THE ANATOMICAL RECORD 24615-29 (1996) Skin Morphology and Cytology in Marine Eels Adapted to Different Lifestyles L. FISHELSON Department of Zoology, George S. Wise Faculty of Life Sciences, Tel Aviv University, Ramat Aviv, Israel ABSTRACT Background: Moray eels (Muraenidae, Pisces) are among the largest benthic predators of littoral habitats, particularly in warm seas and coral reefs. They seek food either by olfaction or visually, moving across the pebbles and rocks. Their skin forms a strong, protective layer. This study examines the comparative morphology and cytology of the skin of moray eels adapted to such lifestyles. Methods: The studied eels were collected in the Gulf of Aqaba, Red Sea and sacrificed by an overdose of MS222. Skin selections from different body sites were dissected and fixed for light and electron microscopy. Results: The skin of moray eels (Siderea grisea, Lycoabntis nudivomer, Gymnothorax undulata, G. hepaticus, Rhirwmuraena amboensis) and the heterocongrid garden eel (Gorgasia sillneri) reveal considerable adaptation of the integument to their different lifestyles, on and within the various bottom substrata. All the eels studied featured skin comprising a multilayered, stratified epidermis and a compact, collagenous dermis, with thickness of up to 2 mm,much thicker than that observed in their free-swimming relatives. The thickness and cytology of the two skin layers differ in the various species on different body sites within the same species and also changes with age. Pronounced differences were observed in the number and type of mucusproducing cells in the epidermis. In S. grisea, the entire body is covered by a multiple layer of goblet cells, whereas in G. sillneri, sacciform cells predominate, particularly on the caudal part of the body where they form an uninterrupted layer, replacing the supporting cells that surround them. These cells are also dominant in R. amboensis. The two latter species are sand-dwelling and the copious production of slime from these cells enables the adhesion of sand granules to their burrow walls. In Gorgasia, a special morphological adaptation was also observed in its pointed tail-end where the very strong dermal collagen forms a rigid device for digging tail-first into the sand. Conclusion: The differing thicknesses and cytological developments in the skin of marine eels protect these crawling and digging creatures against abrasive interaction with their sea bottom habitat. 0 1996 Wiley-Liss, Inc. Key words: Skin Cytology, Moray eels, Comparison An animal’s integument functions as the major protective barrier to its external surroundings. This function is particularly important in fish, as the aquatic medium makes them potentially highly vulnerable to external stressors. Fish skin is highly adapted to blend with the habitat, as well as serving to signal in intraor interspecific communication. The external layers of skin are of particular importance because they enable fish to move on or between hard substrata, or to dig into soft bottoms with minimal abrasion to their body cover. Numerous studies have been published on fish integument (Pickering, 1974; Bullock, 1980; Whitear, 1986), 0 1996 WILEY-LISS, INC. but only a few of these have dealt with eels, the most diversified group of benthic fishes (Bath, 1960; Casimir and Fricke, 1971; Leonard and Summers, 1976; Tesch, 1977). As a continuation of previous studies on the behavior and biology of morays (Fishelson, 1992, 19951, the present study compares skin morphology and cytology of several marine eels, with special emphasis on functional adaptations that enable them to dwell and Received November 14,1995;accepted February 26, 1996. Address reprint requests to Prof. L. Fishelson, Department of Zoology, George S. Wise Faculty of Life Sciences, Ramat Aviv, 69978Israel. 