Andrzej Falniowski, Archiv Zool Stud 2018, 1: 003 DOI: 10.24966/AZS-7779/100003 HSOA Archives of Zoological Studies Review inhabit brackish water habitats, some other rivers and lakes, but vast Species Distinction and majority are stygobiont, inhabiting springs, caves and interstitial hab- itats. Nearly nothing is known about the biology and ecology of those Speciation in Hydrobioid stygobionts. Much more than 1,000 nominal species have been de- Mollusca: Caeno- scribed (Figure 1). However, the real number of species is not known, Gastropods ( in fact. Not only because of many species to be discovered in the fu- gastropoda ture, but mostly since there are no reliable criteria, how to distinguish : Truncatelloidea) a species within the group. Andrzej Falniowski* Department of Malacology, Institute of Zoology and Biomedical Research, Jagiellonian University, Poland Abstract Hydrobioids, known earlier as the family Hydrobiidae, represent a set of truncatelloidean families whose members are minute, world- wide distributed snails inhabiting mostly springs and interstitial wa- ters. More than 1,000 nominal species bear simple plesiomorphic shells, whose variability is high and overlapping between the taxa, and the soft part morphology and anatomy of the group is simplified because of miniaturization, and unified, as a result of necessary ad- aptations to the life in freshwater habitats (osmoregulation, internal fertilization and eggs rich in yolk and within the capsules). The ad- aptations arose parallel, thus represent homoplasies. All the above facts make it necessary to use molecular markers in species dis- crimination, although this should be done carefully, considering ge- netic distances calibrated at low taxonomic level. There is common Figure 1: Shells of some of the European representatives of Truncatelloidea: A believe in crucial place of isolation as a factor shaping speciation in - Ecrobia, B - Pyrgula, C-D - Dianella, E - Adrioinsulana, F - Pseudamnicola, G long-lasting completely isolated habitats. However, neither geomor- - Sadleriana, H - Radomaniola, I - Anagastina, J - Grossuana, K - Hauffenia, L - phological facts (springs are rather ephemeral habitats, there is a net Graecorientalia, M - Trichonia, N - Daphniola, O -Boleana, P - Graziana, Q - Ponto- of interstitial habitats which may be used as the ways of expansion), belgrandiella, R - Paladilhiopsis, S - Litthabitella, T - Pseudobithynia, U - Bythinella, nor the actual genetic structure of (Meta) populations of those stygo- W - Lithoglyphus, X - Emmericia, Y - Marstoniopsis, Z – Heleobia. biont gastropods (not necessarily low intrapopulation polymorphism, not low levels of gene flow) confirm such opinion. Morphological Characters in Species Distinction Introduction In the Gastropod the shell is the most striking and characteristic Hydrobioids, representing a set of gastropod truncatelloidean fami- structure. The shell remains after the death of animal, thus could be lies formerly classified as Hydrobiidae, include minute world-wide easily studied in old museum materials, or even fossil ones. However, distributed gastropods whose shells are 1-4 mm high. Some of them wide ranges of variability of the shells lacking any characteristic fea- ture drastically decrease the usefulness of the shell in species distinc- *Corresponding author: Andrzej Falniowski, Department of Malacology, Institute of Zoology and Biomedical Research, Jagiellonian University, Kra- tion [1-4]. Already, in his classical study on the profundal lymnaeids kow, Poland, Tel: +48 609 111 950; E-mail: [email protected]; of the Lake Leman, demonstrated that the shell plasticity definitely [email protected] excludes, in certain cases, species determination based on the shell: Citation: Falniowski A (2018) Species Distinction and Speciation in Hydrobioid curiously looking shells were found in the gastropods, whose anato- Gastropods (Mollusca: Caenogastropoda: Truncatelloidea). Archiv Zool Stud my - especially of reproductive organs - was typical of the two species 1: 003. inhabiting the littoral of the lake [5]. Later, anatomy of the repro- Received: November 29, 2017; Accepted: January 29, 2018; Published: Feb- ruary 12, 2018 ductive organs became a basis for all the lymnaeid taxonomy [6]. In hydrobioids, the classical papers of Muus on the Baltic Hydrobiinae Copyright: © 2018 Falniowski A. This is an open-access article distributed un- demonstrated the usefulness of soft parts: morphology of the penis der the terms of the Creative Commons Attribution License, which permits unre- stricted use, distribution, and reproduction in any medium, provided the original coupled with head pigmentation, in discrimination of species whose author and source are credited. shell variability