Siphonophores are among the most bizarre animals on Earth. Each colony is not a single organism but a superorganism — a floating, drifting assemblage of genetically identical zooids, each specialized for a different task. Some zooids catch prey, some digest it, some reproduce, and some propel the colony through the water. Rhizophysa eysenhardtii belongs to the cystonect group of siphonophores, which includes the infamous Portuguese Man of War. But unlike the Man of War, which has a large float and no stem, Rhizophysa has a smaller gas-filled float, or pneumatophore, at the apex of a long stem that trails behind it, armed with tentacles like living fishing lines.

The five colonies observed in False Bay were photographed and filmed in situ by Jannes Landschoff and Catherine Corder using an Olympus TG6 camera with a fisheye converter. The smallest colonies, seen in June and July, were contracted and measured between 0.05 and 0.1 meters. The September colonies were far larger — the biggest stretched to 0.8 meters in video footage — and displayed most of their tentacles extended for feeding. The colonies were found at depths of just 1 to 3 meters, near a head of bamboo kelp Ecklonia maxima, in remarkably shallow water for a species previously known mostly from deeper pelagic zones.

Identification was based on key morphological features. The tentacles of R. eysenhardtii are characteristically pink, and the side branches, or tentilla, lack the swollen tips found in its congener R. filiformis. Instead, the stinging cells, or nematocysts, are distributed along the entire length of each side branch. The False Bay specimens matched these traits precisely, with pink tentacles clearly visible in contracted colonies and simple tentilla confirmed in the larger, extended specimens. The pneumatophore of the largest colony was estimated at approximately 13 millimeters, consistent with previously described specimens of this species.

The feeding behavior observed was striking. Fish larvae were found inside the gastrozooids — the asexual feeding zooids of the colony — and the researchers counted between 0 and 8 captured larvae per colony by identifying pairs of eyes visible through the transparent tissues. One colony had also captured a chaetognath, a predatory arrow worm, still stuck to a tentacle and not yet ingested. This diet aligns with earlier work by Purcell, who showed that R. eysenhardtii feeds almost exclusively on fish larvae, making it a potentially significant predator in pelagic ecosystems.

What makes these observations particularly valuable is the light they shed on how the colony attracts its prey. Previous research by Haddock and Dunn revealed that a strip of green fluorescence runs along the length of each gastrozooid, likely luring fish larvae toward the colony in the same way that deep-sea anglerfish use bioluminescent lures. This is added evidence that some siphonophores use fluorescent lures to attract fish prey, a strategy also seen in the physonect genus Erenna, whose five species all feed exclusively on fish.

The pneumatophore itself is a marvel of biological engineering. It contains a gas bladder, or pneumatosaccus, surrounded by a hypocystic cavity that is confluent with the gastrovascular cavity of the stem. Hypocystic villi arise from the base of the pneumatosaccus and contain gas-producing cells that secrete carbon monoxide into the bladder. A chitin layer prevents the gas from escaping, except through an apical pore controlled by a sphincter muscle. In the False Bay specimens, this gas chamber was particularly prominent in photographs, reflecting light rays and making the float stand out sharply against the surrounding water.

Reproductive structures were also visible in the larger specimens. Gonodendra — branched complexes bearing sexual gonophores — were seen between the gastrozooids. In the largest colony, a single gonodendron occurred between two gastrozooids, and these structures increased considerably in size toward the posterior end of the stem. Each gonophore releases gametes of one sex only, meaning the colony must produce multiple gonophores to reproduce effectively. The researchers noted that in some specimens there may be two gonodendra per cormidium, though this was not observed in the False Bay colonies.

The range extension is considerable. Previously, the southernmost published record of R. eysenhardtii was slightly southwest of Hondeklip Bay on the west coast of South Africa, at roughly 31 degrees south latitude. The False Bay observations, at about 34 degrees south, push the known distribution approximately 450 kilometers further south. The researchers suggest the species may have arrived via the variability of the Agulhas Current at Cape Hangklip during the winter and spring of 2022, a mechanism previously discussed for other siphonophore species in the region.

Despite being probably common and potentially ecologically important in pelagic zones, published accounts of rhizophysid siphonophores remain scarce. As Totton noted in 1965, these animals rarely come into the hands of systematists, though fishermen suffer from their stings when tentacles adhere to nets and cordage. The False Bay observations add valuable biological and distributional knowledge to a species that has never been sequenced genetically, and they demonstrate that citizen science and careful snorkeling observation can document rare marine creatures in places where formal surveys rarely reach.

