This site posts digital images (light and scanning electron photomicrographs) and videos of Recent planktonic foraminiferans, such as Orbulina universa.
The morphospecies Orbulina universa is a species complex of three cryptic species that were initially recognized using comparative analysis of DNA sequences but which have subsequently been shown to differ in their biogeographic distributions, association with different water masses, and porosity and thickness of the spherical chamber of the shell (de Vargas et al., 1999; Morard et al., 2009). For the purposes of this article, it is assumed that general biological characteristics of the organism (overall test morphology, cell structure, reproduction, trophic relationships, etc.) apply to all three cryptic species; however, the author recommends that the specific epithet Orbulina universa d’Orbigny, 1826 be restricted to the “Caribbean species” of de Vargas et al. (1999) and Morard et al. (2009).
The test, or shell, of Orbulina universa is multichambered and the early chambers coil in a trochospiral; that is, during growth, chambers are added in a spiral pattern around a central cone (Spero, 1988). In the trochospirally coiled stage, all chambers are visible on the spiral side, while only those of the last-formed whorl are visible on the on the opposite, or umbilical, side. There are up to six chambers in the first whorl of the juvenile test, followed by up to two whorls with four chambers each (Spero, 1988). At the end of its life span, O. universa secretes a single, large, completely embracing spherical chamber around the early coiled portion of the test. Inside of this large spherical chamber, the early coil is held in place only by spines, with the spiral side of the coil positioned closer to the internal wall (Vilks and Wilks, 1974).
The sutures between the chambers in the coiled portion of the test are depressed and radial in orientation. The terminal spherical chamber lacks a suture with the earlier coiled stage of the test. The primary aperture in the coiled portion of the test is interiomarginal in location (that is, it occurs at the suture between the distal wall and preceding coil), and umbilical in position (it opens from the final chamber directly into the umbilicus on the umbilical side of the test). The shape of the primary aperture is a low, broad arch that is bordered by a narrow lip. Supplementary, or secondary, apertures are present above sutures of earlier chambers on the spiral side of the test in the last whorl of the coiled stage. The last chamber of the coiled stage may possess four supplementary apertures, two of which are large, and two of which are small (Vilks and Walker, 1974). The supplementary apertures are small and difficult to see with a compound microscope (Spero, 1988). The terminal spherical chamber possesses multiple circular pores, approximately 9 µm in diameter, that function as apertures (Bé et al., 1969).
The shell is made of secreted calcite and is bilamellar in construction (Schiebel and Hemleben, 2017). The test wall is 2 µm thick in the juvenile test, and up to 15-20 µm in later portions of the test (Bé et al., 1969). Like other perforate calcareous foraminiferans, smaller pores that measure approximately 4.5 µm in diameter penetrate the shell wall. Pore plates, organic membranes perforated by micropores, block the passage of pseudopodia through the pores but allow the diffusion of smaller molecules, like oxygen from the extracellular environment into the cytoplasm.
The wall texture of the test is Globigerina-like, as described by (Hemleben et al., 1991), and coarsely spinose. Round and triradiate spines arise from terraced spine collars, or bases, on the test surface in both the coiled and terminal spherical stages (Vilks and Walker, 1974; Hemleben et al., 1991). Spines are approximately 3 mm in length, and are less densely distributed on the chambers of the coiled stage than the final spherical chamber (Spero and Parker, 1985).
Live individuals surround the shell with a spherical cytoplasmic capsule that is formed from an anastomosing network of granuloreticulose pseudopodia, fine cytoplasmic strands of which are strung between the dense array of calcite spines (Bé, 1982; Bé et al., 1977). During daylight hours, a thick halo of dinoflagellate endosymbionts, 200-400 µm thick, surrounds the spherical chamber (Spero, 1987; Rink et al., 1998). At dawn, the dinoflagellate symbionts are dispersed by the pseudopodia along the surface of the spines; at dusk the algal symbionts migrate down the spines into the interior of the spherical shell and accumulate near the apertures of the coiled portion of the test, then migrate out along the spines at dawn (Spero, 1987). The intracellular dinoflagellate symbionts, Pelagodinium beii, were described as a new species by Spero (1987), and more recently classified in a new genus by Siano et al. (2010). Spero and Parker (1985) estimate the average density of symbionts to be 3,220 per foraminiferal cell (SD=1,500-6,700 symbionts). The algal endosymbionts have been shown to enhance calcification, with calcification rates are two to three times higher in light than in the dark (Lea et al., 1995).
