Danionella dracula , an escape from the cypriniform Bauplan via developmental truncation?

In male Danionella, the gut and genital ducts are redirected so that both open between the pelvic fins.

“These include a redirection of their gut and genital ducts so that both open in between the pelvic fins”

Danionella is unusual in showing organism-wide progenesis or developmental truncation, a heterochronic change that leads to tiny adult organisms with an overall larval appearance.

“Danionella is unusual in that it shows organism–wide progenesis or developmental truncation, a heterochronic change that leads to tiny adult organisms with an overall larval appearance7–10.”

Male Danionella have a large drumming muscle that originates from expanded bony flanges on the os suspensorium of their Weberian apparatus and inserts on a hypertrophied fifth rib.

“a large drumming muscle that originates from expanded bony flanges on the os suspensorium of their Weberian apparatus and inserts on a hypertrophied fifth rib”

Male Danionella have a conical drumming cartilage that probably works like a drumstick on the anterior swimbladder chamber.

“and a conical drumming cartilage that probably works like a drumstick on the anterior swimbladder chamber4–6,9.”

Species of the genus Danionella possess unique morphological novelties in males.

“Species of the genus Danionella are further remarkable in possessing a number of unique morphological novelties in males.”

In D. dracula, the main dimorphism is the robustness of the different bones, which are wider, better ossified and stouter in males.

“Its main dimorphism is thus the robustness of the different bones, which are wider, better ossified and stouter in males9.”

Progenetic miniatures undergo profound developmental truncation, giving rise to striking morphological novelties not shown in their larger relatives, while proportioned dwarfs retain a normal developmental trajectory despite their reduced body size.

“Progenetic miniatures undergo profound developmental truncation, giving rise to striking morphological novelties not shown in their larger relatives, while proportioned dwarfs retain a normal developmental trajectory despite their reduced body size (e.g., refs. 5, 12, 14, 41, and 42).”

The unidirectionality of transitions is most readily explained for progenetic miniaturization, which involves profound and largely irreversible developmental truncation.

“This unidirectionality is most readily explained for progenetic miniaturization, which involves profound and largely irreversible developmental truncation (1, 5, 14).”

Within Cypriniformes, the morphological and developmental correlates of miniaturization have been especially well characterized.

“Within Cypriniformes, the morphological and developmental correlates of miniaturization have been especially well characterized.”

The difference in frequency between the two types likely reflects greater developmental constraints on progenetic miniaturization, which requires extreme heterochronic shifts resulting in simultaneous truncation of multiple organ systems, compared to proportioned dwarfism, which may face fewer developmental and genetic constraints.

“The difference in frequency likely reflects the greater developmental constraints on progenetic miniaturization, which requires extreme heterochronic shifts resulting in the simultaneous truncation of multiple organ systems, compared to the simpler body size reduction characterizing proportioned dwarfism, which may face fewer developmental and genetic constraints (5, 14, 42).”