Revisiting mitogenome evolution in Medusozoa with eight new mitochondrial genomes

Mitogenomics has improved our understanding of medusozoan phylogeny. However, sequenced medusozoan mitogenomes remain scarce, and Medusozoa phylogeny studies often analyze mitogenomic sequences without incorporating mitogenome rearrangements. To better understand medusozoan evolution, we analyzed Medusozoa mitogenome phylogeny by sequencing and assembling eight mitogenomes from three classes (Cubozoa, Hydrozoa, and Scyphozoa). We reconstructed the mitogenome phylogeny using these mitogenomes and 84 other existing cnidarian mitogenomes to study mitochondrial gene rearrangements. All reconstructed mitogenomes had 13 mitochondrial protein-coding genes and two ribosomal genes typical for Medusozoa. Non-cubozoan mitogenomes were all linear and had typical gene orders, while arrangement of genes in the fragmented Cubozoa (Morbakka sp.) mitogenome differed from other Cubozoa mitogenomes. Gene order comparisons and ancestral state reconstruction suggest minimal rearrangements within medusozoan classes except for Hydrozoa. Our findings support a staurozoan ancestral medusozoan gene order, expand the pool of available medusozoan mitogenomes, and enhance our understanding of medusozoan phylogenetic relationships.

Taxonomy and Molecular Phylogeny of the Sea Anemone Macrodactyla (Haddon, 1898) (Cnidaria, Actiniaria), with a Description of a New Species from Singapore

Sea anemones (Cnidaria, Actiniaria) are a successful group of marine invertebrates found in a diverse range of environments globally. In spite of their ubiquity, identities for many sea anemones remain unverified, especially those from the Indo-West Pacific region. Here, we clarify the taxonomy of the poorly known Macrodactyla aspera, a shallow-water species first described from the Torres Straits in northern Australia. We re-describe M. aspera based on new morphological and molecular data gathered from the type specimen, other museum vouchers, and from fresh material collected from Singapore. We tested the monophyly of Macrodactyla using three mitochondrial (12S, 16S and cox3) and one nuclear (28S) marker based on three congeners, recovering this genus to be polyphyletic. As a consequence, we transferred M. doreensis to the genus Heteractis, and describe a new species, Macrodactyla fautinae sp. nov. While both M. aspera and M. fautinae sp. nov. share the same arrangement and number of complete mesenteries, a similar distribution of cnidae, and are not symbiotically associated with any other biota, M. fautinae sp. nov. has perforated, lobe-like verrucae on its column, and lacks nematocyst batteries on its tentacles, unlike M. aspera. These two species also occur in similar habitats in Singapore. Finally, because M. aspera strongly resembles Dofleinia armata, the latter species flagged as a danger to public health due to its ability to inflict painful stings, we tested the relationship between these species and found them not to be closely related. However, tentacles of M. aspera, like D. armata, are densely covered with nematocyst batteries and harbour large nematocysts; we infer that M. aspera may also be capable of delivering stings that endanger public health. This study builds upon a growing number of studies that aim to ascertain identities and systematics of sea anemones historically reported from the Indo-West Pacific. Our findings will facilitate accurate species identification, which is crucial for advancing research, formulating conservation measures, and protecting public health.

Spirobranchus bakau sp. nov. from Singapore: yet another species of S. kraussii-complex (Polychaeta: Serpulidae)

The intertidal serpulid polychaete Spirobranchus kraussii was originally described from South Africa and has since been reported in numerous sub (tropical) localities around the world. Recently, however, S. kraussii was uncovered as a complex of morphologically similar and geographically restricted species, raising the need to revise S. cf. kraussii populations. We formally describe S. cf. kraussii from Singapore mangroves as Spirobranchus bakau sp. nov. based on morphological and molecular data. Despite their morphological similarities, Maximum Likelihood and Bayesian Inference analyses of 18S and Cyt b DNA sequence data confirm that S. bakau sp. nov. is genetically distinct from S. kraussii and other known species in the complex. Both analyses recovered S. bakau sp. nov. as part of a strongly supported clade (96% bootstrap, 1 posterior probability), comprising S. sinuspersicus, S. kraussii and S. cf. kraussii from Australia and Hawaii. Additionally, paratypes of S. kraussii var. manilensis, described from Manila Bay in the Philippines, were examined and elevated to the full species S. manilensis. Finally, we tested the hypothesis that fertilisation and embryonic development of S. bakau sp. nov. can occur under the wide range of salinities (19.630.9 psu) and temperatures (2531C) reported in the Johor Strait. Fertilisation success of ≥70% was achieved across a temperature range of 2532C and a salinity range of 2032 psu. Embryonic development, however, had a narrower salinity tolerance range of 2732 psu. Clarifying the taxonomic status of S. cf. kraussii populations reported from localities elsewhere in Singapore and Southeast Asia will be useful in establishing the geographical distribution of S. bakau sp. nov. and other members of the S. kraussii-complex.