16 L. FISHELSON TABLE 1. Comparison of skin thickness in the studied marine eels of various body lengths Species Siderea grisea Lycodontis nudivomer Gymnothorax undulatus Gymnothorax hepaticus Gorgasiu sillneri’ Rhinomuraena amboensis’ N 8 4 6 3 5 4 Total length (MM) 200-800 350-450 450-1500 220-232 580-870 620-840 Head 0.45-0.90 0.20-0.35 0.40-1.10 0.16-0.20 0.11-0.27 0.18-0.20 Skin thickness (MM) Body 1.15-1.80 0.25-0.40 0.40-2.1 0.17-0.34 0.78-1.32 0.22-0.48 Tail 0.80-1.13 0.20-0.35 0.45-1.8 0.18-0.24 0.50-0.70 0.26-0.40 ‘stationary, cryptic forms. dig in benthic substrata without incurring visible harm. same fish. Thus in S. grisea, the skin on the head is 0.45-0.90 mm thick, on the body 1.15-1.8 mm thick, and on the tail 0.80-1.3 mm (Table 1).Such differences MATERIALS AND METHODS are also seen in Gorgasia, in which the head skin is Four species of moray eels (Muraenidae) were stud- 0.11-0.27 mm thick, that of the anal area is 0.26-0.32 ied, of which three-Siderea grisea, Gymnothorax un- mm, and the tail end is 0.50-0.70 mm. In contrast, in dulatus, and Lycodontis hepaticus-are from the Gulf species of Gymnothorax, such variations are minimal of Aqaba, Red Sea, and the fourth, Rhinomuraena am- and, as in G. undulatus, skin thickness varies from boensis (obtained from fish dealers) is from the Philip- 0.17-0.40 mm in the smallest fish to 1.8-2.1 mm in the pines. In addition, Gorgasia sillneri (Heterocon- largest (Table 1). In all the eels studied, the ventral gridael-the garden eel of the Gulf of Aqaba (Fricke, side of the abdomen possessed the thickest skin, whereas the lateral side possessed the thinnest. 1970; Clark, 1980)-was also studied. The Red Sea fishes were collected using either baited Microstructure of the Epidermis hand-nets or by injecting hypersaline water into their dwelling areas, forcing them to emerge. Sizes of colThe epidermis comprises 3-14 irregular rows of Mallected specimens are given in Table 1. Following cap- pighian cells (Whitear, 19861, most of which are either ture, the fish were anesthetized and sacrificed by a oblong, rounded, or irregular in form (Fig. 1).The exlethal dose of MS222 (Sandoz), and skin samples were ternal cells form a flattened cell-layer, frequently covtaken from various sites of the body for fixation. For ered by a delicate amorphic membrane or cuticle (Fig. light microscopy the samples were fixed in Bouin’s fix- 2, see also Figs. 4, 15) (see also Bullock, 1980), which ative or a mixture of 1:l 90% ethanol and 10%formol. appears t o be secreted by mucoid cells embedded in the At 4-6 hours following fixation, the samples were epithelium. No scales were detected, but in all fish the washed and processed for embedding in paraffin and exposed membranes of the squamous cells were arsectioning. Serial 10-pm-thick sections were then pre- mored by intricate microridges (Fig. 2, see also Fig. 4). pared and stained in Hematoxylin-eosin, Azan-tri- Like other fish, the ridge-mazes on the cell surface chrome, and PAS. Following decalcification, cross sec- (Fishelson, 1984) also vary in eels: in Gymnothorax hetions of the entire body were prepared from smaller paticus, the surface cells are 15-18 pm in diameter specimens of Gorgasia sillneri. For transmission elec- with 8-14 ridges each, whereas in S.grisea, the surface tron microscopy, sections of the samples were fixed in cells are 20-30 pm in diameter and bear 14-18 ridges 2.5% glutaraldehyde, buffered with 0.1M cacodylate (see Figs. 5,6). The ridges are 0.20-0.28 p,m in height pH 7.2, postfixed with 1%osmium tetroxide, dehy- and 0.10-0.12 pm apart (see Fig. 4), and in microsecdrated, and embedded in Epon. The microsections were tion they resemble fingerlike extensions of the cell stained with lead and uranyl acetate and studied with membranes (see Fig. 4). These ridge mazes efficiently a JEOL K80 microscope. The samples for SEM were trap the mucous produced by the skin glalhds (see Fig. critical point-dried, gold-coated, and studied with a 7). JSEM 840A microscope. The structure and density of The cells below the surface layers are 6-10 pm long skin ridges and the number of secretory cells were and 3.5-4.5 pm wide. Their membranes interdigitate studied from SEM and histological sections, using mi- deeply with neighboring cells and are rich in desmocroscopic