Correlations of cranial morphology, ecology, and evolution in Australian suctorial tadpoles of the genera Litoria and Nyctimystes (Amphibia: Anura: Hylidae: Pelodryadinae).

Abstract

Suctorial anuran larvae are highly specialized for living in fast-flowing waters, using their oral disks as adhesive organs to attach to the substrate. The cranial musculoskeletal structure of suctorial larvae of Litoria nannotis, L. rheocola, and Nyctimystes dayi (Hylidae: Pelodryadinae) were compared with congenerics with pond-type larvae (L. caerulea, L. genimaculata, L. xanthomera). Data from two other neobatrachian species with suctorial larvae (Boophis sp., Hyla armata) as well as published descriptions were taken into account. Suctorial tadpoles evolved several times independently in the Neobatrachia and share various features, irrespective of their phylogenetic position. These include the following. Cornua trabeculae are expanded anteriorly and sometimes fused. The lower jaws are robust. The greatest width of the skull is at the level of the jaw articulation. The upper jaw cartilages are partially or fully fused. The palatoquadrate is robust and connected to the skull by a wide commissura quadratocranialis anterior, processus oticus, processus basalis (in some species), and processus ascendens (vestigial or absent in some species). A processus ventralis quadrati provides an extended area of origin for the m. orbitohyoideus. The m. rectus abdominis inserts far anterior and acts on the cranium. The insertion of the epaxial musculature is shifted anteriorly to the anterior parts of the otic capsule. The mm. diaphragmatobranchialis and rectus cervicis cross at their origins. The origin of the m. levator mandibulae anterior has shifted posteriorly. The branchial basket is relatively small and the ceratohyal area is large. Multiple convergent evolution of these features suggests that they may be causally associated with the suctorial mode of larval life. Aside from these characters, however, the suctorial and pond-type neobatrachian species are remarkably similar in their jaw musculature and hyobranchial musculoskeletal composition. In some features, Ascaphus truei differs significantly from the neobatrachian suctorial species, indicating the influence of the historically distant separation of the two taxa. A novel modification of the upper jaw abduction mechanism has evolved in L. nannotis, L. rheocola, and N. dayi. It involves an adrostral cartilage as a pushing-rod element. This mechanism and unique structural similarities of the cartilago labialis superior gives support to the preliminary assumption that the nannotis species group is more closely related to the suctorial Nyctimystes dayi than it is to other Litoria species with pond-type larvae. Suctorial larvae presumably were present in the most recent common ancestor of the Litoria nannotis group and Nyctimystes dayi.