forschungsprojekte | biodiversität | meiobenthos | korallen | fischökologie | fossile riffe | verhalten
 
 
 

Defecation behaviour of Ctenochaetus striatus as a strategy to avoid parasites

Sabrina Hug - Bachelor Thesis - December 2011 - University of Tübingen

Abstract

The Striped Bristletooth surgeonfish (Ctenochaetus striatus) has a specific defecation behaviour, since individuals of this species defecates on specific spots, away from their feeding grounds. These toilets' are mainly located on sand (Krone, et al., 2008) and mean additional distances to swim, when they forage in their feeding grounds and swim away to a defecation spot. A possible explanation for this defecation behaviour could be a strategy to avoid a re-uptake of endoparasites (J. Choat, 1991), which may be contained in the faeces of C. striatus. For that reason 26 samples of faecal pellets were collected and examined for endoparasites. However, no endoparasites could be found in any of the excrements, neither in the egg, larvae, oocyst stage nor in the adult stage. Moreover, the excrements were examined for organisms, which are transported in the faeces. A wide range of meiofaunal organisms were found. Hence, the densities of meiofaunal organisms (1) between the faeces and adjacent sand samples and (2) between faecal samples of two different locations were compared. A significant higher density of meiofaunal organisms in the Canyon reef to the reef of Mashraba could be proven. However, there was no significant difference of the densities of meiofaunal organisms between faecal pellets and sand samples. Nevertheless, the excrements of C. striatus have a positive effect on the density of the endobenthic meiofauna living in the sediment of its toilets' (Förster, 2011). Therefore, the Striped Bristletooth has another important role for the coral reef ecosystem due to the input of organic and inorganic material.

 

 

Sabrina Hug

Ctenochaetus striatus

Ctenochaetus striatus

 

 
 

Effects of Ctenochaetus striatus' Defecation Behaviour on Coral Reefs' Endobenthic Meiofauna

Agnes Förster - Bachelor Thesis - July 2011 - University of Tübingen

Abstract

The Striped Bristletooth surgeonfish (Ctenochaetus striatus) stands out due to his special defecation behaviour. Fish of this species defecate away from their feeding grounds at individually belonging toilets, which are mostly located on sand (Krone et al. 2008) and in sheltered positions (Goatley and Bellwood 2010). Since the constant defecation at distinct spots comes along with an input of organic matter and nutrients (Krone et al. 2010), the following two assumptions were made: (1) The density of meiofaunal organisms in sediments below toilets is higher than in the surrounding area and (2) the composition of meiofaunal taxa differs between sediments below toilets and sediments below nontoilets.
In total, 26 sediment samples with a volume of 15 ml each were taken from 13 toilets and 13 other sand spots. The meiofauna was extracted, counted and identified, and then samples from toilets and non-toilets were compared. To exclude influences of other factors on the results, grain size of the samples was analyzed and exposure to hydrodynamic action, water depth and tides were recorded. A significant higher density of meiofaunal organisms in toilets compared to non-toilets could be proven. Furthermore, a significant change in the meiofaunal composition, which mainly based on a shift in proportions of foraminiferans and copepods was found. Hence, a new aspect of the defecation behaviour of C. striatus was revealed. Thereby this study contributes to the understanding of the Striped Bristletooth's ecological role.

 

  Agnes Förster
 
 

A potential mutualism between the reef sweeper Ctenochaetus striatus and the territorial grazer Acanthurus sohal (Acanthuridae)

Anouk-Isabell Neuhaus - Bachelor Thesis - July 2011 - University of Tübingen

Abstract

Sedimentation is detrimental for coral health and sediment removal thus vital for the resilience of reefs. The surgeonfish Ctenochaetus striatus removes sediment and accumulates it due to its characteristic defecation behaviour. Individuals of this species use certain spots (ʻtoiletsʼ) for repeated defecation events over a longer period of time. Thereby it facilitates a directed sediment transport. Another surgeonfish, the herbivore Acanthurus sohal, might benefit from this effective and lasting removal as it might promote algal growth. A mutualistic relationship may arise from these circumstances. Indeed I found out that A. sohal tolerates the presence of C. striatus in its territory although this large surgeonfish aggressively defends its territory against other species. It could not be confirmed that sediment removal enhances algal growth. In conclusion, a mutualistic relationship between the two surgeonfishes can be considered possible but the advantage for A. sohal could not be ultimately identified in this study and should be the subject of subsequent studies.