eye pieces and micrographs. somes (Figs. 1,3). The nuclei of these cells, oriented parallel to the skin surface, are 3.5-4.0 pm in size, RESULTS mostly oblong, with a dark nucleoplasm and extensive The skin of eels, as in most teleosts, is composed of chromatin along the membrane (Fig. 4); the cytoplasm two main layers: the epidermis, formed by several lay- is granulated, with numerous elongated mitochondria. ers of strongly interconnected cells, and the underlying In Siderea and Gorgasia, increased vacuole formation dermis, formed by a dense layer of collagenous fibers was observed in ageing surface cells. The openings of the glandular epithelial cells are (Fig. 1; see also Fig. 14). These layers, together with the closely attached peripheral musculosa, form the en- detectable among the surface epithelial cells (Figs. velope that surrounds the fish body, strong and elastic 8,9). The number of openings a t each site and the denin free-living eels, more delicate in the cryptic ones sity of secretory cells varied among the five studied such as Rhinomuraena amboensis and Gorgasia sill- species of fish and differs on various sites on the fish’s neri. These differences are illustrated in Table 1. Skin body (see below). The lowest number of such openings thickness in eels differs over various body parts of the were found on the frontal part of the head of Gymnotho- - Fig. 1. TEM vertical section of skin of Siderea grisea. A, nerve axons; BC, basal cells; BM, basal membrane; CD, compact collagenous dermis; NB, nucleus of basal cells; SN, sensory nerve; TC, tonofibrils of Malpighian cells; arrowhead,tonofibrillar layer; arrow, desmosomes;double arrows, interdigitating cell membranes (bar = 4 pm). 18 L. FISHELSON Figs. 2-4. Epidermis of moray eels. Fig. 2. Rhinomuraena amboensis. Fig. 3. Sidereagrisea. Fig. 4. Gymnothorax hepaticus. BN, nucleus of basal cell; D, dermis; ER, endoplasmic reticulum; IM, interdigitating cell membranes of Malpighian cells; M, mitochondria; MR, cross sections of ridges on surface cell membranes; SC, surface cells; SN, surface cell nucleus (open arrows, nuclear pores); TC, translucent cell cytoplasm; V, vacuoles; arrowheads, desmosomes; winged arrow, tight junction; thick arrowhead, basal membrane (bar = 2 Fm). COMPARATIVE SKIN CYTOLOGY IN MARINE EELS Figs. 5-7. SEM of the surface skin in morays. Fig. 5. Gymnothorax undulutus. Figs. 6 and 7. Siderea grzseu. MU, mucus on cell surfaces; OS, openings of secretory cells; arrowheads,cell limits (bar = 5 pm). 19 20 L. FISHELSON rax hepaticus (Fig. 8); G. undulatus possessed a similar number of openings on the head but many more on the body (Fig. 9). In contrast, the skin of the entire body in Siderea, R hinomuraena, and Gorgasia had dense fields of gland openings. The glandular cells in the posterior half of the body of Gorgasia were particularly prominent and disrupted the normal fingerprint pattern observed on the skin of other body parts (Figs. 10,ll). As with other fish, such a high density of gland cells in the epidermis often totally displaced or compressed the normal Malpighian cells of the skin (Figs. 12,13; see also Fig. 18). The stratum spinosum (see Groman, 1982) of the epidermis constitutes the middle layers of eosinophilic cells between which the glandular cells are situated. In the studied eels, these layers can be roughly divided into filament-containing cells (FC-cells of Leonard and Summers, 1976) and mucous-producing cells. The FCcells dominate on the skin of the anterior body (Figs. 1, 31, forming 3-5 layers on the heads of Gorgasia and Rhinomuraena and 5-14 layers on the bodies of Siderea and Gymnothorax, particularly on their abdomens. These cells are usually either irregular or rounded in shape and are 5-7 pm in diameter. Their nuclei, 3.5-4-5 pm in diameter, are pale, round, or elongated and possess a narrow, dense chromatin layer along the membrane (Fig. 1; see also Fig. 15). The cell cytoplasm is clearly divided into two zones: the perinuclear zone, pale and slightly granulated, and the peripheral zone, electron-dense