TURSCH Non-isometric growth in Oliva APEX 12(2-3) 93-100, 20 sep 1997 Non-isometric growth and problems of species delimitation in the genus Oliva * Bernard TURSCH Laboratoire de Bio-Ecologie, Faculté des Sciences, Université Libre de Bruxelles, 50 av. F.D. Roosevelt, 1050 Brussels, Belgium KEYWORDS. Mollusca, Gastropoda, Oliva, taxonomy, non-isometric, growth ABSTRACT. The shape of the shell of some Oliva species undergoes abrupt changes dunng growth If undetected, this often overlooked phenomenon can cause serious errors in the delimitation of morphospecies Examples of non-isometnc growth are reported and discussed RESUME. La forme de la coquille de quelques espèces d'Otiva subit des changements abrupts durant la croissance La non-détection de ce phénomène, souvent néghgé, peut entraîner des erreurs sérieuses dans la délimitation des morpho-espèces Des exemples de croissance non-isométnque sont rapportés et discutés 1. INTRODUCTION characters This is very much the case m the genus Oliva, m which populations of a same species - even Ail species of Oliva —like nearly all species of manne when separated by very short geographical distances - molluscs— have been described solely on the basis of are often fully separable Sympatry should then be morphological characters (mostly features of the shell) reduced to syntopy (see TURSCH 1994, 1995) Other approaches, such as molecular genetics, might be preferable in theory but in practice the taxonomist In these conditions, what should one do with deals nearly exclusively with morphospecies, resting allopatnc populations'^ The set of local populations of upon the demonstration of morphological gaps Such a every Oliva species studied so far appears to form a demonstration is, of course, more objective if it rests morphological continuum, m which no population can upon quantitative characters be separated from all the others (for an example, see TURSCH & GREIFENEDER 1996) The solution is then to Difficulties in taxonomie interpretation may anse for test the presence of morphological gaps not between a vanety of causes For instance, species undergo populations but between sets of populations covering morphological modifications over the course of time the whole distribution ranges (TURSCH 1995) (this case, the chronospecies of the palaeontologist, b.- The discriminating characters should not be will not be considered here) The delimitation of living sex-related In the case of Oliva, no significant sexual species IS fraught with other problems Examples dimorphism could be evidenced (HUART, unpublished abound of animal species in which adults and observations) in the shells of dissected series of the juvemles, males and females and even different rather unrelated species O tignna Lamarck, 1811 populations of a same species have very different (from Madagascar), O amethystma (Roding, 1798) morphological characteristics So, it is important to and O oliva (Linnaeus, 1758) (both from the keep in mind that morphological gaps do Philippines) The only case of sexual dimorphism unambiguously demonstrate the existence of separate reported so far is a slightly differential growth for male species only if three conditions (at least) are met and female Indian specimens of O oliva and is a. The samples to be compared must be strictly restncted to very large specimens of over 39 5 mm sympatnc It is fully expected (see MAYR 1969 and (KASINATHAN, MARUTHAMUTHU & TAGORE, 1987) FUTUYMA 1986) that different populations of a same Recent work (KANTOR & TURSCH, unpublished) could species could be separated by a suitable combination of not evidence any sexual dimorphism in the population ' This IS paper 27 in the senes Studies on Olividae and Laing Island Biological Station contnbution n°345 93 AiTX 12(2-3) 9-?-l()(), 2()sep 1997 Non-isometric growth in Oliva TURSCH of o oliva (from Sisimangum Beach Hansa Bay, ratios YIX as constant for a given phenon These ratios Papua New Guinea) that will be discussed here under can therefore constitute reliable taxonomie c- Separations should rest upon unbiased samples discriminants In practice, it is often difficult to check if this obvious It is important to be aware that the samples used for requirement is met The field collector, especially establishing the regression lines of Figs 1, 2, 3 are when dealing with abundant species, is often inclined strongly biased in order to represent all size classes to collect only the most "interesting' specimens So, the frequency of sizes within the growth series is (extremes m the distribution of some attribute), very different from that of natural populations neglecting the 'uninteresting' intermediates Such biased samples are a frequent cause of taxonomie errors, because shells are generally not described by the same people who have collected them m the field (mm) Authors are therefore often unaware of the collecting 10 J" bias affecting the samples they study For instance, a H sample of Oliva miniacea (Roding, 1798) from . 