Inland waters are susceptible to extreme environmental conditions including freezing, drying or high salinity. In these environments, it is common to find animals with several adaptive strategies to these variable conditions. Freshwater Sponges, Spongillida, for example, produce resisting bodies known as gemmules, which can survive unfavorable conditions. Gemmules can be found in species belonging to three families currently placed in the order Spongillida. Potamolepidae species deposit gemmules with a simple architecture at the Sponge-substrate interface, whereas gemmules of the Spongillidae and Metaniidae are distributed throughout the Sponge body and have complex gemmule morphologies with a well-developed pneumatic layer, and spiny gemmuloscleres (specialized spicules found only in the gemmules). Completely formed gemmules exhibit low metabolic rates and may become dormant. Most Freshwater Sponges undergo a period of dormancy at some time during the year, typically during periods of environmental stress, until hatching occurs. The dormancy of gemmules is likely to be controlled by interactions between exogenous (e.g., temperature, light, salinity, oxygen tension, and pH) and endogenous factors (e.g. concentrations of calcium, and osmotic pressure). It is likely that that such endogenous processes are involved in the control of life cycle phases, and seasonality and length of phases are subject to exogenous limiting factors, such as ice-up and dry-upfreezing and drying.
In a paper published in the journal Iheringia Série Zoologia on 10 January 2020, Ludimila Calheira, Paulo Santos, and Ulisses Pinheiro of the Departamento de Zoologia at the Universidade Federal de Pernambuco, present the results of a study designed to test the capacity of hatching under differnt conditions of the gemmules from two Neotropical Freshwater Sponges, both belonging to the family Spongillidae; Heteromeyenia cristalina and Radiospongilla inesi.
Stressful conditions can be crucial to the success of gemmules hatching in some species. Prvious studies have shown that exposure of gemmules to low temperatures or drying can stimulate hatching when conditions resume later to an ideal environmental condition, respectively in the cases of Eunapius fragilis and Metania spinata. Gemmules of Ephydatia muelleri, Racekiela ryderii, and Spongilla lacustris, also have been shown to be tolerant of low temperatures. Most studies on the hatchability of gemmules have focused in low temperatures and drying, but studies have also tested Eunapius fragilis for salinity tolerance and the ability to survive seasonal anoxia.
Specimens of Heteromeyenia cristalina and Radiospongilla inesi were collected from Fish farming tanks at the Universidade Federal Rural de Pernambuco, fed with water from the Prata River, in northeast Brazil.
Specimen of Heteromeyenia cristalina, in situ. Scale bar is 1 cm. Calheira et al. (2020).
Sponges were individually packed in a container with water of their own environment and transported to the laboratory. Additionally, the tank water was also collected and stored in a glass container. Other samples of water used in the study were all from northeast Brazil: water from the Pirangi River (chosen due to its high diversity of Sponges, with at least four species recorded), water from Araraquara Pond ('humic water' considered to be rich in organic material), and and mineral water from the Emerald Source mineral spring (due to its purity).
Specimen of Radiospongilla inesi, in situ. Scale bar is 1 cm. Calheira et al. (2020).
In the laboratory, gemmules were removed from Sponges and allocated in 24 well (3 ml) plates, containing a single gemmule each, and cultured at room temperature (approximately 25 °C). Five assays were tested: (A1) Water from the Fish farming tanks where the Sponges were collected; (A2) Pirangi River water; (A3) Araraquara Pond water; (A4) mineral water and (A5) dry. Gemmules in test (A5) were stored in Petri dishes covered with filter paper for 10 days to dry. After this period, gemmules were allocated to plates with water from the Fish farming tanks which the Sponges were collected.
For each assay, three replicates with 30 gemmules were used. The gemmules were observed daily during 30 days under a stereomicroscope to register the number of hatching gemmules and how many gemmules develop into juvenile Sponges.
Gemmules of both Heteromeyenia cristalina and Radiospongilla inesi did not hatch when submitted to humic waters (A3). In the other assays (A1), (A2) and (A4) gemmules of both species hatched, with significant differences among assays, and between species.
Gemmule of Heteromeyenia cristalina submitted to Araraquara Pond water. Scale bar is 100μm. Calhiera et al. (2020).
The water of fish farming tanks (A1) yielded high rates of gemmule hatching, 92% for Heteromeyenia cristalina and 80% for Radiospongilla inesi. Gemmules of the two species had different results when submitted to waters from Pirangi River (A2) and mineral water (A4). Radiospongilla inesi showed higher hatching rates with 79% and 78%, against only 8% and 2% for Heteromeyenia cristalina, respectively.
Gemmule of Radiospongilla inesi submitted to Araraquara Pond water. Scale bar is 100μm. Calhiera et al. (2020).
The two species also responded differently to drying. 92% of Heteromeyenia cristalina gemmules placed in water from the tanks from which they were collected without drying hatched, whereas only 66% if those hatched first did so. With Radiospongilla inesi the reverse occured, with 91% of the dried gemmules hatching, but only 80% of the undried gemmules.
Gemmules of both species submitted to Araraquara Pond waters did not hatch, probably due to its conductivity, which was three times higher than that of the other waters tested by Calhiera et al. The high conductivity of this environment can be explained by the organic matter enrichment. Until now, only six species from Neotropical Freshwater Sponges have been found in humic waters: Corvomeyenia thumi, Metania spinata, Tubella variabilis, Dosilia pydanieli, Radiospongilla amazonensis, and Corvomeyenia epilithosa. Except for Corvomeyenia epilithosa, none of these species are restricted to humic waters, and all have a wide distribution, indicating adaptive plasticity and the ability to cope with different kinds of inland waters.
There were significant differences among the assays and between the species. Radiospongilla inesi presented a high hatching rate and sponge development in all non-humic waters tested, whereas Heteromeyenia cristalina had high hatching rate and Sponge development only im its own collection water. These results for Heteromeyenia cristalina were completely unexpected due to its wide distribution on the Neotropical region. Conversely, Radiospongilla inesi has only been known from its type locality until now. Despite belonging to the same family and sharing a complex gemmular morphology, Heteromeyenia cristalina and Radiospongilla inesi have different levels of adaptation to different environments due to the distinct hatchability potential of their gemmules.
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