and rich in tonofibrils that partly extend along the cell membranes and partly towards desmosomesjoining the deeply interdigitating neighboring cells. Higher up in the epidermis, these cells become vacuolated and flattened and there are fewer dense connections between them. As mentioned previously, these intermediate cells are totally compressed and displaced on body sites with very high concentrations of secretory cells, as demonstrated in some parts of Siderea or the tail of Gorgasia (Figs. 12,13; see also Figs. 18,191. The basal stratum (stratumgerrninatiuum), which is the most proximal layer of cuboidal or oblong cells of the epidermis, is situated on the basal membrane that separates the epidermis from the dermis (Fig. 1; see also Fig. 23). Its cells are 3.0-4.5 pm wide and 7.0-8.0 pm in length, weakly eosinophilic; their nuclei are large, with a narrow chomatin layer along the membrane. Following mitosis, these cells produce all the cell types observed in fish epidermis and as also observed in the eels, the newly produced cells move into the Malpighian layers during the process of differentiation. On reaching the upper part of the epithelium, the nuclei of these cells become granulated, with a dense, wide chromatin layer along the membrane (Figs. 1,4). The initiation and differentiation of glandular cells, marked by the accumulation of a secretory product close to their nuclei, can be recognized before these cells detach from their base. The proximal membranes of the basal cells connect via numerous hemidesmosomes to the basal membrane (Fig. 11, forming the link between the epidermal and dermal parts of the skin. Clusters of electron-dense cells are often present close to the basal membrane (see Figs. 15,161. These would appear to be phagocytic cells derived from the underlying connective tissue. Several types of sensory organelles are located within the epidermis and are especially prominent on the head of Siderea grisea (see Figs. 21,22), which exhibits lines of tastebuds that occur in groups of 4-8 among the black blotches of melanin. Between the base of the tastebuds and the collagen layer, a dense aggregation of melanocytes occurs, penetrated by sensory axons (see Figs. 22,231. Isolated buds are sparsely dispersed along the head. In Gorgasia sillneri and Gymnothorax undulatus, different types of sensory buds are also observed (see Figs. 23,24). The overall thickness of the epidermis varied among the species studied and also on different body sites (Fig. 14). For example, in Gorgasia, the epideTis was 100115 pm on the lips, 60-70 p m on the head, 70-80 pm on the body, and 90-140 pm on the tail. Such differences were also seen in Siderea and Gymnothorax. In some eels such as Gorgasia, the thickness of the epithelium is formed by only one or two layers of secretory cells (Fig. 181, whereas in other species it is formed by 3-14 tiers of Malpighian cells, mixed with mucousproducing cells (Figs. 25-28). Secretory Cells of the Epidermis Following the definitions of Bullock (1980) and Whitear (1980, 19861, and the response of epithelial cells to various staining techniques, a t least three types of secretory cells were observed in the studied eels. Goblet or mucus cells. This is the most common type of secretory cell in the skin of fish (Whitear, 1986) and also in the eels studied. In most instances they are rounded, 35-50 p m in diameter (see Figs. 25,26,33), but in Siderea and Gorgasia, pear-shape forms were also encountered (Fig. 17; see also Figs. 28,291, resembling the columnar or clevate cells of Bullock (1980). Because mucus-producing cells exist in various intermediate shapes, it is possible that the rounded ones are goblet cells, and the pear-shaped are the so-called granular cells. In both instances, as the cells ripen, they discharge their contents in the form of isolated mucous microvacuoles. In Siderea, and partly also in other species, goblet cells were observed across the entire Malpighian layer, characterized by an increasing number of centrally located vacuoles that displaced the nucleus to