The genus Podosphaeraster currently comprises seven species, all found on the seabed at depths below approximately 70 metres. Their peculiar spherical morphology has made their phylogenetic placement a subject of ongoing debate since they were first described. Early researchers suggested that these animals belonged to Sphaerasteridae, a family originally established for fossil species. However, subsequent detailed skeletal analyses of the fossil forms indicated that the resemblance was likely a result of convergent evolution, leading to the establishment of a separate family, Podosphaerasteridae, within the order Valvatida. The controversy has persisted, with some taxonomists re-instating the genus within Sphaerasteridae and others maintaining the separate family classification. Meanwhile, molecular analyses have revealed that Valvatida itself is likely a large polyphyletic group, meaning its members do not all share a single common ancestor, leaving the taxonomic position of Podosphaeraster still unresolved.

To address this knowledge gap, the researchers conducted a detailed micro-focus X-ray computed tomography examination of the internal skeletal structure of Podosphaeraster toyoshiomaruae, collected from the seas around Japan. One specimen came from the Oshima-shinsone Bank in south-western Japan at depths of 147 to 149 metres, while another, used for morphological observation, was collected between Okinawa-jima Island and Miyako-jima Island at depths of 95 to 100 metres. Both specimens are deposited at the National Museum of Nature and Science in Japan. Concurrently, the team performed shotgun sequencing using next-generation sequencing technology to determine key molecular markers used in previous phylogenetic studies and, remarkably, the complete mitochondrial genome sequence.

The mitochondrial genome of Podosphaeraster toyoshiomaruae was determined to be 16,247 base pairs in length, with a nucleotide composition that is AT-rich at 65.5 percent. The start and stop codons for the 13 protein-coding genes followed patterns common amongst starfish. However, compared to the general gene arrangement known for the broader group of modern starfish, the COX2 gene was found to be translocated in the Podosphaeraster toyoshiomaruae mitogenome, while the arrangement of all other genes, including transfer RNAs, remained identical to the general pattern. This structural rearrangement represents the first evidence of a major protein-coding gene translocation within Asteroidea, the class that contains all starfish. The discovery is particularly striking because mitochondrial gene arrangements in echinoderms are generally highly conserved, although recent mitogenome sequencing has increasingly revealed lineage-specific rearrangements, particularly in brittle stars.

The genetic divergence is mirrored by significant morphological specialisation. The micro-CT scanning, performed at a tube voltage of 115 kilovolts and a tube current of 70 microamperes for 40 minutes, revealed the arrangement and morphology of the skeletal plates in extraordinary detail. The examined specimen had a horizontal diameter of 9.6 millimetres and a vertical diameter of 6.3 millimetres. The apical system was polygonal, located at the abactinal centre and composed of one central, six interradial, five radial and three additional plates. The central plate was oval and surrounded by six elongated interradial plates, while wedge-shaped radial plates were alternately positioned between them. A madreporite, the sieve-like structure through which water enters the water vascular system, was located at the centre of one of the primary interradial plates.

Perhaps the most significant morphological finding concerns the mesentery, the internal membrane that supports the digestive organs. The presence of calcified reinforcement in the mesentery had been previously implied, but the detailed structure of its skeleton is presented for the first time in this study. The micro-CT images revealed that the mesentery skeleton spans the I-plate, an unpaired actinal plate, and connects to the V-plate and five pairs of plates, including three kinds of marginal plates. This observation suggests that the marginal plates of this species may not be homologous with those of other starfish, as the mesentery skeleton in other asteroids is typically contiguous with abactinal and actinal plates and odontophores, but not with marginal plates. In other words, the structures that look like marginal plates in Podosphaeraster may have arisen independently rather than being inherited from a shared ancestor.

Notably, while the connectivity of the mesentery skeleton varies, there is a lack of significant morphological differentiation between the marginal plate series and the adjacent dorsolateral and actinal plates. One researcher has questioned the homology of marginal plates in Podosphaeraster, suggesting that the fossil family Sphaerasteridae evolved from the Mesozoic family Stauranderasteridae through the development of a spherical body and the subsequent loss of differentiated marginal plates. Although varying degrees of marginal plate reduction are shared with the order Velatida, the researchers consider this to be a case of convergent evolution, where similar features arise independently in unrelated lineages due to similar selective pressures rather than shared ancestry.

On the phylogenetic side, the team analysed mitochondrial 12S and 16S rRNA and nuclear histone H3 gene sequences from 117 operational taxonomic units, aligning a total of 345 sequences from the three gene regions. In their resulting phylogenetic tree, the order Valvatida was resolved as polyphyletic, consistent with previous molecular findings. Podosphaeraster toyoshiomaruae was positioned as the most basal lineage within Valvatida, with the family Poraniidae placed adjacent to it. However, this placement received low statistical support in the Maximum Likelihood tree, with a bootstrap value of 63 percent, and no support in the Bayesian Inference tree. The researchers attribute this instability to long-branch attraction, a phylogenetic artefact caused by the excessively rapid evolutionary rate of the sequenced genes in this species, which can make rapidly evolving lineages appear artificially closely related or basal.

The three genetic markers provided poor resolution for the deep phylogeny of starfish higher taxa, likely because their evolutionary rates are too rapid to accurately resolve divergences that occurred approximately 250 million years ago. Substitution saturation tests indicated that the dataset is generally unsaturated at the family level, but the results suggested that saturation may confound relationships amongst specific taxa. The phylogenetic instability is further complicated by ongoing controversy surrounding outgroup selection and root placement in starfish phylogenetics. The researchers provisionally utilised a Velatida clade to root their tree, following previous studies that suggested Velatida resolves basal to Valvatida in molecular analyses. Their resulting topology revealed several discrepancies with previous studies but consistently supported the polyphyly of Valvatida.

To resolve these deep relationships, future studies must employ a larger set of genetic markers with more conservative evolutionary rates. Additionally, discovering extant taxa closely related to the genus Podosphaeraster will be crucial to breaking the long branch and improving phylogenetic accuracy. The researchers also emphasise that more observations of mesenteric skeletons using micro-CT across various Podosphaeraster species will help assess the homology and diversity of skeletal ossicles amongst Podosphaerasteridae taxa. Strictly speaking, the evolutionary origins of the marginal plates remain unknown without comparative evolutionary-developmental studies. For now, this spherical starfish stands as a testament to how much remains to be discovered in the deep sea, where animals with bodies unlike any of their relatives continue to challenge our understanding of how evolution shapes life on Earth. The volumetric image data from the micro-CT reconstruction is openly available at MorphSource, and the genetic sequences are deposited in the DDBJ, EMBL and GenBank databases, ensuring that this enigmatic animal’s secrets remain accessible to researchers worldwide.

Subject of Research: Evolutionary Biology

Article Title: Phylogenetic position of the enigmatic starfish family Podosphaerasteridae ( Asteroidea , Valvatida ) with a morphological observation of the skeletal structure by micro-CT

Article References: Hiruta, S., Kobayashi, I., & Fujita, T. (2026). Phylogenetic position of the enigmatic starfish family Podosphaerasteridae (Asteroidea, Valvatida) with a morphological observation of the skeletal structure by micro-CT. Biodiversity Data Journal, 14, Article e194006. https://doi.org/10.3897/bdj.14.e194006

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e194006

Keywords: starfish, Podosphaeraster, mitochondrial genome, micro-CT, phylogenetics, convergent evolution, deep sea, Phylogenetic, position, enigmatic, family, Podosphaerasteridae