The histological analyses provided important data on the occurrence of accessory reproductive structures in dendrophylliid corals, being only the trophonema previously reported among anthozoans. Glycoproteic material characterized as mucin was attributed to the carbonate skeleton matrix of scleractinians (Lang et al. 2007), but has never been correlated to the oocytes. Moreover, the nucleolinus remained thus far unknown for cnidarians.
Following the literature, the trophonema is a dense aggregation of specialized fat cells, the trophocytes, evident when the oocyte is completely immersed into the mesoglea (Larkman and Carter 1982; Eckelbarger et al. 2008). Other smaller, feebler mesenterial projections are observed crossing the glycoprotein layer, but the trophonema is thicker, forming a ‘contact zone’ (sensu Eckelbarger et al. 2008), with the oocyte membrane. Among anthozoans, it has been previously mentioned for actiniarians, coenothecalians (Alcyonaria), zoanthids, and antipatharians (Hertwig and Hertwig 1879; Babcock 1990; Ryland et al. 1997; Lauretta et al. 2018).
In Tubastraea corals, the trophonema may play an important role as a ‘bridge’, crossing the thick glycoprotein layer and connecting the gastrodermis to the oocytes. Indeed, a nutritional function has been widely attributed to the anthozoan trophonema (Larkman and Carter 1982; Eckelbarger et al. 2008; Lauretta et al. 2018). In contrast, a ‘contact plate’ composed of trophocytes (not forming a well-defined trophonema structure), has been mentioned for scyphozoans, such as Aurelia aurita (Adonin et al. 2012). It may be analogously compared with the trophonema, if assumed they could be also a source of nutrients for the oocytes (Eckelbarger and Larson 1988). However, Adonin et al. (2012), argued that this contact plate may signalize to the spermatozoon, emphasizing the polarization of the nucleus towards the oocyte membrane (adjacently to the contact plate) and its protein composition similar to the zona pellucida domain.
We observed that the trophonemata found in Tubastraea corals may appear distinctly large and elongated compared to other anthozoans, most likely because the glycoproteic sac encasing the oocyte forms a thicker layer, isolating the cell membrane from the gastrodermis. Tubastraea trophonemata width ranged from 12 to 64 µm, and its length was always equal to the thickness of the irregular glycoproteic sac surrounding the oocyte, from less than 10 µm to greater than 100 µm. Nevertheless, the structure does not appear to grow much as the oocyte develops. The trophonema needs to cross the granulose mucin to nourish the oocyte. Furthermore, the nutritional function of the scleractinian trophonema is here supported by the histological evidence of yolk vesicles in the ‘contact zone’. Because of the Mallory’s trichrome protocol, the trophonema stained darkly red, with occasional large granules of the same color crossing into the oocyte.
First described by Montgomery (1898) in nudibranch eggs, the nucleolinus is a membrane-free nucleolar vesicle, composed of protein and RNA (Alliegro et al. 2010). It has been related to cell division processes, being likely responsible for protein recruitment required by centrosomes (Love 1969; Alliegro et al. 2010; Alliegro et al. 2012). Among invertebrates, the nucleolinus is apparently responsible for oocyte activation in the surf clam Spisula solidissima (Alliegro et al. 2010; Alliegro et al. 2012). Although pointed out as an indicator of parthenogenetic development (see Alliegro et al. 2010), the nucleolinus has never been reported for cnidarians before. Indeed, parthenogenesis has been attributed to asexual larvae production among some scleractinian species, being suggested for Pocillopora damicornis, Tubastraea coccinea, Tubastraea diaphana, Balanophyllia europaea and Porites divaricata (Stoddart 1983; Ayre and Resing 1986; Goffredo and Telo 1998; Lord et al. 2023).
Here, nucleolini were observed in all stages of oocyte development, from early to late maturation phases. Considering previous studies focusing on the Pacific Tubastraea, it would not be unforeseen to confirm the occurrence of parthenogenesis among the introduced species in the Atlantic. Ongoing monitoring and research on the Tubastraea from the Brazilian northeastern coast have indicated a continuous oogenesis with multiple planulation events along a single year, being rare the presence of spermatic cysts (E. Neves unpubl data). Considering that parthenogenesis may be apomictic or meiotic, molecular analyses are expected to confirm the genetic status (clonal vs. aclonal) of asexually produced propagules.
A glycoprotein layer enveloping maturing oocytes, deposited between the mesoglea and the gastrodermis, has not been thus far described for any Scleractinia. The glycoproteic sac is expected to protect the oocytes by preventing autofertilization, dehydration, and mechanical damage (Levitan et al. 2015).
Nonetheless, glandular, mucus-secreting cells are abundant in scleractinians, being usually found concentrated in the external ectoderm of polyps, majorly on the oral disc. The mucus secreted by these cells is mostly composed of mucins with a vast array of functions, namely: feeding and sediment removal, a protective barrier against variations in temperature and salinity, desiccation and UV radiation, and antimicrobial activity, being involved in the immune response of corals (Berzins et al. 2021).
Another structure possibly related to the glycoprotein layer of Tubastraea is the jelly egg sac found in a few invertebrate groups, such as sea anemones and gastropods (Moiseeva et al. 2017). Levitan et al. (2015) described the composition of the jelly egg sac of the sea anemone Nematostella vectensis, and the most abundant protein was found to be mucin 5B, secreted by glandular cells embedded in the surrounding matrix. Mucin 5B is described as having a protective function against predation and microbial infections, also possibly acting as a barrier and/or attractant to spermatozoa (Levitan et al. 2015; Moiseeva et al. 2017).
Staining with the same alcian blue protocol, the structures found in Tubastraea and Nematostella look much alike, with the presence of vacuolated gastrodermal cells and the gelatinous extracellular matrix of glycoproteins (Moiseeva et al. 2017). However further studies are necessary to determine if the glycoprotein layer found in Tubastraea is specifically composed of mucin 5B. Despite the similarities of the glycoprotein layer shared by both organisms, there are inherent differences regarding their modes of reproduction, since Nematostella is a broadcast spawner that undergoes external fertilization, while Tubastraea comprises brooding corals that rely on internal fertilization (Glynn et al. 2008; de Paula et al 2014). Considering that the protective role of the jelly egg sac produced by different organisms is related to external fertilization (probably supporting egg integrity after releasing, by avoiding predation and/or dehydration, and signalizing to spermatozoon), the function of the glycoproteic sac in Tubastraea corals is uncertain. The structure becomes thicker as the oocytes grow, isolating the oocyte prior to maturation, but it is not observed in the late stages of oocyte development. We assume it was gradually reabsorbed, allowing the internal fertilization to occur. In fully developed oocytes the trophonema also receded, once nutrient transferring was no longer needed.
In the Southwestern Atlantic, dendrophylliid corals are geographically expanding, being successfully introduced to Southeastern and Northeastern Brazilian coasts, occurring in artificial and natural environments, including shipwrecks, estuaries, rocky outcrops, and coral reefs. Although studies have thus far focused on the reproduction of Tubastraea coccinea and Tubastraea tagusensis (Glynn et al. 2008; de Paula et al. 2014), important gaps on Tubastraea biological cycles remain so far unanswered and must be investigated. The three structures here described have never been reported among scleractinians, and are likely to be involved in adaptative strategies which may corroborate the dispersion and establishment of the group abroad its natural geographic range.