the cell periphery (Fig. 19; see also Fig. 30). These vacuoles were separated partly by extensions of the electron-dense perinuclear cytoplasm (see pigs. 24,30). The nuclei of these cells are elongated, with a very pronounced chromatin layer along the membrane (see Fig. 32), whereas their periphery is occupied by parallel-oriented bundles of tonofibrils and compressed cell organelles. Upon maturing and discharing their contents, these cells open widely on the skin surface. The secretion, denser than the surrounding water, is released throughout the perforated surface membrane and remains temporarily enclosed in miniature vacuoles (see Fig. 34). Particularly dense aggregations of these cells were observed in the skin of Siderea (Fig. 15) and a t the anterior end of the body of Gorgasia (Fig. 17). According to various authors, this goblet cell secretion contains acidic and neutral substances rich in mucopolysaccharides with sialic components. Club cells. The second type of secretory cell observed in the studied eels also arises from the basal cells, ma- COMPARATIVE SKIN CYTOLOGY IN MARINE EELS Figs. 8-14. SEM, TEM, and LM of eel skin. Fig. 8. Surface cells of Lycodontis nudiuomer. Fig. 9. Surface ceIls of Sidereu grisea. Fig. 10. Tail end of Gorgusia sillneri. Fig. 11. Surface cells of G . sillneri. Fig. 12. Tail cut open of G . sillneri. Fig. 13.Tail cut open of S. grisea. Fig. 21 14.Section of skin in Rhinornuraenu ( X 60). C, surface “cuticle”; D, dermis; E, epidermis; 0, opening of mucus cells; arrow, goblet cells; arrowhead, sacciform cells (bar = in C, 200 p,m; in all others, 20 km). 22 L. FISHELSON Figs. 15-20. Histology of the glandular cells. Fig. 15. Head skin of Gorgasia sillneri ( x 110). Fig. 16. Head skin of Sidereu griseu ( x 120). Fig. 17. Club cells of G. sillneri ( x 180). Fig. 18. Sacciform cells of G. sillneri ( x 180).Fig. 19. Cross section of club cells ( x 220). Fig. 20. Club cell in G. sillneri ( x 180).BC, basal cells; D, dermis; DP, dermal papilla; GC,goblet cells; MA, Malpighian cebls; MC, mucous cells; ME, melanocytes; 0, opening of sacciform cell; X, surface cells; PH, phagocytes; long arrow, membrane of club cells; arrowhead, nuclei. turing within the Malpighian layer (Fig. 20). These are round, 60-75 pm in diameter. From the beginning, they characteristically contain a small, centrally located vacuole, whose content changes during maturation from basophilic to acidophilic (red to blue in Azantrichrome). Large numbers of club cells were observed in Siderea and Rhinomuraena, especially in the skin folds on their ventral sides (Figs. 27,281. These mucusproducing club cells number 4,000-6,000per mm2 and are less prominent in Gorgasia and Gymnothorax. There appear to be several types of functional club cells (Bullock, 1980). According to Whitear (1986), they produce a protein complex that constitutes the fish “cuticle.” A special kind of club cell, 22-29 pm long and 15-17 pm wide, round or oblong in shape, was observed in the tail epithelium of Gorgasia sillneri (Figs. 33,341.The cell membrane is reinforced with bundles of tonofibrils COMPARATIVE SKIN CYTOLOGY IN MARINE EELS 23 Figs. 21-24. Sensory buds in the eels’ skin. Fig. 21. Group of taste buds on head of Sidereu griseu ( x 80). Fig. 22. Neuromast of S.griseu ( x 140) (insert, cross section x 220). Fig. 23. Neuromast of Gorgasiu sillneri ( x 180). Fig. 24. Taste buds on head of Gymnothorax hepat- icus ( x 100). A,axon; C, cupula of neuromast; D, dermis; DP, dermal papilla; E, epidermis; MC, Malpighian cells; ME, melanocytes; MU, mucous cells; N, neuromast; arrowheads, taste buds. containing vacuoles up to 2 pm in diameter and possessing dense membranes. Sacciform cells. This third type of secretory cell was observed in all five eels. These cells are highly elongated, becoming flask-shaped, 80-120 pm long, with an oblong vacuole that, in mature cells, fills up the entire space between the nucleus and cell membrane (Fig. 28). In Siderea, sacciform cells occur dispersed among club cells, whereas in Rhinomuraena, they form continuous