Vertebrate cranial placodes I. Embryonic induction.

Induction and specification of cranial placodes.

A variety of development characteristics, morphological, behavioural, and experiential, contribute to the extreme vulnerability of young fishes to predation. The influence of these characteristics is complicated by the fact that the larval period is one of substantial and rapid change. Yet survival is the ultimate goal;-it is only by reaching maturity that individual fish have the opportunity to reproduce. With such high stakes it is not surprising that predator defences are of major importance during all phases of life. Developmental constraints may limit the defensive options for young fishes. Avoidance behaviours, which reduce the likelihood of encountering a predator or of being attacked by it, are particulaly evident in the youngest stages. Here size, coloration and dispersal are used to help elude the predator's attention. As fishes grow and acquire greater morphological and behavioural sophistication, there is more scope for predator evasion when avoidance fails. Older fishes are increasingly able to respond to external stimuli and can detect and react to predators or join conspecifics in common defence (schooling). Behavioural development is not simply a consequence of growth and the concomitant physical alterations of the body; it is also mediated by experience that comes through interaction with the physical and biotic environment. Predispositions to respond to experience may be a product of evolutionary history. Although mortality rates decline markedly with development and maturity, changes in size or behaviour can render fishes vulnerable to new suites of predators. Effective predator avoidance can compromise other activities, such as foraging, and individuals may be forced to reconcile conflicting demands. Developmental niche shifts that occur, for example, when certain size classes take refuge in less profitable feeding habitats, represent one such trade-off. Niche shifts may also be mediated by the influence of the programme for morphological development on sensory or behavioural capabilities. In addition to all of these developmental consderations, natural variations in environmental conditions — such as temperature, photoperiod, predator density and variety, and presence of alternative prey — represent additional challenges to predator defences during the rite of passage from birth to reproduction.

Development of predator defences in fishes

Phylogenetic studies can be undertaken with a number of aims: (1) to describe the history of traits; (2) to discern form-function relationships that may not be evident from examining the traits in a single species; (3) to generate hypotheses regarding the evolutionary mechanisms responsible for the history of the traits; and (4) to detect gaps in our knowledge of the traits (Gans 1985; Northcutt 1985a, 1986a). The following analysis involves each of these aims but is confined to the distribution and innervation of mechanoreceptive neuromasts. It does not attempt to deal with related receptors and organs or the phylogeny of the octavolateralis system as a whole, although I have discussed electroreceptors and the inner ear in previous publications (Northcutt 1980, 1986a,b).

The Phylogenetic Distribution and Innervation of Craniate Mechanoreceptive Lateral Lines

The lateral line of teleost fish is usually visible externally as a row of small pores along the trunk and the head. These pores lead into an underlying canal, the lateral line canal. According to Parker (1904) the first people to recognise the pores of the lateral line were Stenon in 1664, Lorenzini in 1678, and Rivinius in 1687. In the head of most fishes the lateral line canal has three main branches, one of which passes forwards and above the eye (supra-orbital canal), another forward and immediately below the eye (infra-orbital canal), and a third downwards and over the lower jaw (hyomandibular canal). Together with the ‘free organs’ described later in this chapter, these three head and one trunk canal with their ‘canal organs’ constitute the lateral line system of teleost fish (Figures 7.1A,B).

Role of the Lateral Line in Fish Behaviour

(A) Teleosts . . . . . . . . (B) Elasmobranchs . . . . . . (C) Cyclostomes . . . . . . . (D) Other fish groups . . . . . . (E) Ampullary and tuberous organs . . . (F) Amphibians . . . . . . . (G) Growth of cupulae . . . . . . (H) Other structures . . . . . . V. Development . . . . . . . . (A) Teleosts . . . . . . . . (B) Elasmobranchs . . . . . . (C) Amphibians . . . . . . . IV. Structdre and ultrastructure of the neuromast. .

J. H. S. Blaxter

Papers

Structure and development of the lateral line