CRYPTIC SPECIATION IN THE LIVING PLANKTONIC FORAMINIFER GLOBIGERINELLA-SIPHONIFERA (DORBIGNY)

Two living forms of Globigerinella siphonifera (d'Orbigny), presently identified as Type I and Type II, can easily be distinguished and coll ected by SCUBA divers because of differences in appearance, arrangemen t of the rhizopodial network, and the presence or absence of commensal s. Additional biological differences are apparent from laboratory cult ure experiments; Type I individuals survive significantly longer than Type II under conditions of darkness and starvation and have significa ntly slower chamber formation rates. Stable isotopic analyses of Types I and II also reveal notable differences, with Type I consistently yi elding more negative delta(18)O and delta(13)C values. Results of Mg/C a ratio analyses indicate that Type II specimens precipitated their sh ells in slightly cooler (deeper) surface waters than Type I specimens. These observations and results from DNA sequencing unequivocally demo nstrate that G. siphonifera Types I and II should be regarded as biolo gical sister species. Contrarily, biometric analysis of the empty shel ls reveals few significant differences between G. siphonifera Types I and II. Of all the features measured from X-ray and SEM images of seri ally dissected specimens, only shell porosity yields readily discernib le differences, with Type I adult chambers averaging 10-20% porosity a nd Type II adult chambers averaging 4-7% porosity. Statistically signi ficant differences between Type I and II populations are revealed in m aximum test diameter (Type I is typically larger) and coiling (Type I is typically more evolute), but these differences do not justify speci es level distinction of Types I and II using traditional paleontologic al species concepts. On the basis of the above evidence, and since all specimens were collected at the same location at similar to 3-8 m wat er depth, we conclude that G. siphonifera Types I and II are living ex amples of cryptic speciation, whereby biological speciation has occurr ed 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 Is thmus of Panama reveals a predominance of specimens with Type II poros ity values, with rare occurrence of specimens yielding Type I porosity values. Systematic downcore measurement of shell porosity and tightne ss 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 i solation and speciation of Types I and II include alloparapatric, dept h parapatric, and sympatric speciation. These models could be tested i f further analysis of fossil G. siphonifera shells allows determinatio n of the timing of speciation, the preferred depth distribution, and t he history of geographic distribution of Types I and II.