rows in the ventral skin. Their appearance in Gorgasia is particularly impressive, as they gradu- ally increase in density along the tail, finally producing a continuous, uninterrupted layer a t the tail end (Figs. 12,18), with a density of 1,200-1,600 cells per mm’. Casimir and Fricke (1971), failing to distinguish this continuity, named the observed density at the tail end the “Schwanzdrii~e”(tail gland). As in other fish, micrographs of sacciform cells reveal that the centrally located vacuole is connected to, and apparently receives, presecretory products from surrounding microvacuoles. In all species, the lowest density of glandular cells was observed on the foremost part of the head. Figs. 25-28. Types of glandular cells in epidermis of eels. Fig. 25. Skin of Sidereu grisea ( x 240). Fig. 26. Club cells of Gymnothorax undulatus ( x 280). Fig. 27 ( x 160) and Fig. 28 ( x 300): Skin cross section of Gorgasiu sillneri. BC, basal cells; D, dermis; F, flask cells; GC,goblet cells; MC, mucous cells; ME, melanocytes; SC, secretory cell cytoplasm; SV, secretion vesicle; PH, phagocytic cells; arrow-dividing basal cell; arrowhead, elongation of a flask cell. COMPARATIVE SKIN CYTOLOGY IN MARINE EELS 25 Figs. 29-32. TEM section of glandular cells. Fig. 29. Goblet cell of Sidereu griseu. Fig. 30. Nucleus of such cell. Fig. 31. Sacciform cell of Gorgasia sillneri. Fig. 32. Nucleus of such cell. ER, endoplasmic re- ticulum; 0, opening of the cell; SG, sacciform gland cell; SV, secretory vacuole; thick arrowhead, nuclei of secretory cells; arrow, cytoplasmic extensions (bar = 2 km). Structure of the Dermis served at regular intervals, as well as blood capillaries and nerves that traverse the tissue (Figs. 37,391. In Siderea and Gorgasia, the upper parts of the dermis protrude as papillae into the epithelium (Fig. 16). The thickness of the dermis, like the epidermis, varies in different species and also varies on different parts of In all eels, the dermis, 700-850 km thick, is formed by densely compressed bundles of collagen fibers, 0.20.4 pm thick, that run mostly parallel to the skin surface (Figs. 35-40) forming two sheets that wind along the body. Perpendicular collagen fibers are also ob- 26 L. FISHELSON the same fish. In some eels, such as Rhinomuraena, the dermis is 5-6 times thicker than the epidermis (Fig. 141, whereas on the head of Siderea both layers are of equal width. In the dermis of the skin covering the gill chamber, the collagen fibers are separated into bundles lying parallel to the epidermis. This structure facilitates swelling (suction) and contracting (expelling) of the chamber, helping to move water over the gills. A specially developed dermal tissue occurs in the “usedfor digging” tail end of Gorgasia (Figs. 38,401. Here, the collagen fibers extend into sites normally occupied by muscles and form thick bundles that extend in all directions. Displacing the soft tissue, they form, together with the apical caudal vertebra, a kind of strong and efficient “tailhead for digging. In live fish this part of the tail is permanently covered by a “cuticular” sheet that extends over the glandular epithelium. This particular type of tail modification has not been observed in other species of studied eels. Subdermis This layer of nondense connective and adipose tissue also varies among the different species, as well as in the same species with respect to age. For example, in young and adult Rhinomuraena amboensis, the compact dermal tissue rests on a connective tissue heavily interspersed with fatty cells, whereas in Gorgasia sillneri, there is almost no intermediate fatty tissue between the collagenous dermis and bundles of trunk muscles. In Siderea grisea, the juvenile fish possess no fat deposits below the dermis, but as they reach 450500 pm in length, fat is deposited along the compact dermis, reaching 2.5-3.5 pm a t this site. In fish of 680-760 pm in length, this layer is 5.6-6.7 pm, extending around the body, from the gill region to the anal opening. Gymnothorax spp. revealed a pattern similar to that of