Embryonic development generally progresses according to instructions laid down in the genetic blueprint, but the timing and even details of development are quite sensitive to environmental influences. Pollutants and chemical changes in the water often result in larval abnormalities and can be used to monitor environmental quality (see von Westernhagen 1988, Weis & Weis 1989, and Longwell etal. 1992 for reviews). But even natural variation in temperature, oxygen content, salinity, light intensity, photoperiod, or carbon dioxide can affect development. Meristic traits such as numbers of fin rays, vertebrae, lateral scale rows, myotomes, and gill rakers are known to vary in relation to environmental conditions.
The pattern of variation among meristic traits is not simple. The most commonly found relationship is for fin ray, vertebral, or scale numbers to increase with decreasing temperature (e.g., herrings, minnows, Rainbow Trout, grunions, killifishes, rivulines, and darters). This inverse relationship exemplifi es a general phenomenon, termed Jordan’s Rule, which applies to latitudinal effects on meristic numbers, although the actual determinant is water temperature (Lindsey 1988). However, an opposite pattern of increased meristic values with increased temperature has been observed for fin rays in Guppies and plaice. Another common pattern is the so-called V relationship, in which fewer meristic elements develop in fish raised at an intermediate temperature, but more elements are laid down at higher and lower temperatures (e.g., vertebrae or pectoral rays in Brown Trout, Chinook Salmon, rivulines, sticklebacks, paradise fish, snakeheads, and plaice). An arched, L or A, pattern of higher numbers at intermediate temperatures has been observed for the fin rays of Brown Trout and Chinook Salmon. The actual quantitative difference between experimental groups raised at different temperatures is in the range 0.1–3% difference in number of elements per Celsius degree of temperature. For example, a 1% per degree difference in vertebral count over a 5° temperature range for a fish with 100 vertebrae would translate into a five vertebrae difference between groups.
A critical or sensitive period often occurs during em bryonic or larval development when effects on meristic characters are strongest. Vertebral counts in Brown Trout are most sensitive to temperatures at around the time of gastrulation and again as the last vertebrae are formed. Vertebral number is most sensitive to temperature before hatching in herrings and killifishes, but this sensitivity occurs later in paradise fish and plaice. Vertebrae form before fin rays, and have an earlier sensitive period (Blaxter 1969). Meristic characters may also be sensitive to events prior to fertilization, such as the temperatures at which parents are held and the age of the parents (Lindsey 1988). Causal mechanisms in these patterns are poorly understood. As an embryo develops, it differentiates via segment formation and grows via elongation. Environmental conditions may affect segment formation and elongation differently. Low temperatures may inhibit segment formation more than they inhibit elongation. Hence an embryo developing at low temperatures might be longer when differentiation occurred, causing more segments to be laid down and producing the pattern of more fin rays at lower temperatures (Barlow 1961; Blaxter 1969, 1984; Fahy 1982; Lindsey 1988; Houde 1989).