Sacculinisation

In discussing the question of directedness in evolution, The Nobel-prize winner Konrad Lorenz, one of the founders of the modern study of animal behaviour (ethology), pointed out that “from each already achieved stage of development evolution can go on in any direction whatever, blindly responding to every new selection pressure that turns up” [1]. He illustrated this by the process of retrograde evolution in certain parasites, a process for which he coined the term “sacculinisation”. The term is derived from the parasitic crustacean Sacculina, which has typical free-living crustacean larvae with complex articulated appendages like for example crayfish. After infecting crabs, it matures and all the appendages disappear. What is left is a sac-like appendage (the “Externa”) on the ventral side of the crab’s abdomen which contains the gonads, and a cancer-like growth in the interior of the crab which absorbs nutrients from the host (Fig .1). This “degenerate” evolution is common among crustaceans and certain snails. For example, another crustacean, the isopod Danalia curvata, a hyperparasite (i.e., a parasite of a parasite) of Sacculina, is also reduced to a sac-like appendage (Fig.1). Among snails, certain species have become endoparasitic in sea cucumbers; they have a worm-like appearance with no resemblance with typical snails (Fig.1).

 

Figure 1. A snail parasitic in a sea cucumber; the isopod Danalia curvata hyperparasitic on the parasitic crustacean Sacculina; and the “Externa” of the crustacean Sacculina parasitic on crabs. Note the cancer-like outgrowths of the Externa in the  tissues of the crab. Only the most posterior parts of the crab are illustrated. Original Klaus Rohde. © Klaus Rohde

The Aspidogastrea. Increase in Complexity

Many people believe that parasites are indeed always in some way degenerate. However, parasitism does not always lead to reduction in complexity, on the contrary, it may have the opposite effect, as shown by the Aspidogastrea. Extensive light- and electron-microscopic studies of several species of Aspidogastrea, in particular Lobatostoma manteri and Multicotyle purvisi have demonstrated an amazing variety of sensory receptors, some of them occurring in very large numbers.This variety of receptors is as great or greater, and their number is considerably greater than in related free-living flatworms, the turbellarians. Likewise, the nervous system of the Aspidogastrea examined shows greater complexity than that of turbellarians.

 

The Juvenile/Adult, Sensory Receptors

Two species of Aspidogastrea in particular, Multicotyle purvisi from Malayan turtles, and Lobatostoma manteri from Australian marine fish, were examined using light microscopy, as well as scanning and electron-microscopy. It is important to understand that juveniles of Aspidogastrea from the intermediate host infective to the final host differ little from adults; either stage will therefore give identical results. Rohde (1966), based on the examination under the light microscope of serial sections impregnated with silver, drew attention to the great variety of sensory receptors and their great numbers in Multicotyle purvisi [2][3][4]. Subsequently, numerous studies also using scanning and transmission electron-microscopy, confirmed this not only for Multicotyle, but for Lobatostoma as well.

Scanning electron-microscopy

This technique shows only surface receptors (Fig.2). Rohde (1973) [5] counted the surface receptors on scanning electron-micrographs of one specimen of Lobatostoma manteri supplemented by counts of another specimen impregnated with silver, and reported numbers as follows: in each anterior marginal alveolus 35, in each marginal alveolus in middle of body 50 (total in all 60 marginal alveoli 2700, in all 29 median alveoli 870); in a marginal row of papillae just dorsal to the alveoli 780; on dorsal part of body 1200, along ventral margins of ventral head lobes 1600, on anterior side of median dorsal head lobe 140, on anterior side of ventral head lobes 300, on anterior sides of lateral dorsal head lobes 300, on posterior side of dorsal head lobes 200, on posterior side of ventral head lobe 150, on neck 200. The overall total is 8440. However, it must be stressed that subsurface receptors form only a small proportion of receptors, the total number of receptors therefore is far greater. Considering this, Rohde (1989) [6] [7] estimated that a fully grown worm of this species (4 mm length, unpressed) has a total of 20000-40000 receptors, which appears to be extraordinary for a worm of such a small size.

 

 

Transmission electron-microscopy

Transmission electron-microscopy is not restricted to the surface, but can be used to examine interior structures as well.  Comparison of serial ultrathin sections has shown that juvenile/adult Lobatostoma manteri has at least 8 and possibly up to 14 types of receptors [6], distinguished by the presence or absence of a cilium and its length, by the absence or presence of ciliary rootlets and their shape, by the number of axonemal microtubules in the axoneme of the cilium, and whether they are part of a complex organ or not. Juvenile/adult Multicotyle has 7 and possibly up to 9 types. We illustrate a few major types of L. manteri in the following by single sections. However, for distinguishing different receptor types, in all cases serial sections were used. All receptors are differentiated endings of dendrites (nerve fibres), usually with ciliary structures within them. For example, the receptor illustrated in Fig. 3 has a short cilium at the end of the dendrite, which is embedded in the surface layer of the worm’s surface layer, its tegument. Typically, cilia have a 9+2 structure of the axoneme, i.e., it contains nine pairs (doublets) of microtubules in the periphery and two single microtubules in the centre (Fig.4), but there may be deviations from this pattern. Fig.5, a section through a similar receptor but through a slightly different plane, reveals structures not seen in Fig.3,  such as a basal body at the base of the cilium, and a ciliary rootlet.

