“Indirect development” increases reproductive plasticity and contributes to the success of scyphozoan jellyfish in the oceans

Ecologists and evolutionary biologists have been looking for the key(s) to the success of scyphomedusae through their long evolutionary history in multiple habitats. Their ability to generate young medusae (ephyrae) via two distinct reproductive strategies, strobilation or direct development from planula into ephyra without a polyp stage, has been a potential explanation. In addition to these reproductive modes, here we provide evidence of a third ephyral production which has been rarely observed and often confused with direct development from planula into ephyra. Planulae of Aurelia relicta Scorrano et al. 2017 and Cotylorhiza tuberculata (Macri 1778) settled and formed fully-grown polyps which transformed into ephyrae within several days. In distinction to monodisk strobilation, the basal polyp of indirect development was merely a non-tentaculate stalk that dissolved shortly after detachment of the ephyra. We provide a fully detailed description of this variant that increases reproductive plasticity within scyphozoan life cycles and is different than either true direct development or the monodisk strobilation. Our observations of this pattern in co-occurrence with mono- and polydisk strobilation in Aurelia spp. suggest that this reproductive mode may be crucial for the survival of some scyphozoan populations within the frame of a bet-hedging strategy and contribute to their long evolutionary success throughout the varied conditions of past and future oceans.

Morphological characteristics of ephyrae of Aurelia coerulea derived from planula strobilation

Ephyrae are produced through the strobilation of polyps in the general life cycle of Aurelia coerulea. However, it has been reported that planulae can also metamorphose directly into ephyrae, without passing through the polyp stage. There is a mixture of ephyrae developed from planulae (planula-strobilated ephyrae) and ephyrae developed from polyps (polyp-strobilated ephyrae) in the ephyra population. However, the effect of the planula-strobilated ephyrae on the ephyra population is yet to be determined, since their morphological characteristics have not yet been elucidated. This study aimed to determine the morphological characteristics to distinguish between planula-strobilated and polyp-strobilated ephyrae. The differences in body dimensions, such as total body diameter (TBD), central disc diameter (CDD), lappet stem length (LStL), rhopalial lappet length (RLL), and total marginal lappet length (TMLL) were compared between the two types of ephyra. Thus, we show that body proportions can be used to identify planula- and polyp-strobilated ephyrae. The ranges for identifying planula-strobilated ephyra were 35.0–38.3% for CDD/TBD, 56.7–64.9% for LStL/CDD, 84.7–99.5% for TMLL/CDD, and 31.0–37.5% for RLL/TMLL. This method could be an important basis for devising countermeasures for jellyfish blooms in areas where ephyrae deriving from planula strobilation occur.

Life Cycle of Edible Jellyfish Acromitus hardenbergi Stiasny, 1934 (Scyphozoa: Rhizostomeae) Inhabiting a Brackish-Water Environment

The edible jellyfish Acromitus hardenbergi Stiasny, 1934 is harvested throughout the year at the mouth of the Perak River, Malaysia. Although this species is an important fishery resource in the local area, limited biological studies have been carried out on it. The aim of the present study was to elucidate the life cycle of this unique brackish-water jellyfish in order to conserve the species and develop sustainable jellyfish fisheries. Mature medusae were collected at the mouth of the Perak River. Embryonic and larval development after fertilization was completed within 24 h until the planula stage and within 48 h until the polyp stage. Primary polyps had a long stalk with a small stolon at the base of the calyx. Fully developed polyps were bowl-or goblet-shaped but became an elongated stalk under starved conditions. Asexual reproduction was accomplished only by means of budding, and no podocysts were produced. Strobilation was mono-disc type. These characteristics may be adaptations to the dynamic environmental conditions in the estuary of the Perak River, where salinity fluctuates widely due to strong inflows of highly turbid freshwater coupled with tidal changes. This study suggests that polyps of A. hardenbergi expand their population not by podocysts, but by budding as quickly as possible and forming one large ephyra by mono-disc strobilation without the residuum, because the polyp cannot remain for a long time at its settlement place in the sediment-rich environment with drastic salinity change.

Asexual Reproduction and Strobilation of Sanderia malayensis (Scyphozoa, Pelagiidae) in Relation to Temperature: Experimental Evidence and Implications

Sanderia malayensis is a scyphozoan species present in the Indian and Pacific Oceans, ranging from the Suez Canal to Japan. Although this jellyfish is commonly kept in aquariums around the world, there is a knowledge gap regarding its biology and ecology, especially at the polyp stage. In this study, we tested the asexual reproductive activity of S. malayensis at three different temperatures: 10, 15 and 20 °C. Results showed significant increases of polyps at 15 and 20 °C, and a minimum at 10 °C, corresponding with daily budding rates of 6.61% ± 0.92%, 5.85% ± 2.36% and 0.66% ± 0.24%, respectively. Moreover, a second experiment was carried out to report about the ability of S. malayensis to prey on Aurelia solida at the ephyra stage. Unidirectional predation of S. malayensis ephyrae on A. solida and an absence of inverse predation was observed. These results could give new insights on the potential fitness and survival of this species if it will ever invade the Mediterranean Sea.

