The shape and arrangement of the teeth and multicuspid laminae of the oral disc and tongue‐like piston are described for parasitic lampreys representing the six holarctic genera (Petromyzonli‐dae) and each of the monogeneric Southern Hemisphere families (Mordaciidae and Geotriidae). Particular attention is paid to describing the divergent dentitional characters, the location of attack and the size of the oral disc and buccal glands of the blood‐feeding Petromyzon marinus and the flesh‐feeding Lampetra (Lampetra) fluviatilis and Lampetra (Lampelra) ayresii. The conclusions drawn from these comparisons are used to make suggestions regarding the feeding biology of other parasitic species of lamprey for which less comprehensive data are available. Compared with P. marinus, the flesh‐feeding L.fiuviatilis and L. fluviatils have fewer and smaller oral disc teeth between the circumoral and marginal teeth, a much wider and deeper supraoral. a far larger central cusp on the transverse lingual lamina (which in turn is convex rather than V‐shaped) and a smaller oral disc and buccal glands. It is proposed that in the two Lampetra species, the central cusp on the transverse lingual lamina and the interaction of this lamina with the supraoral are adaptations for gouging and cutting out pieces of host tissue. By contrast, the serrated edges of the lingual laminae in P. marinus are used to create a small but deep wound through which a stream of blood is then drawn. This mode of feeding is facilitated by the ability of P. marinus to remain attached for long periods at a single location on the host and to secrete a flow of anticoagulant ‘saliva’ from its relatively large buccal glands. Since the characteristics of the feeding structures in the parasitic members of the genera lchthyomyzon and Mordacia resemble more closely those of P. marinus than L. fluviutilis and L. ayresii, they would appear to be adapted primarily for the extraction of blood. On the other hand, the reverse is true of Lampetra (Lethenleron) japoniea and Geotria auslralis, indicating that these species ingest predominantly muscle tissue. Species such as Lampetra (Entosphenus) tridentata have an intermediate type of dentition and are apparently more versatile in their feeding habits. It is concluded that: (i) blood‐feeding preceded flesh‐feeding in ‘modern’ lampreys; (ii) endemic freshwater parasitic species typically ingest blood; (iii) the ability to feed on flesh developed in populations which had access to estuarine and marine hosts; and (iv) pre‐Tertiary forms resembling contemporary lehthyomyzon unicuspsis could have given rise independently to both of the divergent and specialized genera of Southern Hemisphere lampreys (Mordacia and Geolria).
The objective of the study was to identify a subset of a set of twenty environmental variables which could explain variations in the density of larval lampreys (Geotria australis) in a south-western Australian stream. Generalised linear modelling, assuming Poisson distributions for the larval counts, led to a different model for each of the four seasons, with variations in larval density being explained in each season by a combination of between five and eight environmental variables. The influence of stream region also had to be taken into account in the model for winter.Four environmental variables (substrate organic material and chlorophyll a, macrophyte roots and low-angle shading) were present in three of the four seasonal models. A further six variables (water depth, substrate depth and profile, medium-sized sands, light intensity, and the presence of an eddy) were each found useful for two models. Two variables (current velocity and substrate profile) were each retained in one model. Eight of the twenty variables were not required for any of the seasonal models. The importance of organic material, shade, eddies, current velocity substrate particle size and a sufficient depth of substrate in our models agree with the largely subjective assessments of larval lamprey habitats made in the field by many previous workers for other lamprey species in diverse geographical localities.Our finding that larval density increased with increases in organic material and unicellular algae in the substrate and with shade, contrasts with the results of a different model based on data collected in a northern European stream. These differences can be related to our use of a more rigorous and comprehensive sampling regime and a more appropriate form of statistical analysis.
Data are presented for a wide variety of different measurements and observations made on representatives of the lamprey Geotria australis caught during the first four months after they had left the sea in seven successive years. Comparisons have been made with the trends shown by similar data obtained from animals subsequently held in the laboratory for a number of months to provide information on the subsequent pattern of changes and the duration of the spawning run. In comparison with anadromous holarctic species, the gonads at the beginning of the upstream migration in July were very small. The eggs, which did not yet contain yolk platelets, measured only 190 μm in diameter and took until October of the following year to reach 1120 μm, which is comparable to the size of the mature eggs of other lampreys. This indicates that the spawning run of G. australis lasts for the exceptionally long period of 16 months, which is consistent with the time of appearance of young larvae. At the commencement of the spawning run, the mean lengths and weights of G. australis were approximately 655 mm and 250 g. Hepatosomic (c. 0·8%) and heart ratios (c. 0·18%) were significantly lower than those of other species. The relatively long length of the trunk, which was associated with a very high mean myomere number (78·8), helped to compensate for a small body depth and permitted the production of a large number of eggs (mean fecundity 57,943). The intestine underwent rapid atrophy immediately after the animal entered fresh water, even though the gonads were not developing rapidly at this time. Migratory movements in the river were most marked on nights when water levels were rising, the temperature lay between 12 and 14.5°C, rain was falling and extensive cloud cover or the dark phase of the moon was present.
The location and arrangement of the pancreatic endocrine tissue in larval and adult Geotria australis (Geotriidae) differ markedly from those exhibited by the comparable stages of Northern Hemisphere lampreys (Petromyzontidae). In larval Geotria australis, the main zones of islet proliferation are located laterally between the oesophagus and the inner edge of the two large intestinal diverticula unique to this species rather than dorsal and ventral to the oesophagus. In adult Geotria australis, the islet follicles are closely packed into a single discrete capsule which could be easily removed surgically, rather than into cranial, intermediate, and caudal cords. The differences in the adult can be related to a lack of involvement of the bile duct in islet formation during metamorphosis. While B cells were found in both larval and adult islet follicles, the PI acidophilic cells and argyrophilic cells, which appeared respectively at stages 3 and 4 in metamorphosis, were present in all adult stages.
Scanning electron micrographs of the teeth and sections and dissections of the head have been used to describe the functional interrelationships between the dentition and associated cartilages, muscles and ligaments in adults of the southern hemisphere lamprey Geotria australis. These studies, together with manipulation of the piston and oral disc in living specimens, elucidated the probable feeding mechanism in this species. The main cutting action appears to result from a scissoring movement brought about by the rapid interlocking of the three sharp and stout cusps of the transverse lingual lamina with large grooves on the rear face of the supraoral lamina. The movement of excised flesh back through the oral passage to the pharynx would be facilitated by the action of the pair of strongly cuspid longitudinal lingual laminae. The unique oral disc teeth of G. australis are apparently adapted to allow the disc to slide forward over the host and yet restrict any tendency to slip backwards.
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