overlapped [7,8]. Citation: Falniowski A (2018) Species Distinction and Speciation in Hydrobioid Gastropods. Archiv Zool Stud 1: 003. • Page 2 of 6 • Anatomical characters, especially of the reproductive system, be- For example, Bichain et al., reported that the species threshold value came a standard in species-level taxonomy in hydrobioids. The anat- of K2P distance in the west European Bythinella was 0.015 [36]. Sev- omy of the female reproductive system as well as the morphology of eral numerical techniques for species distinction applying the genetic the penis was considered in species descriptions and diagnoses [9- distances were introduced [46,47]. Applying mitochondrial DNA one 23]. On the other hand, there are opinions that between the closely should be careful since there are cases of introgression, although not related representatives of the same genus, differences in anatomy are often [48]. not necessarily present, and, in many cases, would not be expected [24]. The ‘lock-and-key’ mechanism, known mostly in arthropods, Morphostatic Mode of Evolution is hardly expected in molluscs, whose copulatory organs lack any In hydrobioids it is common that molecular differences are not sclerotized structures, and are more variable than it is usually noted well reflected in morphology, which confirms the morphostatic model [1,25-28]. of evolution, with numerous cryptic species within the genus [40,49]. In hydrobioids the anatomy does not offer too many characters Morphostatic evolution, as defined by Davis, is often a result of useful for taxonomy. It is simplified as a result of miniaturisation and non-adaptive radiation marked by the rapid proliferation of species without ecological differentiation [50,51]. This results in a flock of made similar because of inevitable adaptations to osmoregulation, in- species that are not differentiated morphologically (and ecologically). ternal fertilization, and embryonic development inside a capsule, all Although traditionally neglected, non-adaptive radiation seems not necessary in a freshwater habitat, but not plesiomorphic for the Gas- to be a rare phenomenon in gastropods [52,53]. Thus the criterion of tropoda. Thus, anatomical similarities are the result of parallelism, or morphological differences between molecularly distinct taxa may not even convergence, at least in many cases, and need not necessarily be useful in the case of morph static evolution. Inpractice, it is not reflect the common ancestry. In general, homoplasies are common in easy to assess the distinctness of species in more or less different yet the Mollusca [29-31]. allopatric taxa. Where there is sympatry combined with well-marked Species Concepts Applied in Hydrobioid; Introduc- differences (morphology included) species distinctness is well sup- ported. At this point, however, one should be more than careful in tion of Molecular Data case of morphology – in springs reproduction takes place throughout From among the concepts of species, I prefer the cohesive one a year, but the conditions - like amount of food (e.g., algae) varies be- proposed by Templeton, not the most often accepted biological one tween summer and winter, which often results in generations striking- [32,33]. The latter defines species as members of populations that ly different in morphology at the same spring, which mimics distinct actually or potentially interbreed in nature. This works perfectly in species [54]. clear situations, where nobody doubts in species distinctness, but is Ways of Speciation hardly applicable in more complex cases and/or if the data are incom- plete. Recent studies on DNA clearly has shown that hybridisation The crucial point in distinguishing species is our knowledge - if is not as rare as had been previously thought. On the other hand, it any - about the ways of speciation in the considered group. And at this is hard to prove hybridisation in nature, and in captivity it may be point, in hydrobioids we have much more myths and legends than real either blocked or promoted by the conditions unusual for the tested knowledge. Wide ranges of variability of the shells lacking any char- animals. This species concept is also absolutely useless in the case of acteristic feature, coupled with common belief in extremely high lev- taxa reproducing asexually (e.g., through parthenogenesis), which are els of endemism (just a distinct species in each spring/cave, or at least, still not less distinct and homogenous than the “normal” species. Ac- spring/cave complex:) resulted in enormously long list of presumed cording to the cohesion species concept, on the other hand, a species species for the Bulgarian stygobiont gastropods) [23,55,56]. There is a population, or set of populations, that has genetic or demographic