Contributions to Zoology, 69 (1/2) (2000)S. Bell; J.E. Bron; C. Sommerville: The distribution of exocrine glands in Lepeophtheirus salmonis and Caligus elongatus (Copepoda: Caligidae)

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Results

Lepeophtheirus salmonis

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DAB was successful in highlighting gland structures in all life-stages of L. salmonis. In chalimus, preadult and adult stages there was considerable variability between specimens of the same sample with regard to their staining characteristics. Pictures of gland distribution in these stages were therefore constructed from multiple specimens. The DAB stain also picked up some other, non-glandular features. In both L. salmonis and C. elongatus retinula cells of the eye and parts of the cerebrum often stained moderately with DAB, as did some of the body musculature, particularly the attachment points of the muscles to the cuticle. In specimens with damaged integuments it was common to see strong DAB-positive staining around the tears and cuts in the cuticle. It is not possible to say whether such staining is due to DAB-positive components in the exposed cuticle or whether this represents an active mechanism where DAB-positive products are aggregating at sites of injury.

Nauplii

Both the first (NI) and second (NII) nauplius larvae displayed the same general pattern of DAB-positive staining, although some slight differences were evident. In addition to two large dorsal and ventral ‘median’ glands, both larval stages consistently displayed three pairs of distinct glandular regions arranged in a bilaterally symmetrical pattern (Fig. 1). These gland populations have been named in accordance with their location within the animal.

FIG2

Fig. 1. DAB-stained second stage nauplius of L. salmonis showing glands and associated ducts e.g. the posterior gland ducts (arrowed). Note the anterior glands are composed of three connected regions. m median glands; ag anterior glands; l lateral glands; th thoracic limb glands precursor; p posterior glands. Scale bar 250 µm.

In addition to these glands, late NII larvae possessed up to 10 small DAB-positive areas, arranged in a symmetrical pattern, in the posterior half of the body between the lateral and posterior glands, position ‘Th’ in Fig. 1. These areas corresponded to the position of glands present in the thoracic limbs of the following copepodid stage. The posterior glands exited via two small pores, one lying close to the ventral side of each balancer.

During the staining process some individual, unhatched eggs of L. salmonis were inadvertently stained along with the hatched nauplius stages. Examination of these eggs showed that pre-hatch nauplii had the same pattern of DAB-positive gland distribution as the NI stage larvae.

Copepodid

Copepodid larvae displayed a similar pattern of DAB-positive glands to that of the preceding naupliar stages (Fig. 2). The posterior, lateral and anterior groups and the dorsal and ventral median glands all showed little apparent difference. In addition, there were 10 distinct glands in the thoracic limbs, four in the first pair of thoracic legs (thI) and six in the second (thII), the precursors of which could be seen under the ventral cuticle of late NII larvae. Some smaller, less distinct regions of staining were also seen i.e. 5 spots around the eyes and 3 spots on the free thoracic segment.

FIG2

Fig. 2. Dorsal view of DAB-stained copepodid larva of L. salmonis. Note granular DAB secretion adhering to cuticle of antennules (x). m median glands; ag anterior glands; l lateral glands; th thoracic limb glands; p perianal glands. Scale bar 250 µm.

The median glands overlay each other, both directly overlying the mouth-tube and were seen to have several ducts associated with them. Although it was difficult to distinguish the precise relationship between these glands and their ducts, some relationships could be visualised, as shown in Fig. 3. The ventral median gland was located at the base of the labrum and had a duct which passed into the labrum to exit on its anterior face approximately one third of the way down from the proximal end. A second DAB-positive duct appeared to lead from one of the median glands and passed in an anterodorsal direction to enter the developing filament. The duct stained clearly and extended to the distal tip of the undeveloped filament. The dorsal median gland had at least one duct arising from it which led to a pore is the dorsal cuticle immediately overlying the gland.

FIG2

Fig. 3. Lateral view of anterior cephalothorax of DAB-stained copepodid larva of L. salmonis. Large regions of dispersed stained granules can be seen within the cephalothorax (x). ag anterior glands ; l lateral glands; mxp maxillipeds; mt mouth-tube; vm ventral median gland; a antenna; dm dorsal median gland; fd filament duct. Scale bar 100 µm.

As the copepodid larvae have a functional gut, in contrast to the naupliar stage, it seems reasonable to re-name the posterior glands of the nauplius the ‘perianal glands’ at the copepodid stage. This fits the terminology of (Boxshall, 1982).