Occurrence of two distinct lineages of the freshwater jellyfish Craspedacusta sowerbii (Hydrozoa: Limnomedusae) in Italy

The freshwater jellyfish Craspedacusta sowerbii Lankester 1880 is a cryptic cosmopolitan invasive species, which occurs in all continents except Antarctica. Recent molecular studies suggest the existence of at least three very different genetic lineages of Craspedacusta: the “sowerbii”, the “kiatingi”, and the “sinensis” lineages. We report the presence of both medusae and polyps of this alien taxon in the Large Lake of Monticolo / Montiggl, a meso-eutrophic natural lake in the Province of Bolzano / Bozen in Northern Italy. Molecular analyses of mitochondrial 16S sequences showed that this population belongs to a different lineage than that recently described for Sicily (Southern Italy). Therefore, there are two different genetic lineages of C. sowerbii in Italy. In the Large Lake of Monticolo / Montiggl medusae were observed in 6 consecutive summers (2015–2020), from July to September. All the examined medusae were males. The stomach content analyses showed that zooplanktonic copepods and cladocerans with size range between 0.3 and 0.8 mm were the preferred prey of medusae. Polyps of C. sowerbii were recorded in the lake on the zebra mussel Dreissena polymorpha in shallow water and on the underside of artificial substrates. The analyses of zebra mussels would therefore be a simple method to check for the presence of the polyp stage of C. sowerbii in various aquatic environments.

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).

Predation behaviour of millimetre-sized hydrozoan polyps on early ivory shell juveniles

Marine hydroids, the polyp stage of hydrozoans, are assumed to be carnivores and play an important role in benthic-pelagic coupling processes by regulating zooplankton populations and transferring energy. The known hydroid diets mainly consist of zooplankton as well as some benthic diatoms and metazoans, almost all of which have smaller dimensions than the polyp. In the present study, a large-scale breeding failure of the early juveniles of the ivory shell Babylonia areolata (Mollusca: Gastropoda: Babyloniidae) was found in a local aquaculture farm in Fujian, China in July 2018. About 640,000 out of 800,000 early juveniles (80%) died due to predation by a millimetre-sized colonial hydroid species that was initially overlooked. The hydrozoan species was identified as Eirene sp. (Cnidaria: Hydrozoa: Eirenidae) via the integration of morphological and molecular data. A laboratory feeding experiment showed that the hydroid predation led to an overall mortality of ivory shell juveniles of up to 92.6% after 48 h. The predation process was observed in the laboratory and a video showing the entire process was prepared. This is the first report of hydroids preying on gastropod juveniles, revealing a previously unknown threat to mollusc aquaculture including ivory shells. Several management practices are suggested to mitigate this newly recognized threat.

Here are the polyps: in situ observations of jellyfish polyps and podocysts on bivalve shells

Most Scyphozoan jellyfish species have a metagenic life cycle involving a benthic, asexually reproducing polyp stage and a sexually reproducing medusa stage. Medusae can be large and conspicuous and most can be identified using morphological characteristics. Polyps on the other hand are small, live a cryptic life attached to hard substrates and often are difficult or impossible to distinguish based on morphology alone. Consequently, for many species the polyp stage has not been identified in the natural environment. We inspected hard substrates in various habitats for the presence of Scyphozoan polyps. Three polyps were found on Dogger Bank, Central North Sea, attached to the inside of the umbo of empty valves of the bivalves Mactra stultorum and Spisula subtruncata. One polyp was accompanied by four podocysts. With this knowledge, the inside of bivalve shells washed ashore in Oostende (Belgium) was inspected and supposed podocysts on the inside of empty valves of Cerastoderma edule and Spisula elliptica were found. Polyps and podocysts were identified to species level by 18S rDNA and mitochondrial COI sequencing. The three polyps found on Dogger Bank all belonged to the compass jellyfish Chrysaora hysoscella. One podocyst from the Oostende beach also belonged to this species but another podocyst belonged to Cyanea lamarkii. These are the first in situ observations of C. hysoscella and C. lamarckii polyps and podocysts in the natural environment. Mactra, Cerastoderma and Spisula species are abundant in many North Sea regions and empty bivalve shells could provide an abundant settling substrate for jellyfish polyps in the North Sea and other areas. Several new strategies to increase the detection of polyps on bivalve shells are presented.

Update on Benthic Scyphozoans from the Brazilian Coast (Cnidaria: Scyphozoa: Coronatae)

Members of the order Coronatae are considered the basal group of Scyphozoa, containing approximately 60 species. Observation of life cycle is critical to unravel the systematics and taxonomy of the order. However, recent studies related only to the polyp stage were able to promote progress using solely morphological characters of the periderm tube. The lack of knowledge about the actual number of coronate species occurring in Brazil and their distribution along the coast is a factor that limits more advanced approaches to interpret the biogeography of these animals. Our goal was to identify and describe the Coronatae polyps from N, SE and S Brazil, taking into account the bathymetric and longitudinal distribution of species. Measurements of the periderm tube under light microscopy and the organization and morphology of the internal cusps observed through Scanning Electron Microscopy allowed the recognition of three morphotypes, recognized only at the genus level: two morphotypes of Nausithoe (eight and 16 cusps) and one form of Atorella.