized larval release with the SIC. Tidal cues were more important than moonlight in entraining the reproductive rhythm, although two populations synchronized spawning to t...

Natural diet and feeding habits of Thalamita crenata (Decapoda: Portunidae)

Thalamita crenata is one of the most common swimming crabs of the mangrove creeks of the East African coast. In Mida Creek, Kenya, this species inhabits the extreme seaward fringe of the mangrove swamp and the intertidal platform in front of the mangal, sheltering in small pools during low tide. Gut content analysis reveals that T. crenata is a generalistic predator, its diet being mainly composed of bivalves and slow-moving crustaceans. Both the stomach fullness and the relative presence of animal prey in the contents were significantly higher in crabs collected at sunset than in those caught at dawn. Stomach fullness seems to depend also on the tidal rhythm; in fact, it is higher during spring tide periods. Females had stomachs slightly fuller than those of males, while there was no difference in diet between juveniles and older specimens. Thalamita crenata forages more actively during daytime, thus differing from the majority of swimming crabs. Both the great abundance of this species and its diet, based on a wide range of slow-moving or sessile species, testify to the importance of the role played by this predator in the mangrove ecosystem of Mida Creek. Thalamita crenata H. Milne Edwards is a common and widespread swimming crab that inhabits the shallow waters of all the Indo-Pacific region. Along the Kenyan coast, large populations of this crab are very often associated with mangrove

An exploratory study on grapsid crab zonation in Kenyan mangroves

Despite earlier efforts to understand the role played by grapsid crabs inmangroves, their importance in the structuring and functioning of suchsystems is fully appreciated, particularly with regard to small-scalestudies. The present study provides some new data on the interaction between mangroves and crabs, namely the link between the distribution of particular mangrove tree species and the distribution of certain crab species at the assemblage level. Floristicand faunistic relevés were made in Gazi Bay (2 sites) and Mida Creek(3 sites), 140 km apart on the Kenyan coast, along five transects in aseries of quadrats covering the width of the mangrove belts. Zonation ofboth mangrove vegetation and brachyuran fauna was described and heightabove datum and distance to the mainland (limit of non-flooded area)measured. The relationship between the presence and the absence of crabs andtrees was analysed using detrended correspondence analysis.Summarized, the mangrove tree zonation pattern contains four assemblageswith a particular dominant species: first a landward Avicennia marinazone, followed by a mixed zone with Ceriops tagal, Rhizophoramucronata and Bruguiera gymnorrhiza, and finally a R.mucronata zone and a Sonneratia alba zone, both of which can mixwith seaward A. marina. Ordination results show that the distributionof Neosarmatium meinerti and Sesarma ortmanni is linked to thelandward A. marina zone, that of Neosarmatium smithii, Sesarma guttatum and Sesarma leptosoma corresponds to the R. mucronata zone and that of Metopograpsus thukuhar and Sesarma elongatum to the seaward A. marina and S. albazone. There appears to be one major underlying factor in the zonation of bothcrabs and trees, with most likely a complex multiple causality. In certaincases the association between crabs and trees are causal, whereas in othercases it can be the result of an independent restriction to the same zonesby a common cause.

Are fiddler crabs potentially useful ecosystem engineers in mangrove wastewater wetlands

The effect of different organic-rich sewage concentration (0%, 20% and 60% diluted in seawater) and absence or presence of mangrove trees on the survival, bioturbation activities and burrow morphology of fiddler crabs species was assessed. After 6 months, males of both species always showed higher survival ( approximately 80%) when compared to females ( approximately 20%). Crabs inhabiting pristine conditions achieved higher survival (67-87%) than those living in sewage-exposed mesocosms (40-71%). At 60% sewage loading, fiddler crabs processed less sediment (34-46%) during feeding and excavated slightly more sediment (45-80%) than at pristine conditions. While percent volume of the burrow chambers increased (13-66%) at contaminated mesocosms for both vegetation conditions, burrows were shallower (approximately 33%) in bare cells loaded with sewage. The results show that fiddler crabs presented moderate mortality levels in these artificial mangrove wetlands, but mainly in sewage impacted cells. However, they still function as ecosystem engineers through bioturbation activities and burrow construction.