 

Figure 3. Ciliated surface receptor of Lobatostoma manteri. A single cilium arises from the terminal dendritic swelling. Original Klaus Rohde. © Klaus Rohde

 

Figure 4. Cross-section through cilium of sensory receptor. Note the 9+2 structure (nine doublets of microtubules in the periphery, two single microtubules in the centre). Original Klaus Rohde. © Klaus Rohde

Figure 5. Sensory surface receptor of Lobatostoma manteri. Note the cilium with a basal body and ciliary rootlet, mitochondria and vesicles, and deep invaginations of the tegument into the dendrite. Original Klaus Rohde. © Klaus Rohde

In another receptor type, a free cilium is lacking and the ciliary rootlet is expanded forming  a disk-like structure (Fig. 6).

 

Figure 6. Receptor of Lobatostoma manteri without a free cilium, but with an expanded ciliary rootlet. Note also the sections through the dense collars around the upper part of the dendrite. Original Klaus Rohde.  © Klaus Rohde

 

Figure 7. Subsurface receptor of Lobatostoma manteri. Note the well developed ciliary rootlet. Serial sections reveal that there is no cilium extending to and through the surface. Original Klaus Rohde. © Klaus Rohde

Figure 8. Non-ciliate receptor of Lobatostoma manteri located in a deep pit, with branching ciliary rootlet. Original Klaus Rohde. © Klaus Rohde

Electron-microscopic studies of Multicotyle purvisi [8] have shown the following receptor types, in many respects similar to those of Lobatostoma, but differing in some  aspects:
1) disk-like receptor with many dense collars and a modified ciliary rootlet forming a disk;
2) non-ciliate receptor with long rootlet;
3) non-ciliate receptor with branching rootlet and dense mass of irregularly arranged microtubules;
4) non-ciliate receptor with rootlet fanning out from basal body, cross-striated in its upper and with electron-dense structures in its lower part;
5) uniciliate receptor with thick layer of cytoplasm around its axoneme;
6) receptor with short cilium, at base of deep invagination of tegument;
7) receptor with short cilium terminating in an electron-denser apical cap;
8) uniciliate receptor with long cilium.

In addition, there may be a small non-ciliate receptor with a long ciliary rootlet at the base of the thick dorsal tegument, and uniciliate receptors differing from the uniciliate receptor with a long cilium in the number of dense collars and the length of the cilium and ciliary rootlet.

 

The Juvenile/Adult, Nervous System

The nervous system of larval and adult Multicotyle purvisi was reconstructed in detail using serial sections impregnated with silver, supplemented by sections stained with various other stains, among them some specific for neurosecretion ([9][10][11] review in [4]). In most Platyhelminthes, the nervous system consists of longitudinal nerves (connectives) connected by transverse nerves (commissures);  the dorsal part of one of the most anterior commissures is often particularly well developed, forming the cerebral commissure or brain. In Multicotyle, the number of anterior connectives is much greater than in any other species of the many turbellarians that have been examined, and there are two rings of commissures, one close to the tegument, the other deeper in the tissue. The dorsal part of an interior commissure just anterior to the pharynx is very large, forming the brain (Fig. 9). More posteriorly, the nerves form a typical system of connectives and commissures (with one pair of dorsal, one pair of lateral and one pair of ventral connectives), as well as a complex pattern innervating the ventral (adhesive) disk (Fig.10).

Figure 9. Nervous system of Multicotyle purvisi in the anterior part of the body. Some nerves on the right have been cut in order to show the arrangement of the nerves in cross-section. Note the two rings of commissures, and the very large brain (cerebral commissure) and main ventral connective (red). Original Klaus Rohde. © Klaus Rohde

Figure 10. Nervous system of Multicotyle purvisi in the middle part of the body, showing a typical arrangement of connectives and commissures in the dorsal part of the body, and an intricate pattern of nerves innervating the ventral (adhesive) disk. Original Klaus Rohde. © Klaus Rohde

Interestingly, a dense network of nerve fibres (nerve plexus) innervates the intestine (Fig.11), and also the connective tissue septum separating the dorsal part of the body from the ventral disk.

 

Figure 13. Statocyst of Luriculus australiensis, as seen under the transmission electron-microscope. Note the three large calcareous bodies (statoliths) in the liquid-filled cavity. Original Klaus Rohde. © Klaus Rohde

Figure 14. Transmission electron-micrograph of the statocyst wall of Luriculus australiensis. Note the axonemes (ax) which apparently are the sensory elements in the statocyst. Original Klaus Rohde. © Klaus Rohde

 

Conclusion

The evidence presented here clearly shows that the diversity of receptor types of some Aspidogastrea is at least as great and possibly greater than in free-living flatworms; the total number of receptors is far greater, only partly explained by the larger body size of the aspidogastreans examined, and the complexity of the nervous system is greater. Thus, not sacculinisation due to parasitism, but the opposite effect is apparent. Possible reasons are discussed in the third knol on the group: The Aspidogastrea: a Parasitological Model III.

Note

All figures are originals by the author. Further details, in particular reconstructions (diagrams) of the various receptor types and the nervous system, can be found on my Tree of Life page on the Aspidogastrea.