Siderea. Remarks on Skin Regeneration A surprising ability to regenerate damaged skin was found in the morays, especially S. grisea. Living together in aquaria and often attacking each other, they sometimes inflict deep wounds that can even expose the vertebral column. Undisturbed, these wounds stop bleeding within a matter of minutes, and within 28-36 hours the wound is covered by a delicate epithelium. Infection was not observed on such injured areas. Within a short period of time, from a few days to 2-3 weeks, even the deepest wounds heal, often leaving no visible scars. DISCUSSION The adaptive value of dermal specializations has been widely discussed in fish literature, particularly by Bullock (1980), Roberts and Bullock (19801, Whitear et al. (19801, and Whitear (1986). Most of these studies focused on fish armored by scales or dermal plates, which serve as a protective device against abrasion and ~ Figs. 33-34. TEM section of a special form of club cell in epidermis of Gorgasiu sillneri. Fig. 33. Entire cell. Fig. 34. Partly open surface of such cell. LS, expelled mucus in microvacuoles; SV, secretory vacuoles; arrow, cytoplasmic extensions; arrowheads, parts of burst-open cell membrane (bar = 2 pm). Figs. 33-34. COMPARATIVE SKIN CYTOLOGY IN MARINE EELS Figs. 35-40. Structure of dermis in eels. Fig. 35.Dermis and muscles of Gorgmia sillneri ( x 600). Fig. 36. Dermis of Gymnothoraz hapaticus (bar = 20 pm). Fig. 37. Dermal bundles in skin of Siderea grisea ( x 260). Fig. 38. Cross section of dermis in S. grisea (bar = 4 pm). Fig. 39. Basal epithelial cells and dermal fibers of S. grisea (bar = 2 pm). Fig. 40. Longitudinal section of collagen fibers in the tail end of G. sillneri (bar = 2 pm). A, axon; BC, basal cells; BM, basal 27 membrane; C, transversal dermal collagen bundle; CB, collagen bundles parallel t~ epithelia; CS, cross section of a collagen bundle; DC, dermal compact layer; E, epithelial layer; EN, enveloping connective tissue; MA, Malpighian cells; NE, nerve; arrow, tonofibrillae in basal cells; open circles and arrowheads, cross and longitudinal sections of collagen. 28 L. FISHELSON predation. In such fish, the skin, including the stratified epidermis and underlying dermis, forms a relatively delicate cover above these sclerifications. This situation contrasts with fish possessing nonsclerified skin, such as Anguilla (Leonard and Summers, 1976; Tesch, 1977). In the morays studied here and in the related heterocongrid, G . sillneri, the increase in skin thickness compensates for a lack of armour. Thus, e.g., in the scaled species, the epidermis is 25-35 pm thick, whereas in the studied eels, this layer reaches up to 140 pm thick and is composed to 4-15 cell layers. The dermis in the eels is also very thick, formed by a continuous envelope of compact collagen fibers of 70-850 pm. These two factors would appear to be a general adaptive structure in both these and other fish, which not only live and crawl on the bottom, but also dig between the rocks and sand. The skin of such fish is constantly subjected to abrasion and stress by the substrate. The epithelial cells of the epidermis, such as FC, squamous, polygonal, or Malpighian, are highly interdigitated, attached by desmosomal junctions, and strengthened by tonofibrillae. This type of interaction produces structures that are highly resistant to sheer stresses. As the epidermal Malpighian cells approach the surface, they not only change structure but also become denser and their membranes become covered by reinforcing microridges. The second device for skin protection is the “mucus”-secreting cells distributed between the Malpighian cells. Both of these cell types develop from the basal layer of epithelial cells. Secretory cells of various types form an important constituent in the skin of fishes (Fishelson, 1972; Pickering, 1974; Bernadsky and Rosenberg, 1992). They are particularly prominent in eels that produce a copious amount of mucus (Bath, 1960; Randall et al., 1981). Several types of secretory cells were described for fishes by Mittal and