Carlgren’s hesitation allayed: redescription and systematics of Heteranthus verruculatus Klunzinger, 1877 (Cnidaria, Actiniaria), with a redefinition of Heteranthidae Carlgren, 1900

Species boundaries delineating tropical sea anemones (Cnidaria, Actiniaria) of the zooxanthellate genus, Heteranthus Klunzinger, 1877, are unclear. There are currently two valid Heteranthus species: type species Heteranthus verruculatus Klunzinger, 1877, first reported from Koseir, Egypt, and H. insignis Carlgren, 1943, from Poulo Condore, Vietnam. In describing the latter from a single, poorly preserved specimen, zoologist Oskar Carlgren expressed apprehension with traits he had used to establish this species. Carlgren’s doubts persisted later in writing when he found a similar-looking sea anemone from the Great Barrier Reef. Crucial details to positively identify either species have since remained limited. Here, we re-diagnosed Heteranthus and re-described its type species based on observations of specimens we have obtained from Singapore and Pulau Ambon (Indonesia), and of museum material collected elsewhere across the Indo-West Pacific region (n > 180). Supported by molecular phylogenetic evidence, the family Heteranthidae Carlgren, 1900 was reinstated and re-diagnosed. Heteranthus verruculatus is encountered in the lower intertidal region amongst seagrass, in rocky crevices, or coral rubble. It occurs as solitary individuals or in clonal clusters, well-camouflaged against the substratum. Individuals were observed to frequently propagate by longitudinal fission, resulting in a varied appearance. Type material of H. verruculatus and H. insignis were re-examined and as we found no differences between them, the two were synonymised. We inferred that Carlgren probably misinterpreted cnidae and histological data in defining H. insignis as a distinct species. This revision clarifies the taxonomy and geographic range of H. verruculatus, an Indo-West Pacific species that is found from the Red Sea to subtropical Australia and Hawaii.

Sea anemones (Cnidaria, Actiniaria) of Singapore: redescription and taxonomy of Phymanthuspinnulatus Martens in Klunzinger, 1877

Despite the ubiquity of sea anemones (Cnidaria: Actiniaria) in tropical ecosystems, our understanding of their biodiversity and taxonomy is limited. Here we re-establish the identity of an intertidal zooxanthellate species, Phymanthuspinnulatus Martens in Klunzinger, 1877. Originally described from a single preserved specimen in the Berlin Museum by CB Klunzinger, his brief footnote lacked crucial details to positively identify the species. Our redescription is based on more than 50 living individuals of P.pinnulatus collected from its type locality, Singapore. These were examined and compared with type materials of the species and its congeners. Specimens of P.pinnulatus differ from syntypes of species described as Phymanthuslevis Kwietniewski, 1898 from Indonesia, as well as Phymanthussansibaricus Carlgren, 1900 and Phymanthusstrandesi Carlgren, 1900, both described from East Africa. Phymanthuspinnulatus was encountered on the lower intertidal, among coral rubble and between rocky crevices. It is vibrantly coloured and has 96 marginal tentacles with branching outgrowths along each, resulting in a ‘frilly’ appearance. The anemone has a flat expanded oral disc, with discal tentacles that are inconspicuous and reduced, unlike syntypes of its congeners. Details of its live appearance, musculature, and cnidom are also provided for the first time. Overall, types of cnidae and capsule sizes differ from other known species of Phymanthus documented elsewhere. It is inferred that P.pinnulatus has a wide distribution that extends eastwards from Singapore, as far as Ambon and the Torres Straits. Some individuals reported as Phymanthusmuscosus Haddon and Shackleton, 1893 and Phymanthusbuitendijki Pax, 1924 are probably P.pinnulatus. This morphological analysis provides new insights into the characters used to delimit P.pinnulatus, clarifies its geographical distribution, and contributes to an ongoing revision of the genus Phymanthus.