Osteology of Paedocypris, a miniature and highly developmentally truncated fish (Teleostei: Ostariophysi: Cyprinidae)

Abstract

Species of the cyprinid genus Paedocypris are among the smallest and most developmentally truncated fishes and vertebrates. Our analysis of their skeletal structure reveals a puzzling combination of extreme developmental truncation and an increased morphological complexity in sexually dimorphic characters. The skeleton of Paedocypris is characterized by reduction and loss and resembles in many aspects that of a larval/early juvenile stage of its close relatives. We found 61 characters that have been affected by developmental truncation. A comparison with the skeletal development of a close relative, the zebrafish Danio rerio, demonstrates that the majority of the absent bones or skeletal structures in Paedocypris are those that appear late in the ossification trajectory of the zebrafish. Thus, their absence in Paedocypris seems to be due to the simple developmental truncation of terminal stages in the ossification sequence. Our study of the sexually dimorphic structures in Paedocypris demonstrates that predominantly the male exhibits the more complex state. In relation to the female, male Paedocypris uniquely possess a cleithrum with a pointed posterior process that covers the scapula laterally, and a more medially situated posterior flange that contacts the dorsal area of the coracoid; a massive and heavily ossified uppermost pectoral radial tightly bound to the scapula; thickened and enlarged three uppermost pectoral-fin rays; a large triangular, dorsolaterally directed process on the outer arm of the massive os suspensorium; and a enlarged and shovel-like anterodorsally directed basipterygium; and a hypertrophied first pelvic-fin ray with additional anterior flanges that support keratinized pads of skin. Female Paedocypris show only one structure that is better developed than in males: the first proximal-middle radial and the anteriormost fin ray of the dorsal fin are more massive and more heavily ossified. Although the function and biological role of these dimorphisms is still unknown, we hypothesize that they are related to a special reproductive behavior. Paedocypris is a prime example for the recent claim that miniaturization among cyprinids is associated with evolutionary novelty only in developmentally truncated miniatures and not in proportioned dwarfs. Paedocypris offers a strong challenge to Schindleria as the most extreme example of developmental truncation known among fishes. We highlight the difficulties that developmentally truncated taxa frequently pose to the resolution of their phylogenetic position and propose an approach to overcome this problem. Our phylogenetic comparison to determine the systematic position of Paedocypris among cyprinids reveals that it shares not only a number of unique absences, but also highly unusual progressive characters with Sundadanio and Danionella, two other Asian miniature cyprinids. We hypothesize that the three genera form a monophyletic group. We further found that Paedocypris and Danionella share a number of uniquely derived characters pointing to a sister g...

Miniaturization in Otothyris Myers, 1927 (Loricariidae: Hypoptopomatinae)

“…This illustrates the complexity of the miniaturization process in the genus. Some of these features are also shared with Pseudotothyris, the sister to Otothyris, suggesting that miniaturization may be widespread in the clade and must be interpreted as a gradual process, as already reported for cyprinids by Britz & Conway (2009).…”

Spectacular morphological novelty in a miniature cyprinid fish, Danionella dracula n. sp.

“…Of these nine, two characters (absence of neural spine 4, intermusculars) are uniquely shared among Asian miniature cypriniforms with Paedocypris and another two (absence of post-temporal and ectopterygoid) with Paedocypris and Sundadanio. Along with additional characters, these losses have been interpreted as evidence for a close relationship of the three genera, with Danionella as the sister group of Paedocypris (see discussion in Britz & Conway 2009). Absence of the urohyal and the metapterygoid is a derived character of all Danionella species and absence of the coronomeckelian bone seems to be an autapomorphy of D. dracula.…”

“…The large number of undeveloped skeletal structures highlights the larval-like neurocranial and branchial skeleton of D. dracula and makes it even more developmentally truncated than Paedocypris, which lacks 41 bones or bone parts when compared with the zebrafish (Britz & Conway 2009). This theme of loss and reduction in D. dracula contrasts with pronounced hyperossification restricted to its sexually dimorphic skeletal structures, the jaws, hyopalatine arch and pelvic fin skeleton.…”

“…In vertebrates, this relationship is best documented for amphibians and fishes, the most intriguing recent example being the cypriniform genus Paedocypris ( Kottelat et al 2006;Britz & Conway 2009). Among cypriniforms (carp-like fishes), the most unusual morphological novelties are found in miniatures showing a high degree of developmental truncation (Rü ber et al 2007;Britz & Conway 2009). Here, we report the discovery of a new miniature species of cypriniform, Danionella dracula n.…”

Masters of miniaturization: Convergent evolution among interstitial eukaryotes

“…Miniaturization is best documented in vertebrates [4] such as fish [6,7], frogs [8][9][10], and salamanders [11,12] (but also see examples in terrestrial arthropods [13,14]), where it illustrates evolutionary themes such as the origin of morphological novelty [15], heterochrony [7], and convergence [6]. Here, we suggest that meiofauna might provide some of the deepest insights into these fundamental evolutionary processes, and, because these miniature species are ubiquitous throughout the metazoan tree of life, we suggest that meiofauna are uniquely poised to shed light on the origins and diversification of animals.…”