Landmark-Based Morphometric Analysis of the Body Shape of Two Sympatric Species, Ctenopharynx Pictus and Otopharynx Sp

Landmark-based morphometric analysis of the body shape of two sympatric species, Ctenopharynx pictus and Otopharynx sp. "heterodon nankhumba" (Teleostei: Cichlidae), from Lake Malawi Daud D. Kassam1*, Tetsu Sato2, and Kosaku Yamaoka1 1 Laboratory of Aquatic Ecology, United Graduate School of Agricultural Science, Ehime University, B 200 Monobe, Nankoku, Kochi 783-8502, Japan (e-mail: DDK, [email protected]) 2 WWF Japan, Nihonseimei Akabanebashi Building, 6 Fl., 3-1-14 Shiba, Minato-ku, Tokyo 105-0014, Japan Received: May 17, 2001 / Revised: May 22, 2002 / Accepted: June 22, 2002 Abstract Morphological differences in body shape of two sympatric benthophagous cichlid species Ichthyological from Lake Malawi, Ctenopharynx pictus and an undescribed species, Otopharynx sp. “heterodon Research nankhumba,” were investigated using geometric morphometric methods. From digitized data of land mark points on lateral profiles of fishes, the shape of each species was compared by the thin-plate spline ©The Ichthyological Society of Japan 2002 method. Statistical analyses revealed significant variation in both uniform and nonuniform compo nents of shape between the two species. From the splines generated, it was revealed that most of the Ichthyol Res (2002) 49: 340-345 significant variation between the two species occurs in the head region. Specifically, C. pictus has a longer and deeper head than Otopharynx sp. In addition, the mouth of C. pictus is larger than that of Otopharynx sp. In the trunk region, C. pictus has a shorter abdominal cavity, which may indicate possession of shorter intestines than Otopharynx sp. The variation in gross head morphology and intestinal length may reflect interspecific differences in trophic ecology, possibly facilitating the coexi stence of the two species through resource partitioning. Key words Geometric morphometric • Sympatric species • Ctenopharynx pictus • Otopharynx sp. “heterodon nankhumba” • Trophic specialization he adaptive radiation and explosive speciation of However, not much research has been conducted on the the cichlid fishes of the Great Lakes of Africa (Lakes body shape of Lake Malawi’s cichlids, and most research has Victoria,T Tanganyika, and Malawi) has fascinated ecol focused on feeding apparatuses only (Reinthal, 1989, 1990). ogical, behavioral, and evolutionary biologists world Body shape has been investigated by the multivariate analy wide. Speciation in Lakes Malawi and Victoria is believed sis of distance measurements, the shortfall of this method to be relatively recent when compared to that of Lake being the restriction to the directions sampled by the mea Tanganyika. For example, Snoeks (2000) reported that Lake surement scheme (Bookstein, 1991). No study has yet exam Malawi contains about 800 cichlid species, but the lake itself ined the relationship between body form and feeding habit is reportedly less than 1 million years old (Meyer et al., in these cichlids using the novel landmark-based technique 1990). of geometric morphometrics (GM). Adaptive radiation of the haplochromine cichlids of Lake Bookstein (1991) defined morphometrics as the statistical Malawi has resulted in the coexistence of extremely diverse study of biological shape and shape change. Morphometric assemblages of closely related species (Fryer and Iles, 1972). methods are needed whenever one wants to describe or Such adaptive radiation has mostly been attributed to mor to compare shapes of organisms or of particular structures. phological adaptations, in particular that of the feeding ap Such descriptions are useful in the understanding of growth, paratus (Fryer and Iles, 1972; Greenwood, 1974, 1981; Liem, experimental treatments, or evolution. Geometrically, the 1974; Barel et al., 1977; Witte, 1981; Yamaoka, 1987). shape of objects is studied well after removal of the effects Although several studies have shown that morphological of location, size, and orientation. Marcus et al. (1996) re differences in the feeding apparatus have directly contrib ported many applications of the GM method in different uted to the success of the cichlids (Fryer and Iles, 1972; biological fields including fisheries. GM data are the ho Greenwood, 1974, 1981; Liem, 1974; Barel et al., 1977), mologous landmark points (anatomic points with biological examining the whole organism’s body shape may contribute label) in either two-dimensional or three-dimensional to a greater understanding of species diversity. Body shape forms. GM has several merits compared to traditional is a highly variable characteristic among cichlid fishes and morphometrics (i.e., classical morphometrics of distance determines function, behavior, and habitat (Fryer and Iles, measurements), hence our choice of the former approach. 