Chalimus

All stages of chalimus larvae appeared to possess all the DAB-positive glands located in the preceding larval stages, but during development from chalimus I to chalimus IV the distribution of glands changes. Chalimus larvae often exhibited variable staining which hindered the interpretation of a stage-specific pattern. The simplest pattern of gland distribution was found in chalimus I and II larvae: these still possessed the perianal, lateral and anterior glands of the previous stages, although the thoracic limb glands had become very indistinct. The anterior glands appeared to be composed of one/two components in chalimus I and chalimus II, as in Fig. 4a, whereas it appeared to consist of two/three components in the nauplius stage (see Fig. 1). This change may represent either a regression, or amalgamation, of the gland units. Other regions of staining, not apparent in the copepodid stage had developed in the chalimus larvae. A prominent feature of stained specimens was a complex of approximately six separate elements lying close to the cuticle of the dorsal surface immediately anterior to the eyes. This was termed the ‘frontal gland complex’ (FGC). This complex appeared to consist of some of the elements of the filament-producing glands described by Bron, Sommerville, Jones & Rae (1991). In particular, the A-glands described by Bron et al. (1991) stain strongly with DAB, as did the three mucous glands which lie immediately in front of the eyes and extend anteriorly towards the A-glands. Other, smaller, associated structures also stained but their origin and significance could not be determined, although it is possible that they are part of the filament-producing complex.

FIG2

Fig. 4a. Ventral view of DAB-stained second stage chalimus larva of L. salmonis. Ducts are visible leading anteriorly from the anterior glands (arrowed). f existing filament; df developing filament; l antennule; ag anterior glands; fcr first cuticular ridge; cg circum-oral glands. Scale bar 200 µm.

Two pairs of symmetrical glands (or one pair of bi-lobed glands) surround the base of the mouth-tube on the ventral surface and were termed ‘circum-oral glands’. These glands are evident in Figs. 4a and 4b. It is possible that these are the ventral reaches of the mucous glands of the FGC.

A similar pattern was seen in all three later chalimus larvae, but with a few, small differences. By chalimus III a pair of large glands close to the bases of the maxillae was apparent under the ventral cuticle. From each of these glands a fine duct ran in a medioventral direction and was presumed to exit close to the bases of the maxillules and maxillae. Although no exit pore could be detected in this region, there was evidence that a pore did lie in this area and that it was probably obscured by overlying appendages. This evidence was the presence of considerable quantities of DAB-positive material of a granular nature adhering to the cuticle close to the bases of the maxillules where the stained duct was seen to lead. These glands are here termed the ‘post-oral glands’ until their homology with previously described glands has been determined. The possibilty that these glands correspond to the widely described maxillary glands found in other species of copepod and attributed with an excretory function, should be considered.

In some specimens examined there was a large number of discrete DAB-positive granules/vesicles present within the body cavity. These regions are shown in Fig. 4b. These regions correspond to the cells of the mid-gut epithelium.

FIG2

Fig. 4b. Ventral view of DAB-stained second chalimus stage larva of L. salmonis. ag anterior glands; df developing filament; cg circum-oral glands; fcr first cuticular ridge; v vesicles lining gut. Scale bar 350 µm.

What were presumed to be the vestiges of the lateral glands could be discerned in the lateral regions of the cephalothorax, roughly level with the maxillipeds. These glands were very faint, consisting of loosley-packed grains. An anteriorly leading stained duct was associated with each of these glands.

In chalimus II some small, discrete regions of staining first appeared, lying close to the dorsal cuticle of the cephalothorax. There was an increase in their number during each successive chalimus stage, but no definite pattern characteristic of any one stage could be determined. There was no evidence to suggest that these regions of staining were glands per se as no secreted material was seen on the surface of the cuticle, and they shall therefore be referred to only as ‘dorsal surface regions’ (DSR).

Preadults

The gland distribution pattern of the preadult stages develops from that of the chalimus IV and showed an increase in complexity. There was also an increase in the number of DAB-positive glands from preadult I to preadult II in both sexes.

All of the main gland structures located in the chalimus larvae were also found in the preadult stages i.e. perianal, frontal gland complex, circum-oral, postoral and dorsal surface regions, although no evidence of the lateral glands could be found.

In the preadult stage there were two large groups of glands (or lobes of the same gland) present in the anterolateral regions of the cephalothorax. These groups, which were visible from both the dorsal and ventral surfaces, showed an increase in number of regions from preadult I to preadult II in both males and females. These gland complexes were given the name ‘anterior gland complexes’ (AGC) on account of the similarity of their location to the glands of the same name in earlier stages.

