Synodontis angelicus, known commonly as the polka-dot squeaker or angel squeaker, belongs to a family of African catfishes called the Mochokidae (Teleostei: Siluriformes). ‘Squeaker’ and ‘upside-down catfish’ are common names given to species in this genus and the two closely related genera, Hemisynodontis and Brachysynodontis. They are quite common in the pet trade due to stunning pigmentation patterns and the peculiar habits from which the common names are derived. The angel squeaker exhibits a prime example of such a striking pigmentation pattern.
The name squeaker refers to the fact that, when agitated, many Synodontis species are capable of making a squeaking noise (recorded by Dr. John Friel, Cornell University) by stridulation of the pectoral spine against the pectoral girdle. Even more peculiar is the habit of some species that are known to swim upside-down. This habit is most certainly correlated with feeding for insects and other food items while upside-down at the waters surface, but upside-down catfish will rest and swim upside-down on a regular basis.
Synodontis is by far the largest genus in the family Mochokidae and, indeed, one of the largest among all catfishes (about 120 species, a close second to Corydoras, about 150 species). And so, Synodontis is often the stand-in when people think about mochokid catfishes. But, in fact, the family (10 genera, ~190 species) is much more morphologically diverse when you consider some of the other recognized genera. Euchilichthys, Atopochilus and Chiloglanis are particularly notable for their ventrally-directed sucker-shaped mouths. Others like Mochokus and Acanthocleithron are very poorly known, but exhibit unique morphologies of their own. Synodontis angelicus, like many mochokids, is typified by branched mandibular barbels, well-developed nuchal plates, a large cleithral process and S-shaped mandibular teeth set in a deep cavity/cup of the dentary.
Synodontis species vary greatly in adult size, from 5 cm total length (TL) in the recently described S. acanthoperca (Friel & Vigliotta, 2006) up to 80 cm in other species. Larger Synodontis are important food fishes in many parts of Africa. Species in other mochokid genera are even smaller as adults (e.g., Microsynodontis and Chiloglanis), but none are larger than the largest Synodontis. Synodontis angelicus may reach about 25 cm TL (Poll, 1971).
Members of the genus Synodontis, and more broadly the family, are known to exhibit obvious sexual dimorphism. For example, many species in the genus Chiloglanis show dimorphism of the caudal fin (Roberts, 1989; Seegers, 1996). Mochokids are also known to exhibit spiny ornamentation of the skull roof bones, opercular series and pectoral girdle. In the case of S. acanthoperca, a spine found at the rear of the operculum is, itself, sexually dimorphic. The spines of males are much larger than those of females (Friel & Vigliotta, 2006).
Synodontis catfishes span the entire geographic range of mochokids; they inhabit a large proportion of African freshwaters including most of sub-Saharan Africa and the Nile River basin and can be found in large rivers, smaller fast-flowing streams and the massive African rift lakes. Some species are known to swim in mid-water and some will shoal; others are primarily benthic and relatively solitary. Synodontis angelicus is distributed throughout the Congo River drainage (Poll, 1971). Anecdotal reports tell of angel squeakers using holes in submerged logs for shelter; this practice is probably more widespread in the family.
Fossil mochokids (almost exclusively Synodontis) are widespread in Africa and date as far back as the early Miocene (Stewart, 2001). Interestingly, fragments of Synodontis pectoral spines dating from the early Oligocene have been found in Oman, an area where mochokids do not exist today.
Beyond diet, there isn’t a great deal known about the ecology of Synodontis or other mochokids. Synodontis stomach contents have included insects, nematodes, crustaceans, mollusks, annelids, seeds, algae, fish scales and incidentally sand (Bishai & Abu Gideiri, 1965; Sanyanga, 1998; Winemiller & Winemiller, 1996). The diets of the three sucker-mouthed genera (see above) may contain a higher proportion of plant matter, but it seems likely that all members of the family are omnivorous.
One species of Synodontis is known for an interesting bit of its ecology; S. multipunctatus is a brood parasite of mouth brooding cichlids such as Haplochromis (Sato, 1986; Wisenden, 1999). Adults of this species will ‘dump’ their own fertilized eggs into the mix while cichlids are breeding and the catfish eggs are taken into the mouth of a cichlid. The Synodontis eggs hatch first and will eat the host eggs before they leave the host mouth. Visit this link to see video of S. multipunctatus spawning with cichlids, and this link to see pictures of S. multipunctatus fry emerging from a cichlid mouth.
The relationship of species within Synodontis is almost entirely unstudied, but the genus was taxonomically reviewed by Poll (1971); the systematic relationship of Synodontis to other mochokids and the relationship of the family to other catfishes is also currently unresolved. Many recent morphological phylogenetic studies indicate a relationship between mochokids and the neotropical families Doradidae and Auchenipteridae (Chardon, 1968; Diogo, 2005; Ferraris, 1988; Higuchi, 1992; Lundberg, 1993; Mo, 1991; de Pinna, 1993, 1998), but this relationship was proposed at least as far back as Günther (1864). The phylogenetic hypothesis is based largely on the presence of well-developed nuchal plates and certain elaborations of the Weberian apparatus, including an elastic spring apparatus. More recent molecular works have not born out this relationship, however, and place the Mochokidae with unexpected siluriform groups, for example the North American Ictaluridae (Hardman, 2005). Very recent work (Sullivan et al., in press) has suggested that the Mochokidae are most closely related to several African catfish families including the Amphiliidae (loach catfishes) and Malapteruridae (electric catfishes). Regardless, one thing is clear; molecular and morphological phylogenies do not agree with respect to the placement of the mochokids. Placement of the Mochokidae will be greatly enhanced by a deeper understanding of the relationships within the family (previously unstudied). These intrafamilial relationships, the putative monophyly of the family and the general taxonomy of the group are currently under study by Vigliotta (in progress), among others.