54 



ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



Table 6. Occurrence of Multiple Primordia in Fish Otoliths (see 

 Text for Explanation). 



Order Mormyriformes 

 Mormyridae 



Order Salmoniformes 

 Esocidae 

 Umbridae 



Salmonidae (including Coregoninae) 

 Osmeridae 



Order Cypriniformes 

 Characidae 

 Cyprinidae 

 Catostomidae 



Order Siluriformes 

 Ictaluridae 

 Bagridae 



Order Atheriniformes 

 Exocoetidae 

 Oryziatidae 

 Cyprinodontidae 

 Belonidae 

 Anablepidae 

 Poeciliidae 

 Atherinidae 



Order Syngnathiformes 

 Gasterosteidae 

 Syngnathidae 



Order Scorpaeniformes 

 Cyclopteridae (Cyclopterinae and Liparinae) 



Order Gobiesociformes 

 Gobiesocidae 



Order Perci formes 

 Istiophoridae 

 Stichaeidae 

 Percichthyidae 



Order Pleuronectiformes 

 Pleuronectidae 



variable than seen for adults. Similarity between taxa is greatest 

 in the youngest and smallest individuals, in which the otoliths, 

 particularly the sagittae and lapilli, tend to resemble flattened 

 spheroids or hemispheres. Landmark features used in char- 

 acterizing adult otoliths such as the form of the sulcus, rostral 

 projections, cristae, colliculi, ostia etc. are initially not evident 

 or weakly developed in most fishes. Exceptions to this gener- 

 alization may prove to be useful taxonomic characters (e.g., in 

 various istiophorids, the sulcus acousticus is clearly developed 

 in larvae only 6 mm SL). Exaggerated or distinctive morpho- 

 logical features of adult otoliths of some taxa may also begin to 

 develop in the early larval stages. For example, if a species has 

 a markedly elongate sagitta, such as found in some callionymids 

 or fistulariids, then the larval otolith may show a tendency for 

 greater growth along the anterio-posterior axis. Unfortunately, 

 such early evidence for adult otolith characters is often not 

 present, particularly for the many species which show an abrupt 

 change in otolith growth patterns at the end of the larval phase. 

 Nevertheless, there are other unique or distinctive larval otolith 

 features in many taxa, and they are potentially valuable for 

 systematic studies. 



Aside from shape, there are at least two other "external" 

 otolith characters which may be used for taxonomic work; these 

 involve the relative sizes and times of formation of the different 

 otoliths; the sagittae, lapilli, and asterisci. In certain taxa, such 

 as the Ostariophysi, the sagitta is highly modified from the 

 typical teleost condition, being smaller and very elongate; and 

 the asteriscus is relatively enlarged. In clupeids, the lapillus is 

 unusually small and distinctively shaped. Differences of this sort 

 exist to a lesser degree at lower taxonomic levels and may be 

 used in larvae for distinguishing groups. The time of appearance 

 of the otoliths in development is also a variable feature offish 

 ontogeny. Many or perhaps most species have sagittae and lapilli 

 at hatching, the former usually noticeably larger even at this stage. 

 There is a general positive relationship between egg size, time 

 to hatching and state of otolith development at hatching. Fishes 

 with very large eggs and corresponding hatching size may also 

 have the asterisci present at this early stage, however for the 

 majority of fishes, these otoliths appear later, and are sometimes 

 not apparent until the end of the larval stage. The asterisci are 

 distinctive in other respects as well; all species I have looked at 

 have a poorly defined core with multiple primordia; the calcium 

 carbonate is deposited as vaterite (Lowenstam and Fitch, 1981) 

 rather than the aragonite of the sagittae and lapilli; and there 

 are qualitative differences in the appearance of growth incre- 

 ments. 



Internal structures other than the primordium and core may 

 also have direct or indirect systematic applications. It is well 

 documented that otoliths grow by the addition of layers which 

 are deposited on a diel cycle (see earlier references on larvae, 

 plus review by Pannella, 1 980; also Barkman, 1 978; Wilson and 

 Larkin, 1980; Steffensen, 1980; Victor, 1982; Victor and Broth- 

 ers, 1982). These daily growth increments are usually simple 

 bipartite structures composed of one protein-rich and one pro- 

 tein-poor calcareous layer. In certain situations (especially fast 

 growth and large otoliths) subdaily increments (formed over 

 shorter time intervals) of similar structure may also be present. 

 The timing of the production of the defining boundary of the 

 core, which also corresponds to the onset of incremental growth 

 around the core, is another "internal" character that varies be- 

 tween taxa. Some groups start incremental growth before hatch- 

 ing, others at hatching, and still others at about the time of yolk 

 absorption and the onset of exogenous feeding (Brothers et al., 

 1976; Radtke and Waiwood, 1980; Radlke and Dean, 1982; 

 Radtke, 1984). There appear to be clear taxonomic trends in 

 these characters which are also related to other trends in egg 

 size and developmental rate and pattern. 



Some Examples of Taxonomically Related 



Trends in Larval Otolith Form: 



External Morphology 



The development of the general form of the adult sagitta is a 

 gradual process in many species, whereas in others there may 

 be one or more relatively abrupt changes in growth form, par- 

 ticularly around the time of transformation from larva to ju- 

 venile. This change involves the development of "secondary 

 growth centers" which first appear externally as angular to 

 rounded protuberances on the sagitta surface (Fig. 21; internal 

 structure is discussed below). The result of the expanding growth 

 around these centers is the eventual surrounding of a discrete 

 larval otolith and the stronger development of form and surface 

 characters of the adult sagitta. In examining the otoliths of over 