Subject of Research: The rare siphonophore Rhizophysa eysenhardtii Gegenbaur, 1859 (Hydrozoa, Siphonophora, Cystonectae) from False Bay, South Africa

Article Title: The rare siphonophore Rhizophysa eysenhardtii Gegenbaur, 1859 ( Hydrozoa , Siphonophora , Cystonectae ) from False Bay, South Africa

Article References: Mapstone, G. M., & Landschoff, J. (2024). The rare siphonophore Rhizophysa eysenhardtii Gegenbaur, 1859 (Hydrozoa, Siphonophora, Cystonectae) from False Bay, South Africa. African Invertebrates, 65(2), 213-221. https://doi.org/10.3897/afrinvertebr.65.136267

Image Credits: AI Generated

DOI: 10.3897/AfrInvertebr.65.136267

Keywords: rare, siphonophore, Rhizophysa, eysenhardtii, Gegenbaur, Hydrozoa, Siphonophora, Cystonectae, False, South, Africa, scientific research

Cladonema pacificum has recently emerged as a tractable laboratory model for studying regeneration in medusae, the free-swimming, sexually reproducing stage of the cnidarian life cycle. Unlike the sessile polyp Hydra, which can display negligible senescence under laboratory conditions, medusae follow a finite and reproducible lifespan, making them an ideal system in which to ask how aging reshapes tissue homeostasis within a single life-history context. The jellyfish retains robust regenerative capacity during early adulthood, so the researchers could track precisely when and how that capacity begins to fail. Their findings, published in Aging Cell, reveal a coordinated breakdown across multiple cellular systems rather than a single point of failure.

The team began by monitoring medusae under standardized culture conditions, taking longitudinal measurements of the three major organs: the umbrella that powers swimming, the manubrium that handles feeding and gametogenesis, and the tentacles used for prey capture and defense. As the animals aged, all three structures deteriorated. Umbrellas shrank significantly, manubria regressed relative to body size, and tentacles became markedly shorter. The researchers classified animals up to 40 days old as young, those between 41 and 60 days as middle-aged, and those 61 days and beyond as aged, a scheme grounded in the consistent, measurable changes they observed across dozens of individuals.

Histological analysis deepened the picture of decline. In young medusae, the manubrium displays a well-organized bilayered structure with oocytes nestled in its interior; in aged animals, the outer ectodermal layer thinned dramatically and oocyte numbers appeared reduced. Egg production, tracked longitudinally in the same individuals, peaked during the young phase and then fell steadily. The tentacles told a similar story: the photoreceptor organs called ocelli, neatly circular in youth, became irregular and disrupted with age, while the spherical clusters of stinging nematocytes at the tentacle tips grew less prominent. Critically, the accumulation of phosphorylated histone H2AX, a widely used molecular marker of DNA damage, rose in the tentacle bulbs as the animals aged, linking the visible morphological decay to cellular-level damage.

Because tentacles are the best-characterized regenerative tissue in Cladonema, the researchers focused their cellular analysis there. Hydrozoan tentacles harbor multipotent interstitial stem cells, or i-cells, concentrated near the tentacle base, which act as resident homeostatic stem cells supplying both progenitors and differentiated cells. Using fluorescence in situ hybridization and immunostaining, the team found that mature nematocytes, identified by the poly-γ-glutamate they synthesize, declined significantly in aged tentacles. Their precursors, the nematoblasts marked by the genes Mcol1 and Nanos2, were also reduced, and an independent, marker-independent histological scoring of the nematoblast-rich region confirmed the shrinkage of this lineage. Neuronal cell bodies near the tentacle base, labeled with anti-FMRFamide staining, likewise dropped in number.

The stem cells themselves showed clear signs of functional decline. Quantification based on β-catenin staining and nuclear morphology revealed a modest reduction in the proportion of i-cell-like cells in aged tentacle bulbs, alongside an increased share of epithelial cells, and FISH for the stem cell marker Piwi showed a marked drop in Piwi-positive cells. Proliferation collapsed across the board: EdU incorporation assays revealed far fewer S-phase cells in aged bulbs, and phospho-histone H3 staining showed fewer mitotic figures. Notably, when the researchers combined EdU labeling with Piwi FISH, they found that DNA synthesis declined both in Piwi-positive stem-like cells and in the broader Piwi-negative population, indicating that the proliferative slowdown was not confined to the stem cell compartment but swept through the entire tissue.

