Publication Date:
2015-08-05
Description:
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 S'^O and 8"C 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.
Type:
Article
,
PeerReviewed
Format:
text
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