Munshi (1971) and Roberts and Bullock (1980) and were also observed in the studied eels. Following Whitear (19861, these can be identified as mucous (or goblet) cells, club, and sacciform cells. The first type, mucous cells, the most common cell on the anterior parts of these eels, produces glycoproteins and is pale-orange in PAS preparation. In S. grisea, the mucous cells totally displace the Malpighian cells, forming fields of 4,000-16,000 secretory cells per mm’. Mixed with club and sacciform cells, they are especially dense along the sides and abdomen of the fish. When these eels are trapped in nets or by hand, they are able to produce 3-5 g of slime in < 3 min. When attacked by the more predatory Gymnothorax, Siderea produces a copious and emulsifying slime that generally discourages the predator. The other two cell types, club and sacciform, react positively to the PAS technique, displaying a strong, violet color. According to Mittal and Munshi (1971), these cells produce a more proteinacious secretion. A prominent goblet cell population was observed in the skin of Lepadichthys Zineatus (Fishelson, 1972). Bullock (1980) claims the secretion of the club cells is also unpalatable to predators, and Randall et al. (1981) describes it as toxic. The skin of Rhinomuraena, and especially of Gorgasia, is rich in club and sacciform cells. In Gorgasiu, club cells are particularly prominent along the ventrolateral folds of the body, whereas the sacciform glands start to extend first on the anal area and then gradually occupy the entire tail epidermis, with cells 90-120 pm long. The cells at the pointed tail-end, Gorgasia’s digging device, are especially dense and large. Gorgasia sillneri dwell in narrow tunnels in sandy sediment in which, according to Fricke (1970) and Clark (19801, the walls of the sand granules adhere by means of the mucous produced by the tail of th0 fish. These specialized functions of the fish mucus seem to be apomorphic, derived from their primary functions as protective cover and as a lubricant in water. As previously described, the surface cells of the fish epidermis are covered by microridges, often forming different patterns in various species of fish (Fishelson, 1984). Such differences were also observed in the eels. It is postulated (see also Pickering, 1974 that these ridges not only strengthen the surface membrane against abrasion, but the microridge mazes temporarily anchor the excreted slime, thus forming a dragreducing layer for swimmers (Fishelson, 1984) and providing an additional “cuticlelike” protection for the underlying epithelium for bottom dwellers. This protective cover is also well developed around the tail-end (digging device) of Gorgasiu. The eel epithelium remains alive right up to the flattened surface cells, as evidenced by the detection of DNA in these cells using DNA-specific Gallocyanin stain (see also Roberts and Bullock, 1980). The stratified epidermis rests on a compact collagenous stratum that forms a continuous envelope around the eels. Dermal thickness differs in various species of eels and on various parts of the same fish, as well as increasing with age, as shown for Anguilla restrata by Leonard and Summers (1976). A very strong and versatile serpent-form body is thus created, well adapted to the harshness of the benthic habitat. Provided with several types of cutaneous neuromasts and other sensory units, these fish are also very sensitive to surrounding stimuli. Their extraordinary ability to regenerate following deep injury (pers. obs.) completes the picture of this highly successful predator and scavenger of shallow water sea bottoms. P ACKNOWLEDGMENTS I am indebted to the Interuniversity Institute in Eilat for help and space during collections and studies. Thanks are due to Mr. A. Maroz, Mr. A. Kushnir, and Ms. R. Tannenbaum for assistance in eel collection, and Mr. N. Sharon for help in maintaining them in our aquaria. Thanks are also due to colleagues whose remarks helped to improve the manuscript. 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