 

 

 

 

Anouk Neuhaus

Acanthurus sohal - skalpell

Acanthurus sohal

 
 

Characterisation of surgeonfish Ctenochaetus striatus defecation sites suggests possible functions of this behaviour

Ana Rodriguez Perez - Bachelor Thesis - August 2011 - University of Tübingen

Abstract

The characteristics of spots used by Ctenochaetus striatus for defecation were compared to "toilets" of three different habitats. Moreover, long time monitoring of some of the defecation sites was performed in order to find out if the "toilets" were used constantly over the surveyed period of time. The study revealed that all toilets were to some extent sheltered, which was the most relevant characteristic. Moreover, when sand was available, it was preferably chosen as substrate type as it was stated before. There was also a tendency for the less sheltered defecation sites being oriented towards the open sea. Finally, the surveyed "toilets" were continuously used over the monitored period of five weeks as defecation patches. Altogether, these findings reveal two main functions resulting from the defecation behaviour of this species. Firstly, C. striatus is not only, as mentioned in previous studies, directly responsible for sediment transport off-reef by feeding on the upper reef crest and defecating in deeper areas, but also indirectly through defecation in sites which are less sheltered towards the open sea; therefore, sediment drift through hydrodynamic agents is likely to occur in this direction. Secondly, another main function seems to be the transport of sediment from areas of high hydrodynamic exposure to sheltered patches. Taken together with the findings of the long time monitoring, namely that the defecation sites were used over at least five weeks, this behaviour could enhance the microbial growth on faecal sediment, as abrasion is a limiting factor for the expansion of microbes. These microorganisms could process nutrients of C. striatus faeces not accessible for other organisms and provide a direct trophic link between the microbes and organisms that ingest them. Hence, through this defecation behaviour, the internal nutrient flow within the reef ecosystem could be supported. However, this novel hypothesis needs to be verified.

 

  Ana Rodriguez
 
 

Habitat Preferences of Fish Assemblages in Masbat Bay, Dahab (Red Sea, Egypt)

Johanna Sophie Zimmerhackel - Bachelor Thesis - August 2010 - University of Hamburg, Section Ichthyology

Abstract

Possible effects of habitat characteristics on fish assemblages were investigated in the Masbat Bay in South Sinai, Egypt using snorkelling techniques. Therefore, the Masbat Bay was divided into squares containing transects for data acquisition and in a time period of six weeks a total number of 40 transects was examined. The visual census method by ENGLISH et al. (1994) was used to estimate the number of all fishes inside these transects. A total of 114 different species belonging to 37 families was counted. The quality and quantity of substrate was measured using the line intercept method (LIT) by ENGLISH et al. (1994) and ten environmental parameters, namely the time of day, water depth, wave height, distance from the shore, distance from the end of the reef, water temperature, current direction and intensity, tide and visibility were recorded. The high correlation found by regression of substrate diversity and fish diversity indicated that habitats in which the substrate was more structurally or topographically complex were favoured by the majority of observed fish species. Multivariate analyses were performed to explain the variation of fish assemblages in relation to the measured variables using the Canonical Correspondence Analysis (CCA) by TER BRAAK (1987). A large part of the variability of reef fish assemblages was explained by distance from shore and substrate diversity. Upon further examination, the differences in fish assemblages of a number of habitat types that were found in Masbat Bay were shown to be mostly influenced by the substrate types: branching corals, sand, algae and rubble.
The results of this study may gather extensive information about the diversity and abundance of fish assemblages in Masbat Bay, and should thereby provide knowledge as an applicable tool for the nature conservation management of the Masbat Bay area.

 

  Johanna
 
 

Fish Fauna Diversity in Relation to Habitat Structure at an Isolated Fringing Reef at Masbat Bay, Dahab, South Sinai, Egypt -Bachelor Thesis- Katharina Fietz

Abstract

Diversity of a fish assemblage was determined and compared to substrate structure, to the amount of coral damage, as well as to defined environmental variables on an isolated fringing reef off the coast of Egypt. A total of 95 different species belonging to 35 families were counted in the reef adjacent to the tourist resort town Dahab. Labridae and Pomacentridae dominated fish assemblages in terms of species diversity while Pomacentridae were most abundant with 1262 counted individuals out of a total of 4165.

