Functional ontogenetic changes in Branchinecta ferox (Milne-Edwards) (Crustacea: Anostraca)

Abstract

The development of the anostracan Branchinecta ferox is described in a manner that emphasizes how functional continuity is maintained from the earliest nauplius (1) (length generally < 0.5 mm) to the large adult (length at least 45 mm). The first nauplius subsists on yolk. Feeding begins at stage 2(2). Morphological specializations of the stage 2 nauplius include the use of a posteriorly located gnathobasic spine on each mandible, a purely naupliar feature, as the sole means of sweeping food particles forward between the mandibles (8, 9). The gnathobasic spines lie across the molar faces which, even if they were sufficiently developed, are therefore debarred from participation in food handling. The self-same mandibular mechanism, based on sweeping and rolling, as persists throughout life is employed. This specialization is restricted to one instar only; at stage 3 the gnathobasic spines no longer lie across the molar surfaces, which are now functional and operate in essentially the same manner as they do throughout life, but at this stage with few of the later refinements. The gnathobasic spines, however, remain as a conspicuous feature of the naupliar mandible (10) and assist the gnathobases throughout the early post-naupliar stages. They persist until an almost complete set of functional trunk limbs has been acquired. The hitherto little-known anatomy of anostracan nauplii has been elucidated, particularly in stages 3 and 4, and is described and illustrated in detail (12-19). Particular attention is given to the complex skeleto-muscular system, whose arrangement is intimately related to locomotion and feeding. Both these activities are analysed. Locomotion is powered entirely by the antennae (20). Because the nauplius is small it inhabits a viscous medium: a low Reynolds number environment. It therefore has no momentum and in effect levers itself through the water with the antennae, whose tips, although having a wide amplitude of beat, actually swing posteriorly for only a short distance during a cycle of movement (21, 22, 24). In the early naupliar stages the return movement of the antennae actually causes the animal to move backward slightly at this phase of the cycle (20, 21). The antennal musculature is described (26). The exopodite spines of the antennae have a purely natatory function. They are not concerned with food collection as some have contended. The antennae are provided with specialized food-collecting spines, the distal masticatory spines (2, 5), which extract particles from suspension as the antennae beat with a frequency exceeding 5 cycles per second at room temperatures. These spines carry food particles to the vicinity of the labrum (20, 25). The labrum is provided with three sets of labral glands whose secretions, stored in conspicuous reservoirs (16, 18, 19), are discharged in a convenient position for entangling collected food particles. The distal masticatory spines are cleaned by setae of the mandibular palps (6) - transient naupliar features - as they leave the vicinity of the labrum (20). Stout, mobile, proximal masticatory spines, located near the base of each antenna (2, 12, 16, 17), sweep material forward to the mandibles. These spines are brush-like from the first feeding stage (nauplius 2) (4), and are bifid from stage 3 (12) until they cease to function at the time the full complement of trunk limbs becomes active. The mandibles pass food to the oesophagus. Post-naupliar development is anamorphic and very gradual. Twenty instars have been identified before the acquisition of a full complement of functional trunk limbs at stage 21 (27). Maxillules and maxillae are added to the mouthparts, and trunk limbs are gradually incorporated into the locomotory/feeding mechanism, usually at the rate of one pair per instar. Such incorporation initially supplements the naupliar mechanism which continues to function until the adult mechanism is fully developed, but different naupliar elements cease to operate at slightly different times (27). There is no simultaneous development of the first six pairs of trunk limbs as has been claimed for Artemia, and no sudden cessation of operation of the naupliar mechanism when six pairs of trunk limbs have become active. Just before the adult system takes over, an almost complete adult mechanism and a naupliar mechanism operate hand in hand. Various aspects of development from the nauplius to the adult are examined, particularly from a functional standpoint, beginning with some consideration of segmentation and early differentiation (28-31). With the assumption of the adult condition, naupliar devices either are lost, as in the case of the mandibular palps, or are transformed, as in the case of the antennae which lose their role in both food handling and locomotion. The antennary glands, a conspicuous feature of the naupliar stages (14, 17, 19), also degenerate to be replaced by the maxillary glands of the adult. Development of the posterior mouthparts (12, 16, 32-34) and of the trunk limbs (33, 35, 36) is described. All these appendages become functional before they achieve the adult condition. Concomitant changes in body form occur as size increases, and the development of other structures, such as the telson (38-41), are noted. In order both to understand functional aspects of the adult and to make intelligible the processes involved in its development, it is necessary to have an understanding of the adult skeleto-muscular system. Its thoracic elements are therefore described in detail, particularly with reference to young adults in which the muscles, being less massive than in large individuals, are less congested (42-50, 57, 61, 62, 65, 70). Although the skeleto-muscular system is complex, its general pattern, which is metamerically repeated in the thorax, can be relatively easily appreciated from illustrations. The development of the muscles is reported (29, 30, 35). The role in the adult of the hitherto...