Feeding clusters and olfaction in the mangrove snail Terebralia palustris (Linnaeus) (Potamididae : Gastropoda)

Large numbers of the snail Terebralia palustris (Linnaeus) (Potamididae; Gastropoda) are frequently observed feeding in a cluster on a single fallen mangrove leaf, yet none are present on leaves nearby. Consequently, we investigated the food-finding ability of T. palustris in a Kenyan mangrove forest using field experiments. We estimated the attractive effect of different cues and analysed the paths (video-recorded) of snails when approaching a food-related odour. This intertidal snail can potentially use both air-borne and water-borne odours to locate food. T. palustris is attracted to conspecifics feeding on leaves, while intact leaves as well as non-feeding snails are not attractive. Moreover, the guiding stimulus appears to be compounds released when the leaves are damaged.T. palustris also seems able to discriminate between different food items; it is more strongly attracted to green mangrove leaves than senescent or fallen ones or mangrove propagules, probably because green leaves release a greater amount of attractive cues.Feeding snails thus recruit more snails to feed on the same leaf. The ecological implications of this behaviour are discussed: a large number of snails on the same leaf counteracts the ability of crabs to remove the leaf being eaten by the snails.

An ordination study to view vegetation structure dynamics in disturbed and undisturbed mangrove forests in Kenya and Sri Lanka

The mangrove vegetation of a disturbed and undisturbed site in bothKenya and Sri Lanka was investigated in the field for three vegetation layers:adult trees, young trees, and juvenile trees. A minimum of 25 sample points, inwhich the vegetation was described and environmental factors (salinity, lightintensity, land/water ratio, abundance of herbivorous crabs and snailabundance)were measured or estimated, were taken on each site. Detrended correspondenceanalysis (DCA) and canonical correspondence analysis (CCA) were used tosummarize the data bulk, to investigate the vegetation dynamics(e.g., comparability of species distribution in the threevegetation layers), and/or to link the vegetation data to the environmentalfactors. Results showed that species clusters were relatively easy todelineate,whether mangrove zonation was present or not. Among the environmental factors,the abundance of propagule predators (mostly sesarmid crabs) contributedsignificantly to the variation in vegetation and could be an explanatoryparameter for the observed vegetation data in a majority of sites. In the sitewhere it was not, the most important factor in the ordination was theland/waterratio, which is important at the ecological level as well (link between waterlevel and vegetation dynamics). However, none of the environmental factorscouldsuccessfully explain the total variability in the vegetation data suggestingthat other, more determining factors exist. Our results further provideinformation on the dynamic or non-dynamic nature of a forest and on its abilityto rejuvenate, and may contribute to appropriate forestry management guidelinesin the future.

Tree fidelity and hole fidelity in the tree crab Sesarma leptosoma (Decapoda, Grapsidae)

An integrated unit associated with the shower head adapted for mounting in a shower stall. The unit contains conventional aerosol cans or containers for fluids such as soap and/or shampoo and the like. The aerosol cans are positioned to dispense into cavities, there being channels controlled by valves, providing communication between these channels, and a tube or pipe leading to the shower head for mixing the fluid controllably with the shower water. Associated with the cavities are manually actuatable plungers which are operable to cause dispensing of fluids from the cavities. Manually actuatable means can be provided for actuating the aerosol can valves to dispense.

Improved heat tolerance in air drives the recurrent evolution of air-breathing

The transition to air-breathing by formerly aquatic species has occurred repeatedly and independently in fish, crabs and other animal phyla, but the proximate drivers of this key innovation remain a long-standing puzzle in evolutionary biology. Most studies attribute the onset of air-breathing to the repeated occurrence of aquatic hypoxia; however, this hypothesis leaves the current geographical distribution of the 300 genera of air-breathing crabs unexplained. Here, we show that their occurrence is mainly related to high environmental temperatures in the tropics. We also demonstrate in an amphibious crab that the reduced cost of oxygen supply in air extends aerobic performance to higher temperatures and thus widens the animals thermal niche. These findings suggest that high water temperature as a driver consistently explains the numerous times air-breathing has evolved. The data also indicate a central role for oxygen- and capacity-limited thermal tolerance not only in shaping sensitivity to current climate change but also in underpinning the climate-dependent evolution of animals, in this case the evolution of air-breathing.