DOI: 10.5061/dryad.tqjq2bw4t
title: Lack of host specialization despite selective host use in brood parasitic cuckoo catfish

description: Host-parasite dynamics involves coevolutionary arms races, commonly
 leading to host specialization. General understanding of evolutionary
 trajectories of specialization in brood parasites is compromised by
 restricted focus on bird and insect lineages. We studied host utilization
 and host specificity in a natural population of the cuckoo catfish
 (Synodontis multipunctatus) which is an obligate parasite of parental care
 of mouthbrooding cichlids in Lake Tanganyika. On a sample of 779 host
 broods from 20 cichlid species, we detected four host species (with
 prevalence of parasitism of 2-18%). Phylogenetic analysis based on genomic
 (ddRAD sequencing) and mitochondrial (Dloop) data from cuckoo catfish
 embryos showed an absence of host-specific lineages, despite former
 indications of two morphological forms of the cuckoo catfish. This was
 corroborated by analyses of genetic structure and co-ancestry matrix. All
 host species were from the tribe Tropheini, maternal mouthbrooders that
 spawn over a substrate (rather than in open water). Parasitized host
 individuals carried smaller clutches (as cuckoo catfish prey on cichlid
 eggs), but did not differ in their body size or habitat use from
 non-parasitized conspecifics. We conclude that the cuckoo catfish is an
 intermediate generalist, selecting a subset of available cichlid species
 as hosts but not forming host-specific lineages. Brood parasitism in the
 cuckoo catfish arose in a lineage which lacks any parental care and we
 discuss costs and benefits of host specialization in this species and
 brood parasites in general.
description: Sampling was conducted by
 SCUBA diving and snorkelling. During each dive, we set a benthic screen
 net (20 m long and 1.5 m high, mesh size 10 mm) along the bottom at a
 particular depth and searched for mouthbrooding cichlid fish in close
 vicinity. Mouthbrooding fish are easily recognized by their extended
 buccal cavity. Immediately
 after each dive, the collected females were identified to species level,
 their body length was measured to the nearest mm, and the broods were
 collected by gently washing them out of the maternal buccal cavity using
 the water jet of a Pasteur pipette. The offspring of each brood was
 photographed, counted, and the age was estimated based on offspring
 developmental stage. Parasitic eggs and embryos were clearly visible
 during brood inspections (smaller size, characteristic colour and shape).
 When parasitic offspring were detected, they were counted, measured to the
 nearest 1 mm, euthanised and stored in 96% ethanol.
 For each brood, we recorded
 host species, host size, depth of host collection, host brood size,
 developmental stage of the brood, occurrence of parasitism and, when the
 brood was parasitized, the number and size of parasitic offspring. Host
 brood developmental stage was estimated visually based on embryo and yolk
 sac size. We considered this estimate reliable until the age of 3 weeks.
 To mitigate uncertainty around our brood age estimates, we then converted
 brood age into a new variable, termed brood stage. Brood stage comprised
 five categories; eggs (age 1-5 days), early embryo (6-10 days old), mid
 embryo (11-15 days old), late embryo (16-20 days old), and juvenile (21
 days and older).