The diameter of the adult test (without spines) ranges from 385-825 µm, with a mean diameter 610 µm (Caron et al., 1990). Test size varies with water temperature and the availability of food (Spero, 1988). Orbulina universa is an omnivorous suspension feeder that feeds on both algal and animal prey based on laboratory experiments and transmission electron microscopical studies of digestive vacuole contents (Caron et al., 1987; Caron et al., 1990; Anderson et al., 1979). Prey items are trapped in the meshwork of pseudopodial covered spines, and digested by the foraminiferan within 2-16 hours of capture (Spero and Parker, 1985). Tracer studies using the stable isotope 15N have shown that 50-57% of the foraminiferan’s nitrogen is translocated from the endosymbionts and 43-50% of the nitrogen originates from its captured diet. In nitrogen-deficient environments, 90-100% of the foraminiferan’s nitrogen is translocated from its algal endosymbionts, indicating that the primary role of the captured diet may be to provide the system with phosphorus when environmental sources of nitrogen are scarce or inaccessible (Uhle et al., 1999).
The final spherical chamber is secreted at reproductive maturity. Prior to the formation of gametes, the spines are shed, the symbionts are digested, and a gametogenic crust, or a veneer of calcite, is added to the test wall (Spero, 1988; Caron et al., 1990; Hemleben et al., 1989; Schiebel and Hemleben, 2017). This calcite crust represents between 4-30% of the final shell mass (Hamilton et al., 2008). Approximately 300,000 biflagellated gametes are released through the large circular pores of the spherical chamber (Bé and Anderson, 1976; Spero, 1988; Caron et al., 1990). Gametes possess two flagella of unequal length, approximately 5 µm and 16 µm long, respectively (Le Calvez, 1936). The gametes do not contain the dinoflagellate endosymbionts and thus the zygote must reacquire them after syngamy (Hamilton et al., 2008). Gametogenesis occurs with lunar periodicity in synchrony with the synodic lunar cycle (Schiebel et al., 2005; Hemleben et al., 1989).
Orbulina universa lives in the sunlit waters of the euphotic zone in oceanic waters that are shallower than 100 m and is globally distributed throughout the tropical to temperate zones of the world ocean (Bé, 1968; Vilks and Walker, 1974). In laboratory experiments, O. universa has been shown to exhibit temperature tolerances between 17-23 ºC (Bé, 1968). Molecular systematic studies have shown that the morphospecies O. universa is comprised of three cryptic species: a Caribbean species, a Sargasso species, and a Mediterranean species, with the Sargasso and Mediterranean species being more closely related to each other (sister groups) than to the Caribbean species (de Vargas et al., 1999; Morard et al., 2009). The Caribbean species (=O. universa) dwells in stratified and oligotrophic subtropical waters and possesses relatively large pores and a mean porosity of 12.2% (Morard et al., 2009). The Sargasso species occupies stratified and oligotrophic subtropical waters, possesses relatively smaller pores, a mean porosity of 6.3%, and a thinner shell than either the Mediterranean or Caribbean species (Morard et al., 2009). The Mediterranean species is most abundant in vertically mixed and nutrient-rich waters at low to mid latitudes, possesses relatively smaller pores, and a mean porosity of 6.0% (Morard et al., 2009).
The age of the first occurrence of Orbulina universa in the fossil record is 15.1 million years ago during the middle of the Miocene Epoch (Aze et al., 2011; Kucera and Schönfeld, 2007; Desai and Banner, 1985). Molecular clock estimates of the divergence of all three cryptic species of the Orbulina universa complex are 12.1 million years (de Vargas et al., 1999), 3 million years later than the first appearance of, indicating that the first fossil “Orbulina universa” is a stem lineage, and not part of the Orbulina crown clade.
The morphospecies Orbulina universa was originally described by d'Orbigny (1839)from Recent shore sands of Cuba. Orbulina universa is the type species of the genus Orbulina by monotypy (Loeblich and Tappan 1964, Loeblich and Tappan 1988).
The types of Orbulina universa d’Orbigny, 1839 are deposited in the Paleontology Collection of the Muséum National d’Histoire Naturelle (http://www.mnhn.fr) in Paris, France. Le Calvez (1977: p. 57)states that the lectotype was chosen from d’Orbigny’s original material by Loeblich and Tappan in an unpublished manuscript. The museum number for the lectotype is MNHN.F.FO252 (MHNH 2018a). Five paralectotypes are deposited under the museum number MNHN.F.FO263 (MHNH 2018b); three paralectotypes are deposited under the museum number MNHN.F.FO264 (MHNH 2018c).
This site lists the current taxonomy and lists synonyms for the species Orbulina universa d’Orbigny, 1839.
This site lists the current taxonomy and lists synonyms for the species Orbulina universa d’Orbigny, 1839.