0 porphyria Kwajalein Atoll, strongly biased for extremes in size, 90 *• led to the description of the small specimens as a t separate species, O berti Terzer, 1986 (see TURSCH & 70 GREIFENEDER 1996) <* There is still an additional, often overlooked risk of 50 V errors the growth pattern of many species undergoes D rather abrupt changes If this phenomenon is •• %- undetected, there is an obvious danger of splitting 30 - .<- conspecific young and old specimens into separate, f artificial "species" reflecting different, succesive 10- patterns of growth L (mm) -1 —1 \ 1— 1- The present paper draws attention to the problems 5 25 45 65 85 105|t raised by some Oliva species in which the shape of the shell IS greatly modified dunng growth It only aims at illustrating the existence of complex cases, not at their Fig. 1. Example of isometric growth 0\i\ia porphyria ngorous statistical analysis Scatter diagram of teleoconch measurements H and D vs shell size, expressed as the length of the shell, L 2. OBSERVATIONS and INTERPRETATION DN (mm) 2.1. The general case: isometric growth. - '• Massive expenmental evidence (see, amongst others, 8 •r TURSCH & GERMAIN 1985, VAN OSSELAER & TURSCH 0 sayana 1993a, VAN OSSELAER & TURSCH 1993b) has shown < that most linear measurements on Oliva shells can be approximated as being m isometnc relation The 6- relationship between any two teleoconch linear •r measurements X and Y can be generally approximated 4 • as being linear A few examples, selected amongst many others, are given here Fig 1 illustrates the f relation of D (the maximum diameter) and H (the total 2i • length) with the length of the lip, L (for a definition of • these measurements, see TURSCH & GERMAIN 1985) Fig 2 depicts the relation of DN, a measure of the H (mm) •1 ^ -|__ 1 width of the anterior notch (for definition, see VAN 70 OSSEIAER& TURSCH 1993a) with L Fig 3 shows the vanation of the fasciolar /one measurements PLI, UF Fig. 2. Example of isometric growth Oliva sayana and BW (for definitions, see VAN OSSLLAI R & Scatter diagram of anterior notch measurement DN vs TURSCH 1993b) with H shell size, expressed as the length of the shell, H It can also be seen that, in all these examples, the intercept with the Y axis is negligible, the regression Instead of reducing cxtensi\c \ariables to ratios of lines passing practically through the origin of the axes linear measurement (as in Figs 1 2. 3) to Meld This means that, in most cases, one can consider the descriptors of .shape, one could also use as internal 94 TURSCH Non-isometnc growth m 0/;va APEX 12(2-3) 93-100, 20 sep 1997 reference PNW (the number of postnuclear whorls) to like in the "normal" example of Fig 1 The same, yield descriptors of growth Due to the helico-spiral except for a greater dispersion of expenmental points growth of the shell, one cannot expect a linear (see Fig 5), goes for radial measurements (all teleoconch measurement Y to vary in a linear relation perpendicular to the shell axis), such as D, X and R with PNW As an example, specimens of a population (defined in TURSCH & GERMAIN 1985) of O rufula Duclos, 1840 from Hansa Bay show a In sharp contrast, if one plots radial against regular vanation of D and H with PNW (see Fig 4) longitudinal measurements, then the expenmental As for most Oliva shells, the observed curvature is curves exhibit strong deviations from lineanty (see rather weak Although the curve in Fig 4 has an Fig 6) An important change of shape (D and X exponential aspect, this is not a case of allometnc measurements quite suddenly increase more than growth (because one is not companng two linear expected) takes place when the shell reaches a length measurements) With the possible exception of the (H) of very roughly 18 mm (about 60% of the slight sexual dimorphism reported for very large maximum size) A diffuse transition zone between the specimens in an Indian population of O oliva (see § two growth patterns starts at shell length of about 15 lb), no case of strict allometry has been yet reported in mm Oliva shells This does not mean that the growth of Oliva shells is always isometnc In several species, the shape of the shell obviously vanes dunng growth Two different - mm cases will be reported and discussed here • 30 0 rufula • (mm) (H) • • 70 • 20 - •9- ' BW a* -• • 0 sayana 0 50 ® aaa- w a a 10 Mr a 1-^ a a a* ' a so­ PNW • ••• • ^ a • 30 35 40 45 a • a- cr^ Fig. 4. 0\\ya rufula Scatter diagram of H and D vs lo- -D cr PNW, the number of postnuclear whorls nS ,00 H (mm) ) H 1 10 30 50 70 Fig. 3. Example of isometric growth Oliva sayana X(mm) Scatter diagram of fasciolar zone measurements PLI, a UF and BW vs shell size, expressed as the length of • the shell, H 0 buelowi ^* 0- a 2.2.