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Cryptic speciation in the living planktonic foraminifer Globigerinella siphonifera (d'Orbigny)
- Brian T. Huber, Jelle Bijma, Kate Darling
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- Journal:
- Paleobiology / Volume 23 / Issue 1 / Winter 1997
- Published online by Cambridge University Press:
- 08 February 2016, pp. 33-62
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Two living forms of Globigerinella siphonifera (d'Orbigny), presently identified as Type I and Type II, can easily be distinguished and collected by SCUBA divers because of differences in appearance, arrangement of the rhizopodial network, and the presence or absence of commensals. Additional biological differences are apparent from laboratory culture experiments; Type I individuals survive significantly longer than Type II under conditions of darkness and starvation and have significantly slower chamber formation rates. Stable isotopic analyses of Types I and II also reveal notable differences, with Type I consistently yielding more negative δ18O and δ13C values. Results of Mg/Ca ratio analyses indicate that Type II specimens precipitated their shells in slightly cooler (deeper) surface waters than Type I specimens. These observations and results from DNA sequencing unequivocally demonstrate that G. siphonifera Types I and II should be regarded as biological sister species.
Contrarily, biometric analysis of the empty shells reveals few significant differences between G. siphonifera Types I and II. Of all the features measured from X-ray and SEM images of serially dissected specimens, only shell porosity yields readily discernible differences, with Type I adult chambers averaging 10–20% porosity and Type II adult chambers averaging 4–7% porosity. Statistically significant differences between Type I and II populations are revealed in maximum test diameter (Type I is typically larger) and coiling (Type I is typically more evolute), but these differences do not justify species level distinction of Types I and II using traditional paleontological species concepts.
On the basis of the above evidence, and since all specimens were collected at the same location at ∼3–8 m water depth, we conclude that G. siphonifera Types I and II are living examples of cryptic speciation, whereby biological speciation has occurred in the absence of discernable change in shell morphology. However, it is not clear when or where this speciation took place. Preliminary study of deep-sea cores from the Caribbean and Pacific sides of the Isthmus of Panama reveals a predominance of specimens with Type II porosity values, with rare occurrence of specimens yielding Type I porosity values. Systematic downcore measurement of shell porosity and tightness of coiling needs to be extended back to the middle Miocene, when G. siphonifera first appeared, to determine the timing of the Type I and II morphological divergence.
Postulated mechanisms for reproductive isolation and speciation of Types I and II include alloparapatric, depth parapatric, and sympatric speciation. These models could be tested if further analysis of fossil G. siphonifera shells allows determination of the timing of speciation, the preferred depth distribution, and the history of geographic distribution of Types I and II.
A model for planktic foraminiferal shell growth
- Miguel Signes, Jelle Bijma, Christoph Hemleben, Rolf Ott
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- Journal:
- Paleobiology / Volume 19 / Issue 1 / Winter 1993
- Published online by Cambridge University Press:
- 08 February 2016, pp. 71-91
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In this paper we analyze the laws of growth that control planktic foraminiferal shell morphology. We assume that isometry is the key toward the understanding of their ontogeny. Hence, our null hypothesis is that these organisms construct isometric shells. To test this hypothesis, geometric models of their shells have been generated with a personal computer. It is demonstrated that early chambers in log-spirally coiled structures cannot follow a strict isometric arrangement. In the real world, the centers of juvenile chambers deviate from the logarithmic growth curve. Juvenile stages are generally more planispiral and contain more chambers per whorl than adult stages. These traits are shown to be essential in order to keep volumes of consecutive chambers in geometric progression. We are convinced that the neanic stage marks the constructional bridge from a juvenile set of growth parameters to an adult one. The adult stage can be strictly isometric, that is, the effective shape is constant and the increase in volume after a chamber addition is proportional to the preexisting volume of the shell.
The shell volume is related to the biomass, the ratio of outer shell surface area to shell volume is related to the respiration rate and the ratio of the total shell surface area to shell volume is related to the total calcification effort. The influence of the parameters of the model on these relationships is investigated. Only the initial radius and the rate of radius increase affect the relationships between shell volume and surface area. The other shape parameters merely provide a fine tune-up of these relationships. Size itself plays a major role during foraminiferal development.
A fixed axis coiling model for the living planktonic foraminifer Globigerinella siphonifera
- Jelle Bijma, Brian T. Huber, Christoph Hemleben
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- Journal:
- The Paleontological Society Special Publications / Volume 6 / 1992
- Published online by Cambridge University Press:
- 26 July 2017, p. 29
- Print publication:
- 1992
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Two morphotypes of Globigerinella siphonifera (Types I and II) can be clearly distinguished in their natural environment based on differences in symbiont distribution, which is dependent on the structure of the pseudopodial network. Laboratory experiments have demonstrated that the life cycle and ecological requirements of Types I and II differ considerably as well. However, qualitative observation of the empty shells reveals no significant differences between these two morphotypes. Therefore, a “fixed-axis” coiling model has been developed to simulate foraminiferal shell morphology with a computer. The model is based on the assumption that isometry is the primary rule implemented in planktonic foraminiferal development. Four parameters (rate of radius increase, number of chambers per whorl, translation rate, and relative distance from the center of any chamber to the coiling axis) and two scaling factors (initial chamber size and number of chambers) suffice to generate geometric models of the shells of these planispirally coiled organisms.
Values for the four parameters extracted from digitized SEM microphotographs of dissected specimens of G. siphonifera demonstrate significant differences between the Types I and II. These are primarily due to a different rate of radius increase and a different number of chambers per whorl. Type I has a higher rate, which in combination with its lower number of chambers per whorl results in a more lobate test and a 22% smaller adult shell size than Type II. We suggest that the smaller surface area-to-volume ratio in the Type I population can be explained by increased respiration due to higher oxygen production during symbiotic photosynthesis; TEM has demonstrated that Type II contains twice as many symbionts than Type I and each symbiont contains a higher concentration of chloroplasts.
The fixed axis model was also used to describe the ontogeny of G. siphonifera. The model shows that early chambers in log-spirally coiled structures must deviate from a strict isometric arrangement. To maintain exponential growth, the juvenile stage of Types I and II is more planispiral, more umbilicated, and contains more chambers per whorl than the adult stage. Future investigations will focus on the transformation of the shape parameters during later ontogenetic development and during cladogenesis.