The Diversity of Cichlid Fishes
Abstract
Introduction: Cichlids are a large group of freshwater fish. They are deep-bodied and have one nostril on each side of
the head and rounded tails. The lateral line is discontinuous, and there are three or more anal spines. They usually do
not grow longer than ~30 cm (12 inches). In many species, the rear edges of the dorsal and anal fins are pointed and the
pelvic fins are elongated. Most cichlids are found in sub-tropical America, South America, mainland Africa, Madagascar,
and southern Asia. Scientists estimate that there are at least 1,350 species worldwide, and possibly hundreds more new
species yet to be described. Cichlids occupy a wide range of aquatic habitats, and where water temperatures are greater
than about 68℉ (20℃). Cichlids exhibit a great diversity of feeding adaptations and consume numerous food types:
phytoplankton, zooplankton, soft bottom deposits, benthic algae, higher plants, insects, molluscs, fish scales and fins, fish
eggs and larvae, among others. Specialization of the jaws and dentition have allowed cichlid species to occupy a wide
range of habitats and feed on a diversity of food types. The diversity of cichlids may be the result of a phenomenon called
adaptive radiation. In order to adapt to their surroundings and quickly prey on new food sources, their body and head
of cichlids exhibit extremely diverse morphologies, which allow them to achieve great success in specific diets in specific
habitats.
Objective: Because the cichlid’s varied head shape is related to a number of factors, I chose one of the possible factors –
diet. Based on the cichlid’s diet, my goal was to work out how the length, width, and depth of the cichlid head associate
with diet?
Methods: In order to achieve the goal, I first placed eighteen landmarks on different types of cichlids’ CT scans by using
MeshLab. Then I used numerous statistical analyses to assess differences between and among groups and to examine
possible associations between diet and head shape and extract linear measurements from the landmarks(i.e., a t-test,
analysis of variance (ANOVA) and a Tukey honest significant difference (HSD) test, and a linear regression). I used a t-test
to understand if the means of two populations are different. I combined ANOVA and Tukey HSD to understand if there is a
difference in means among multiple groups. And regression is used to examine correlations between two continuous (i.e.,
number) variables. All of these steps can be shown on the R programming language.
Results: I observed differences between cichlid groups in their head lengths. The Utaka sand dwelling group typically
exhibited longer heads relative to the Mbuna rock dwellers. The ANOVA revealed differences in head width between
cichlids based on their diets (P = 0.0125), and a Tukey HSD test later revealed these differences were driven by the insect
dietary group, in which cichlids exhibited much wider heads than both the plant and zooplankton groupings (P = 0.018
and P = 0.043 respectively). There was a strong correlation (r2 = 0.85) between the height of the skull crest (where feeding
muscles attach) and the height of the skull bar (which resists feeding forces), P <0.001. Typically, cichlids that eat insects
exhibit larger crests and bars compared to cichlids that eat plants and zooplankton. The figures below enhance the
results.