1972). First, the GM approach is much more effective in capturing Geometric morphometric study of Lake Malawi cichlids 341 Fig. 2. Landmarks collected from the left side of the fish. 1, tip of the premaxilla; 2, 3, anterior and posterior insertion of the dorsal fin; 4, 6, Fig. 1. The two species used: A Ctenopharynx pictus; B Otopharynx sp. upper and lower insertion of caudal fin; 5, posterior extremity of the “heterodon nankhumba” (Courtesy of Dr. Ad Konings) lateral line; 7, 8, posterior and anterior insertion of the anal fin; 9, insertion of the pelvic fin; 10, insertion of the operculum on the profile; 11, posterior extremity of the gape; 12, upper insertion of pectoral fin; 13, posterior extremity of the operculum information about the shape of an organism, as the geom etry of the whole organism is taken into consideration. The approach also has the capability to show shape variation Materials and Methods graphically, as deformations in a manner similar to D ’ Arcy Wentworth Thompson’s transformation grids (Thompson, Sample collection.—From September through November 1917), which is easier to interpret than the tables of numeri 1999, and in April 2000, 37 specimens of Ctenopharynx cal coefficients of the traditional morphometrics approach pictus fUsa Kochi University; UKU 387001001-387001037; (Rohlf and Marcus, 1993). 38.2-111.3mm standard length, SL) and 37 specimens The two study species (Fig. 1), Ctenopharynx pictus of Otopharynx sp. fUKU 387001038-387001075; 39.7 Trewavas and Otopharynx sp. “heterodon nankhumba” 116.7 mm SL) were collected from West Thumbi Island in (sensu Konings, 1990), are both benthic feeders (Ribbink et the Cape Maclear region of Lake Malawi, East Africa al., 1983; Konings, 1990) and are sympatric (T. Sato, personal (14°00' S, 34°50' E). Specimens were collected by scuba observation). The breeding behavior of C. pictus is interest divers using hand nets and gill nets. Immediately after cap ing in that it relies on mutualistic association with the catfish ture, fish were killed and placed in 10% formalin solution. Bagrus meridionalis, locally called Kampango. Research has The body cavity of each specimen was also injected with the shown that predation of catfish fry has been significantly formalin solution. After fixation, specimens were trans reduced in broods that incorporate C. pictus fry, whereas ferred to 70% ethanol until examination. C. pictus fry benefit by feeding on the eggs produced by Morphometric and statistical analyses.—Images were adult catfish (McKaye, 1985). However, the presence of taken for each specimen using a digital camera (Fuji Finepix Otopharynx sp. at this locality needs to be further investi 500). Thirteen landmarks taken from the left lateral side of gated, as it is not clear whether this species is a parasite as each specimen (Fig. 2) were digitized using computer soft well. ware, TPSDIG version 1.19 (Rohlf, 1996). The two species look morphologically similar, except that All 74 specimens were superimposed using the general Otopharynx sp. has spots at the base of the dorsal fin. As ized least squares (GLS) method (Rohlf and Slice, 1990). both species are benthic feeders, there may be intense com This method removes nonbiological variation by scaling petition for food resources. Alternatively, differences in all specimens to unit size, translating them to a common morphology could have resulted in trophic specializations location, and rotating them so that their corresponding that enable the species to coexist. Neither detailed ecologi landmarks line up as closely as possible. From this superim cal nor morphological study has been done of these two position, the reference configuration was computed that was species to understand their relationship, hence our first at used in later analyses. The thin-plate spline algorithm was tempt. This study aims to describe variation in body shape used to calculate the uniform and nonuniform components between C. pictus and Otopharynx sp. by utilizing the GM of shape for each specimen (Bookstein, 1989, 1991). approach, and subsequently to explain the mechanism for Superimposition of specimens and the calculation of the coexistence of the two species. both uniform and nonuniform shape components were 342 D.D. Kassam et al. done using computer software TPSRELW version 1.20 anterad shift of landmark 1 versus the posteroventrad shift (Rohlf, 1997). To determine if shape varied significantly of landmark 11 (Fig. 3a,d). In contrast, Otopharynx sp. has a between species, a multivariate analysis of variance smaller mouth as shown by an opposite shift in the same (MANOVA, computed with JMP statistical package, ver landmarks, with landmark 1 shifting posterad against the sion 3.2; Sall et al., 1999) was performed

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