Diversity in rest-activity patterns among Lake Malawi cichlid fishes suggests novel axis of habitat partitioning

Abstract

Animals display remarkable diversity in rest and activity patterns that are regulated by endogenous foraging strategies, social behaviors, and predator avoidance. Alteration in the circadian timing of activity or the duration of rest-wake cycles provide a central mechanism for animals to exploit novel niches. The diversity of the 3000 cichlid species throughout the world provides a unique opportunity to examine variation in locomotor activity and rest. Lake Malawi alone is home to over 500 species of cichlids that display divergent behaviors and inhabit well-defined niches throughout the lake. These species are presumed to be diurnal, though this has never been tested systematically. Here, we measure locomotor activity across the circadian cycle in 12 cichlid species from divergent lineages and distinct habitats. We document surprising variability in the circadian time of locomotor activity and the duration of rest. In particular, we identify a single species, Tropheops sp. red cheek that is nocturnal. Nocturnal behavior was maintained when fish were provided shelter, but not under constant darkness, suggesting it results from acute response to light rather than an endogenous circadian rhythm. Finally, we show that nocturnality is associated with increased eye size, suggesting a link between visual processing and nighttime activity. Together, these findings identify diversity of locomotor behavior in Lake Malawi cichlids and provide a system for investigating the molecular and neural basis underlying the evolution of nocturnal activity.

Weak genetic signal for phenotypic integration implicates developmental processes as major regulators of trait covariation

“…LF therefore appear to develop a more athletic heart, running counter to our initial expectation that LF is a more efficient forager and should therefore expend less overall energy while feeding, compared to TRC (Conith et al, 2019a). Both species are territorial, with males vigorously defending breeding territories (Ribbink et al, 1983), and a recent study examining activity levels across cichlid species found that LF and TRC exhibit high and indistinguishable levels of activity relative to other species (Lloyd et al, 2020). LF do occupy a greater depth range relative to TRC, foraging up to depths of 18 metres, while TRC often occupy depths up to 6 metres (Ribbink et al, 1983).…”

Trophic specialization on unique resources in one of the most celebrated examples of sympatric speciation, Barombi Mbo crater lake cichlids

“…1 & 3). This may reflect the rare hunting strategy of this species for nocturnal shrimp prey (also see Lloyd et al 2021 for a nocturnal Malawi cichlid). S. mongo were only observed and captured by seine net after twilight hours beginning around 19:00 hours.…”

“…However, the remaining resource specializations are particularly rare among African cichlids. Nocturnality has only been documented once out of thousands of Malawi cichlids (Lloyd et al 2021) and spongivory is only qualitatively reported from Coptodon spongotroktis in Lake Bermin, Cameroon (Stiassny et al 1992). In general, our findings align with the major trophic strategies and specialists outlined by Trewavas et al (1972), providing quantitative data on the differences in dietary component proportions between species.…”

Behavioral rhythms are bad predictors of general organismal rhythmicity

“…A diversity of lifestyles can increase the chances of survival in heterogeneous habitats and can help securing the persistence of the population. Phenotypic diversity can further reduce competition due to a diversification of resource usage and spatial/temporal niches 67, 68 . Distinct individual phenotypes may be more beneficial in heterogeneous or variable habitats like coral reefs, intertidal shores, forest or urban areas that provide a higher potential for different lifestyles 22, 69 .…”