Klaus Stiefel, Abner A. Bucol

The minimum body size of vertebrate species lies just above 6 millimeters, in stark contrast to the minimum sizes attained by species of other major taxonomic groups. This paper presents two connected hypotheses explaining this minimum size obtainable with a vertebrate Bauplan.
Firstly, the complex bodies of vertebrates might not be amendable to reduction below a certain level of complexity. We hypothesize that this lower limit for organ complexity hold especially true for the vertebrate body’s most complex organ, the brain. This is at least partially due to poor scaling of the brain to small sizes, and a coding strategy known as population coding.
In the context of poor scaling of a complex Bauplan, we discuss the relative sparsity of paedomorphism and parasitism in vertebrates.
Second, these constrains will disproportionately affect the smaller bodies and brains of juvenile or larval animals. This, in turn requires a certain minimum egg size to for a juvenile or larva to reach a size where it can independently function. Due to that minimum egg size, the number of eggs per female will decrease with decreasing female body size, indirectly limiting adult size. That problem is likely aggravated since extremely small animals are likely to be low in the food web and in need of high reproductive rates to offset high mortalities caused by predation. Hence, below a certain body size a female will not be able to produce enough eggs to reach the population replacement value. We demonstrate the scaling relationships relevant for this argument with data from gobiid fishes.
The first argument is about animal body complexity; the second argument stems from ecology.