Did this cellular erosion translate into regenerative failure? To find out, the team amputated tentacles from medusae aged 30 to 70 days and monitored regrowth. Young tentacles elongated steadily, formed branches, and rebuilt their nematocyte clusters, but regeneration grew progressively weaker with age, with the decline becoming especially pronounced between 50 and 60 days. Functional recovery fared even worse. Seven days after amputation, every regenerated tentacle in young medusae captured and ingested prey, whereas most aged tentacles failed to capture prey at all and none completed ingestion. Even the earliest step of regeneration, wound closure driven by remodeling of the actin cytoskeleton, was substantially delayed: while most young tentacles achieved complete epithelial closure within two days, few aged tentacles managed it even by three days.

The mechanistic culprit appears to be the blastema, the localized mass of proliferative, undifferentiated cells that assembles at the wound site and orchestrates regrowth. Previous work established that Cladonema blastema formation depends on repair-specific proliferating cells, a lineage largely distinct from the resident homeostatic stem cells, and that blocking proliferation anywhere in the system impairs regeneration. In the aged medusae, EdU incorporation at wound sites was severely reduced, mitotic cells were scarce, and FISH revealed far fewer Piwi- and Nanos1-positive cells accumulating at injuries. Morphology-based classification confirmed a loss of i-cell-like cells near aged wounds. Intriguingly, a substantial population of morphology-defined stem-like cells remained at the injury sites even in old animals, but these cells had largely lost the molecular markers and proliferative activity that define a functional blastema, suggesting that aging impairs the ability of existing cells to enter the regenerative state rather than simply depleting them.

The authors interpret blastema failure not as a primary cause of regenerative decline but as an integrated outcome of broader aging defects: reduced baseline proliferation, weakened injury responses, and deteriorating stem cell support all precede or accompany the blastema’s collapse. They also note that changes in the stem cell niche, the local tissue environment that provides instructive cues, may contribute alongside intrinsic changes in the stem cells themselves. Because regeneration-specific cell lineages are distinct from homeostatic ones, aging may selectively compromise the activation or coordination of repair programs, perhaps through alterations in cellular state or the surrounding tissue architecture.

The comparative implications are striking. In the African turquoise killifish, aging disrupts progenitor activation and early regenerative events in brain and fins; in axolotls, limb regeneration slows with age rather than disappearing outright. Cladonema now extends this pattern to a cnidarian, an animal lacking dedicated mobile immune cells and classical inflammatory responses, implying that aging targets core components of regenerative systems, stem and progenitor cell dynamics and their coordination with surrounding tissues, across the breadth of metazoan evolution. The study also joins a growing body of work showing that strong regenerative capacity does not guarantee escape from senescence: sexual Hydra oligactis deteriorates after induced reproduction, and sexual planarians show clear aging despite their legendary plasticity. Cladonema, with its short life cycle, easy laboratory culture, and experimentally accessible tentacles, now offers researchers a powerful new platform for dissecting how the biology of aging collides with the biology of repair, and why, in the end, regeneration alone cannot hold back the tide of time.

Subject of Research: Age-related decline in tissue homeostasis and blastema-mediated tentacle regeneration in the cnidarian medusa Cladonema pacificum

Article Title: Aging Disrupts Tissue Homeostasis and Constrains Blastema‐Mediated Regeneration in the Cladonema Medusa

Article References: Kanehisa, R., Nakatani, H., Takatori, S., Tomita, T., Miura, M., & Nakajima, Y.-I. (2026). Aging Disrupts Tissue Homeostasis and Constrains Blastema‐Mediated Regeneration in the Cladonema Medusa. Aging Cell, 25(10), Article e70727. https://doi.org/10.1111/acel.70727

Image Credits: AI Generated

DOI: 10.1111/acel.70727

Keywords: Cladonema pacificum, aging, regeneration, blastema, stem cells, cnidarians, medusa, tissue homeostasis, nematocytes, DNA damage, proliferation, hydrozoa