Advanced Cambrian hydroid fossils (Cnidaria: Hydrozoa) extend the medusozoan evolutionary history

Primitive cnidarians are crucial for elucidating the early evolution of metazoan body plans and life histories in the late Neoproterozoic and Palaeozoic. The highest complexity of both evolutionary aspects within cnidarians is found in extant hydrozoans. Many colonial hydrozoans coated with chitinous exoskeletons have the potential to form fossils; however, only a few fossils possibly representing hydroids have been reported, which still require scrutiny. Here, we present an exceptionally well-preserved hydroid found in the Upper Cambrian Fengshan Formation in northern China. It was originally interpreted as a problematic graptolite with an uncertain systematic position. Based on three characteristic morphological traits shared with extant hydroids (with paired hydrothecae, regular hydrocaulus internodes and special intrathecal origin pattern of hydrocladium), we propose this fossil hydroid as a new genus, Palaeodiphasia gen. nov., affiliated with the advanced monophyletic hydrozoan clade Macrocolonia typically showing loss of the medusa stage. More Macrocolonia fossils reviewed here indicate that this life strategy of medusa loss has been achieved already as early as the Middle Devonian. The early stratigraphical appearance of such advanced hydroid contrasts with previous molecular hypotheses regarding the timing of medusozoan evolution, and may be indicative for understanding the Ediacaran cnidarian radiation.

Growth and ingestion rates of the freshwater jellyfish Craspedacusta sowerbii

The invasive freshwater hydromedusa Craspedacusta sowerbii Lankester, 1880 (phylum Cnidaria, class Hydrozoa, family Olindiidae) is native from East Asia but for more than 20 years, reports of this species have been increasing in Europe, North America, and Australia. Due to the sporadic presence of the medusa stage and difficulties in the sample polyp stage, there is a lack of data on the physiological parameters of C. sowerbii and its potential impact. We present data on growth and ingestion rates of the medusa stage measured at 29°C, temperature at which polyps are maintained in the Cinéaqua Aquarium (Paris). Medusa growth increased from 0.60 ± 0.08 to 9.0 ± 2.1 mm (mean: 0.28 ± 0.26 mm d−1), and gonads appeared after 11.5 ± 3.0 days. Ingestion rates increased significantly from 28 ± 16 prey ind−1 d−1 (<1 mm) to 442 ± 170 prey ind−1 d−1 (>7 mm).

Scolionema sanshin sp. n., a new species (Hydrozoa, Limnomedusae, Olindiidae) from the Ryukyu Archipelago, southern Japan

A new species of hydrozoan jellyfish belonging to the order Limnomedusae is reported from the Ryukyu Archipelago, Southern Japan. The species belongs to the genus Scolionema, which prior to this study includes just a single valid species, S. suvaense. This name, however, has several junior synonyms and considerable diversity has been reported for different populations from Mediterranean, Indian ocean and Pacific ocean localities, including Central Japan. The species described in this paper, Scolionema sanshin sp. n., can be differentiated from all other described populations of Scolionema based on shape of gonad, number of tentacles of its medusa stage, and/or genetic sequences. A discussion of the diversity presently united under the name S. suvaense suggests that additional work is necessary to clarify the true number of Scolionema species.

The hydroid and early medusa stage ofOlindias formosus(Cnidaria, Hydrozoa, Limnomedusae)

Olindiasspp. medusae are found worldwide in sublittoral tropical and sub-tropical coastal regions; their occurrence near shore can result in human envenomation events. While behaviour of medusae and human contact with medusae has been documented for the genus, the hydroid (polyp) phase of theOlindiaslife cycle has eluded investigators for over a century. Given the recent debate among public media and scientific communities that jellyfish blooms are increasing worldwide, there is a growing urgency to understand how and why jellyfish populations bloom. In order to understand jellyfish population dynamics, the asexual benthic phase must be studied to determine when, where, and how juvenile medusae are produced. In this study, husbandry management strategies, including the creation of artificial habitat forOlindias formosusmedusae in aquaria were developed to encourage spawning and larval settlement. The resultant hydroid colony ofOlindias formosuswas discovered in November of 2012, utilizing the natural fluorescence of the medusa as a detection method. A description of the hydroid and early medusa stage is presented. These techniques provide a basis for locatingin situthe benthic hydroid phases within this genus and other fluorescent medusae, the discovery of which may lead to a better understanding of the causative factors for jellyfish blooms.

Detection of a new Clytia species (Cnidaria: Hydrozoa: Campanulariidae) with DNA barcoding and life cycle analyses

The genus Clytia is distributed worldwide, but most accepted species in this genus have been examined either only at the hydroid or medusa stage. The challenge in identifying Clytia species reflects their complex life cycles and phenotypic plasticity. In this study, molecular and morphological investigations of Clytia specimens from the coastal waters of China revealed an as yet unreported species, designated C. xiamenensis sp. nov., that was considered as conspecific to two nearly cosmopolitan species, C. hemisphaerica and C. gracilis. DNA barcoding based on partial mitochondrial cytochrome c oxidase subunit I (COI) and large subunit ribosomal RNA gene (16S) confirmed the highly distinct lineage of C. xiamenensis sp. nov. These results were corroborated by the detailed observations of its mature medusae and its colonies, which showed that C. xiamenensis sp. nov. was morphologically distinct from other species of Clytia. Thus, based on our findings, the nearly cosmopolitan distribution attributed to some species of Clytia might rather be due to the misidentification, and it is necessary to elucidate their whole life cycle in order to establish the systematic validity of all species within the genus Clytia.

The genus Hybocodon (Cnidaria, Hydrozoa) in the southwestern Atlantic Ocean, with a revision of the species recorded from the area

The genus Hybocodon includes seven species, two of which (H. prolifer and H. unicus) have been recorded in the south-western Atlantic. These reports were based on the medusa stage and only a few diagnostic characters were described, ren-dering some of these records rather doubtful. Additionally, H. prolifer has a boreal distribution, suggesting that the reportsof this species from the southern hemisphere could correspond to other Hybocodon species. Recently, we sampled benthicand planktonic specimens of Hybocodon in the southwestern Atlantic and two species were identified: polyps andmedusae of Hybocodon chilensis Hartlaub, 1905, and medusae of Hybocodon unicus (Browne, 1902). These samples al-lowed us to review and discuss the geographical distribution and validity of previous records of Hybocodon species forthis area. We conclude that H. chilensis and H. unicus are the only Hybocodon species presently known for the southwestern Atlantic Ocean.

Life cycle of Bougainvillia nana (Cnidaria: Hydrozoa: Bougainvilliidae) from Italy, including a discussion of Bougainvillia muscus in the Mediterranean Sea

The life cycle of a species of the genus Bougainvillia (Cnidaria: Hydrozoa), found in the southern Mediterranean Sea, Italy, is here described. Hydroid colonies produced immature medusae with two tentacles and two ocelli per bulb and four unbranched oral tentacles. The number of tentacles and ocelli, which remained constant during the entire life cycle, are here considered diagnostic characters to identify the present as a new species. Female medusae lived up to 47 days (with an average of 30 days) when reared at 17°C, while males, reared at the same temperature, were short-lived, concluding their life cycle in no more than 15 days. The medusa of this species resembles B. ramosa var. nana described by Hartlaub in 1911 on the basis of few specimens and no polyp stage. After the complete life cycle has been observed, and given its peculiar medusa stage, Hartlaub's subspecies (variant) must be promoted to species rank as Bougainvillia nana.