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Feeding-Site Utilization in Three Sympatric Species of Petrotilapia (Pisces, Cichlidae) from Lake Malawi

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Feeding-Site Utilization in Three Sympatric Species of Petrotilapia (Pisces, Cichlidae) from Lake Malawi Biological Journal o f the Linnean Society (1985), 25: 331 -338 Feeding-site utilization in three sympatric species of Petrotilapia (Pisces, Cichlidae) from Lake Malawi A. C. MARSH* Department o f Zoology, University o f the Witwatersrand, Johannesburg 2001, South Africa AND A. J. RIBBINK J.L .B . Smith Institute o f Ichthyology, Private Bag 1015, Grahamstown 6140, South Africa Accepted fo r publication November 1984 Three sympatric sibling species of Petrotilapia, with similar dietary and macrohabitat requirements, occur near Monkey Bay, Lake Malawi. Males and females of all three species fed selectively on the rocky shores. Males utilized less than 6% of the space they defended as a breeding territory for feeding and although interspecific overlap of territories was considerable, only 2.5% of the feeding space used by territorial males was shared. Males primarily utilized rich food patches which occurred within the territories of highly aggressive Pseudotropheus species, whereas females tended to feed in undefended areas. Although Petrotilapia species are not highly aggressive fishes, there was a well-defined social dominance hierarchy amongst them and it is suggested that feeding-site utilization is dependent on social rank. Inter- and intraspecific differences in feeding-site utilization probably facilitate species coexistence and thus contribute to the maintenance of the high species diversity which is a characteristic of the ichthyofauna of Lake Malawi. KEY WORDS:— Ecology - diversity - species persistence - coexistence - microhabitat utilization - competition - social dominance - sibling species. CONTENTS I n tr o d u c tio n ..................................................................................................................................................331 M aterials and m ethods..................................................................................................................................332 R e s u l t s .......................................................................................................................................................... 333 D iscussion..........................................................................................................................................................336 A c k n o w le d g e m e n ts ..................................................................................................................................337 References.......................................................................................................................................................... 337 INTRODUCTION The Great Lakes of Africa contain speciose fish faunas which are dominated by the Cichlidae and which have evolved in a short time (Fryer & lies, 1972; •To whom all correspondence should be addressed. 331 0024-4066/85/080331 4-08 $03.00/0 (g) 1985 The Linnean Society of London 332 A. C. MARSH AND A. J. RIBBINK Greenwood, 1974). For example, Lake Malawi, which is only about two million years old (Dixey, 1926; Fryer, 1959; Greenwood, 1974; Banister & Clarke, 1980), has almost 300 described endemic species of cichlid and it is estimated that a further 200 species await description (Ribbink et al., 1983). A knowledge of species formation and persistence is a prerequisite for understanding the high species diversity encountered in these lakes (Lowe-McConnell, 1969). Species persistence is primarily an ecological problem, one of its essential components being the coexistence of sympatric species. To comprehend the manner and extent of differences in morphology and habit among closely related sympatric species is essential to an understanding of coexistence and hence diversity (Schoener, 1974). The rocky shores of Lake Malawi are dominated by a group of closely related cichlids known collectively as Mbuna (Fryer, 1959; Ribbink, Marsh, Marsh, Ribbink & Sharp, 1983). The Mbuna contain a number of sympatric sibling species complexes (Holzberg, 1978; Marsh, Ribbink & Marsh, 1981; Ribbink el al., 1983) which, by definition, are closely related and have very similar or identical morphologies (Mayr, 1963). Since it is likely that closely related species have similar ecological requirements, problems of coexistence are likely to be highlighted in sibling species complexes. This study focuses on three sympatric sibling species of Mbuna belonging to the genus Petrotilapia. Previous work has shown that the three species, in the vicinity of Monkey Bay (14° 03'S, 34° 55'E), appear to have very similar dietary and spatial requirements (Marsh et al., 1981; Marsh, 1983; McKaye & Marsh, 1983; Ribbink