Diets and culture of lampreys

The life cycle of lampreys consists of two distinct and divergently specialized phases. During the larval or ammocoete stage, the lamprey is relatively sedentary, remaining burrowed in the silt deposits of streams and rivers from which it derives its microphagous diet. After metamorphosis, several species enter a parasitic phase in either a marine, lacustrine, or large fluviatile environment where they feed voraciously, mainly on the blood and tissue of host fishes. However, the majority of lampreys do not feed at all after they complete the larval period and reach sexual maturity, 6 to 10 months after the onset of metamorphosis. These so-called nonparasitic, or brook, lampreys are believed to have been derived from parasitic lampreys, and in many cases there is convincing morphological, cytogenetic, and biochemical evidence that pairs of ancestral and derivative species can be recognized in the contemporary fauna

Physiological adaptations of the living agnathans

The modes of life and environments of the extant agnathans (cyclostomes) are discussed in relation to their adaptations to temperature, light, oxygen and salinity. As their antitropical distribution indicates, both hagfishes and lampreys are cold water groups. Since hagfishes live in deeper waters than lampreys, they are not exposed to the marked seasonal changes in temperature and light which influence major events in the lamprey life cycle. Both groups tend to be nocturnally active, either burrowing during daylight as in the case of larval lampreys (ammocoetes) and most hagfishes, or showing cryptic behaviour as in the case of adult lampreys. Olfaction plays a major part in the location of prey, presumably aided in adult lampreys by their eyes and sensitive electrosensory system. Rates of standard oxygen consumption, ventilatory frequency and heart rate of adult lampreys increase at night. Standard oxygen consumption is relatively low in ammocoetes (as it also is in hagfishes) but increases markedly during metamorphosis into the adult lamprey. Ammocoetes and hagfishes, and to a lesser extent adult lampreys, are resistant to reduced environmental oxygen tensions. Differences in the oxygen dissociation curves of ammocoetes, adult lampreys and hagfishes can be related to differences in the characteristics of their monomeric haemoglobins and their environments and modes of life. The extraordinary tolerance of the hagfish heart to hypoxia is a reflection of a robust capacity for glycolysis, an LDH isozyme geared towards anaerobic functioning and a low work output. The hagfishes, which are restricted to marine waters, are osmoconformers. The osmolality of their blood, which is almost wholly attributable to inorganic ions, is virtually identical to that of full strength sea water (c. 1000 mOsmkg−1). By contrast, the osmolality of the blood of larval and adult lampreys when in fresh water is only 205-260 mOsm kg−1, i.e. about one quarter to one fifth of those of hagfish, and these rise only to 240-270 mOsm kg−1 in the adults of anadromous lampreys in sea water. The regulation of ions by adult lampreys is achieved by mechanisms similar to those adopted by teleosts. The implications of the contrasting ionic and osmotic physiology of the two living groups of agnathans are discussed in relation to their possible environmental history and against the background of their Carboniferous fossil representatives

Gonadogenesis and sex differentiation in the southern hemisphere lamprey, Geotria australis Gray

Gonads of larval Geotria australis destined to become ovaries can be readily identified well before the start of metamorphosis, whereas the majority of presumptive testes of large larvae and some metamorphosing individuals remain indistinguishable from the undifferentiated gonads of smaller larvae. Gonial proliferation in the female germ line, which occurs earlier and is more intense than in the male line, produces cystic gonads containing large numbers of meiotic germ cells. The growth rate of those oocytes surviving the larval cytoplasmic growth phase (25%) increases by about three times at metamorphosis. Although mean cell counts for the undifferentiated and future male testes rose slightly with increasing body length, many larvae of metamorphosing length, as well as transforming animals, possess a minute gonad with cell numbers no greater than those of small ammocoetes. Differences in the timing of gonial proliferation and the onset of oogenesis during the larval life of holarctic lampreys have been correlated with differences in the fecundity and body size of the mature adult. Geotria conforms to this trend and parallels most closely landlocked Petromyzon marinus, a form whose fecundity and body size lie between those of the largest anadromous parasitic lampreys and the small nonparasitic species. However, the pattern of gonadogenesis and sex differentiation in Geotria differs markedly from holarctic lampreys. Both male and female gonads are relatively ‘immature’ and more variable at the onset of metamorphosis. At this stage, the ovaries contain small oocytes (43·1 μm diameter), frequently with undeveloped cytoplasmic basophilia, and in many cases still possess premeiotic cysts. During metamorphosis, the future testes show no acceleration of mitotic activity and remain undifferentiated, small and variable. Geotria also exhibits a more direct type of sex differentiation by lacking the characteristic juvenile hermaphrodite phase of holarctic lampreys. Thus no evidence was found for the origin of testes through the extensive regression and atresia of gonads of the ovarian type. Indeed, meiotic gonia and oocytes were seen in only 17% of future male testes

A comparison of the metamorphosing and macrophthalmia stages of the lampreys Lampetra fluviatilis and L. planeri

Comparisons of metamorphosing and macrophthalmia stages of the closely related species, L. fluviatilis (L.) and L. Planeri (Bloch), have shown that these can be distinguished within a few weeks of the onset of metamorphosis by characteristic differences in colouration and body form. Measurements of several body intervals have disclosed differences between the macrophthalmia stages of the two species. A sharp distinction between the blunt teeth of L. planeri and the supposedly sharp teeth of L. fluviatilis has not been confirmed in these early stages, but significant differences have been found in the numbers of teeth in the anterior field of the oral disk and in the lateral and posterior marginal series. For material from several rivers, the range of length of metamorphosing and macrophthalmia stages of L. fluviatilis is83–119 mm (mean 99.3 mm). Weights varied from 0.71-2.5 g (mean 1.51 g). Regression coefficients for weight on length are much lower in the macrophthalmia of L. fluviatilis than in comparable stages of L. planeri. Total oocyte counts on macrophthalmia of L. fluviatilis gave values from8000–20,000 which are in general agreement with egg counts for adults of this species in the river Severn. Observations on the earliest metamorphosing forms have shown that it is not possible at this stage to distinguish the males of the two species by the structure of the testes. The development of the lumen in the adult foregut of L. fluviatilis has been shown to be variable and in some instances the gut does not become patent until early Spring. The possibility has also been raised that, in exceptional cases, a temporary lumen may also be present for a short period in L. planeri. Observations on the foregut and dentition, together with field data, suggest that the climax of downstream migration and the onset of parastic feeding takes place in late March or April