The postoral glands were again prominent and were situated close to the AGC in a region termed the ‘lateral sinus’. When viewed from the ventral surface this lateral sinus was located immediately lateral to the maxillules and posterior to the post antennary processes. The posterior margin of this sinus was bounded by the ‘first cuticular ridge’ – a ridge of thickened cuticle extending posterolaterally from close to the mouth-tube to the edge of the cephalothorax approximately half way along its length.

Circum-oral glands were present but their appearance differed from those in the chalimus stages: a large gland was evident at the base of the labrum with two further glands lying immediately anterolateral to it, within the body cavity. These glands appeared to connect to the gland at the base of the labrum.

In common with the chalimus stages, the thoracic limbs showed very little staining. However, in contrast to the chalimus stages, the DSR were prolific and accounted for the bulk of DAB-positive staining found in preadults: »85 in preadult I females, »140 in preadult II females, »56 in preadult I males and »63 in preadult II males. They were distributed in a bilaterally symmetrical pattern across the cephalothorax, and were especially concentrated around the margins. There were similar regions of staining on the dorsal and ventral surfaces of the genital segment. No secreted material was ever seen in association with any of these regions although quantities of DAB-positive material were commonly found on the setae of the caudal rami (particularly in preadult males) and was presumed to have originated in the perianal glands.

Adults

The greatest number of DAB-positive areas were found in adult specimens. The pattern of distribution was similar to that of the preadults but with some slight differences. The bulk of the staining could be accounted for by the DSR (Fig. 5).

FIG2

Fig. 5. Dorsal view of DAB-stained adult female L. salmonis. Note the DAB-positive material adhering to the marginal membrane of the cephalothorax (arrowed) and the regions of staining associated with muscle attachments (x). p perianal glands; dsr dorsal surface regions; agc anterior gland complex; fgc frontal gland complex. Scale bar 1mm.

Adult specimens retained all of the main glandular complexes described in the preceding stages. They had well developed perianal glands which consisted of one large region with up to six pairs of smaller associated regions. The secretions from these glands were often seen to coat the setae of the caudal rami. The circum-oral glands take the same form as those seen in the preadult stages, as did the frontal gland complex. The anterior gland complexes were large and composed of many elements, and together with the postoral glands filled the lateral sinus. The post-oral glands in adult stages appeared to have the same structural morphology seen in previous stages.

In adults DAB-positive glands become prominent in the first four pairs of thoracic legs, the setae of which were often seen to be covered in granular stained material. The staining of these glands was heavy and consistent between specimens.

The DSR at this stage, numbered at least 140 in females and 120 in males. The DSR were the most prominent feature of the stained animals. There was an obvious bilateral symmetry to the distribution of these areas, the greatest number of which were located around the margins of the cephalothorax. There appeared to be a sexual dimorphism with regard to DSR distribution, which was especially pronounced in the genital segment.

The marginal membrane often had DAB-positive material adhering to it, although it was not clear whether this was secreted material, stained epibionts adhering to it, or regions of damaged integument.

Caligus elongatus

The nauplius and copepodid larvae of C. elongatus showed the same pattern of DAB-positive gland distribution found in the corresponding stages of L. salmonis.

Both adult male and female C. elongatus possessed all of the main DAB-positive regions seen in L. salmonis adults. Some differences were noted i.e. the ‘swellings’ (Boxshall, 1974) on the ventral cuticle between thI and thII always stained heavily with DAB and a slight structural difference of the frontal gland complex was noted.

Specimens of C. elongatus had considerably more dorsal surface regions than their L. salmonis counterparts with adult males having »280 distinct regions while females had »160 (Fig. 6). A sexual dimorphism of these regions was evident, at least in the genital segment. These regions again showed a bilateral symmetry in their distribution, with the greatest proportion found around the extreme margins of the cephalothorax, close to the point of marginal membrane attachment.

FIG2

Fig. 6. Dorsal view of DAB-stained adult female C. elongatus. Note stained muscle attachment points on dorsal cuticle (x). dsr dorsal surface regions; agc anterior gland complex; fgc frontal gland complex. Scale bar 500 µm.

Other species

None of the other species of aquatic arthropod stained using the technique employed for L. salmonis and C. elongatus showed comparable positive staining. A. tonsa, E. affinis and D. magna all had a pair of granular DAB-positive regions present in the buccal cavity region. Both A. tonsa and E. affinis often showed a region of staining within the gut, but whether this represented secretions from the buccal cavity glands or food material could not be determined.

The egg strings of female E. affinis were generally the only other regions of the body to stain with DAB. Specimens of A. salina, and M. rosenbergii showed no definite staining, although a slight darkening of the general body tissues was often seen.