et al., 1983). Sharp (1981) found that Petrotilapia territorial males appear to feed primarily within the territories of two highly aggressive, undescribed Pseudotropheus species: P. elongatus ‘aggressive’ and P. tropheops ‘orange chest’ (throughout this manuscript we have used the informal names from Ribbink et al. (1983) for these undescribed species). Furthermore, Sharp (1981) showed that the territories of P. elongatus ‘aggressive’, the more aggressive of the two Pseudotropheus species, support approximately 2.25 times more epilithic algae than similar undefended sites, with larger individuals occupying larger and more productive sites than smaller individuals. Similarly, P. tropheops ‘orange chest’ territories support approximately 1.6 times more epilithic algae than undefended sites. Thus, abundance of epilithic algae varies spatially and the richer the food patch the less accessible it is to most Mbuna in view of the aggression of the tenant of the food patch (= territory). These differences in food abundance and availability could form the basis upon which the Petrotilapia species partition feeding space. Here we show that there are interspecific and intraspecific differences in feeding-site utilization and aggression. It is suggested that the feeding sites are partitioned amonst the Petrotilapia individuals according to their position in a social dominance hierarchy. MATERIALS AND METHODS The study took place on the northern and eastern shores of the island of Thumbi East, Monkey Bay, where rocky habitats extend down to a sandy bottom at 30-35 m. The nature of the rocky substratum varies but most areas comprise a rubble of rocks and boulders, ranging in diameter from 0.3 to 2 m, with very large flat rock slabs occurring at irregular intervals. Rock surfaces are covered by a thin mat of algae. FEEDING SITES OF PETRO TILAPIA 333 The following method was used to obtain information on feeding site utilization at the species level. Whenever an adult Petrotilapia was seen feeding, the feeding site was classified into one of four categories: (1) territories of P. elongatus ‘aggressive’ individuals > 100 mm t l , (2) territories of P. elongatus ‘aggressive’ individuals < 100 mm t l , (3) territories of P. tropheops ‘orange chest’, and (4) undefended areas. Feeding-site niche breadths were calculated using the formula W = r® (Petraitis, 1979), where r is the number of feeding-site categories sampled and r E - pij logr q3~ £ pj logrpij, . J=] w here p i} is the frequency with which a particular Petrotilapia form used one of the four feeding sites and q is the frequency of occurrence of a particular feeding-site category in the environment (data for q were obtained from Sharp, 1981). To obtain detailed information on the utilization of feeding space by territorial individuals, an area 12 x 10 m was divided into cells 50 x 50 cm, using nylon cords laid across the rocks, and the position of a fish could accurately be assigned to a cell of 25 x 25 cm. Each territorial male Petrotilapia that occurred within the grid was observed for a total of 6 h over a period of 10 consecutive days during May 1979, and its territory boundary and feeding sites were mapped. Once a month during 1980 eight randomly selected territorial males of each species were observed continuously for 15 min each between 08.00 and 10.00 hours in 1-5 m depth. Aggressive encounters between these males and other Petrotilapia individuals were observed to determine which fish was dominant in each encounter. Like most cichlids, Petrotilapia species possess a stereotyped agonistic behavioural repertoire involving the use of ‘lateral displays’, ‘tail beating’, ‘flank biting’ and ‘mouth fighting’ (terms after Baerends & Baerends van-Roon, 1950). Typically subordinant Petrotilapia individuals fled when challenged and the latter three behaviours were seldom evident. During agonistic displays dominant males intensified their livery, while subordinant individuals frequently adopted a duller coloration. Thus, it was a relatively simple matter to decide which protagonist was dominant. Laboratory experiments were performed to supplement the field observations. Territorial male Petrotilapia were placed, with a few Psuedolropheus zebra individuals, in 280 1 glass aquaria on either side of an opaque partition. Each aquarium always held two species of Petrotilapia and because there are interspecific differences in the size of mature males in nature (Marsh et al., 1981), no attempt was made to match the protagonists for size. Once the Petrotilapia males had established dominance over the P. zebra, and developed the intense coloration associated with territoriality in nature, the partition was removed and the responses of the two Petrotilapia males were noted. Similar experiments involving Petrotilapia males and P. elongatus ‘aggressive’ and P. tropheops ‘orange chest’ were also performed. RESULTS X2
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