Abiotic cover was the most frequented in substrate analysis followed by life coral cover and algae cover. The reef divided into three main zones: the coral area in the centre showed highest live coral cover dominated by Acropora spp. and highest fish abundance. The area closest to shore showed high algae cover and high abiotic cover and was dominated by Porites, while a high abiotic cover and low coral cover defined the outer edge of the reef area. Variation in total number of fishes between transects, variation in reef fish diversity, as well as distribution of the most abundant fish species and significant changes of abiotic cover between transects further clarified this differentiation of the reef.

Multivariate analyses showed that substrate structure influenced fish assemblages strongest of all investigated environmental variables. Water depth, distance from shore, and distance from the beginning of the reef also revealed a good correlation to fish assemblages. Analysis of coral damage correlation to fishes suggested little influence of damaged coral colonies on variation in fish assemblages. Results of coral damage data analyses, however, can only limitedly be interpreted due to a lack of relative amounts of damage in the reef. Diurnal changes in fish assemblage composition were not detected.

 

Kathi

Siganus rivulatus - feeding

Acanthurus sohal - skalpell

 
 

Cryptobenthic Fish of the El Quadim Bay, El Quseir, Egypt

-Bachelor Thesis- Andreas Scharl

Abstract

This study establishes a preliminary species list of cryptobenthic fishes: Apogonidae (four species), Blennidae (eight species), Gobiidae (thirteen species) and a Plesiopidae, on Egypt's mainland Red Sea coast. Different methods of capturing and/or identifying these fishes are evaluated, among them interference visual census and anaesthetisation by clove oil as well as fish traps and a ground-net filled with coral rubble. Clove oil proved to be most effective and a valuable tool when studying small and cryptic fishes. Additionally the habitat preferences of the studied species have been listed revealing hard substrates such as coral rubble, rock and live corals to be of greater importance to cryptobenthic fish than sandy bottoms. A certain bias towards species living on the reef flat has to be acknowledged as it was easy to catch fish hiding in pools and channels at low tide and the time dedicated to capturing fish during research dives was limited.

 

Christian

Exallias brevis

 
 

Schwangeres Haiweibchen aus dem Meer gezogen- wie Mitarbeiter des RSEC einen brutalen Vorfall auf Dahabs Promenaden beobachteten

Es war der 21.Juni 2008 gegen 19 Uhr als ein Fischer in der Masbat- Bucht von Dahab ein eindeutig schwangeres Haiweibchen an Land zog. Der Hai, vermutlich ein Grauer Riffhai, hing an zwei Haken und war noch am Leben.

Der Verdacht, dass dieser Fischer schon seit längerem gezielt auf Hai-Jagd geht, wurde dadurch bestätig, dass sich keine andere Fischfang-Utensilien, wie Netze, an Bord seines kleinen Bootes befanden. Innerhalb von gerade mal 6 Minuten wurde das zappelnde Tier aus dem Wasser geholt, über die Promenaden geschleift und auf einen „Pick up“ geworfen, der in einer Seitenstraße parkte.
 
Beobachtet wurde das ganze sowohl von RSEC- Mitarbeitern, als auch von mehreren schockierten Touristen, die versuchten den Vorfall zu dokumentieren und die Polizei zu alarmieren. Da sich alles so schnell abspielte und von den mitwirkenden Einheimischen quasi abgeschirmt wurde, war es den RSEC- Mitgliedern nicht möglich einzuschreiten, auch wenn sie dies gewollt hätten.
Die schnelle Abwicklung des Abtransportes des Tieres spricht offensichtlich für eine gezielte Organisation für den Verkauf in lokalen Restaurants als „:Spezial catch of the day“, wohlwissend, dass dies illegal ist.
Einzig positives Fazit aus dem tragischen Vorfall: dem verantwortlichen Fischer wurde die Lizenz entzogen, sein Boot konfisziert und von der Nationalpark- Behörde eine Geldstrafe in Höhe von 3.300 US-$ abverlangt.
Graue Riffhaie bekommen bis zu 6 Junge pro Wurf- das Weibchen und ihre Nachkommen  sind jedoch unwiederbringlich verloren.

 

Shark fishing

Shark fishing

Shark fishing

 
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