Sea Spiders (Pycnogonida) – Center for Invertebrate Biology

All About Sea Spiders (Pycnogonida)

INTRODUCTION

Pycnogonids (Phylum Arthropoda, Subphylum Chelicerata, Class Arachnida, Order Pycnogonida) (Bain, in press) or sea spiders as they are commonly known, are marine chelicerates ranging in size from less than 1 mm to over 50 cm in leg span. They are quite common in many different marine habitats, especially the intertidal zone, but are seldom seen because most of them tend to be small (less than 2.5 cm) and are often cryptically colored. The larger and more conspicuous pycnogonids, especially members of the genus Colossendeis, live in deeper waters worldwide and are common near shore in the Antarctic.
Some species live on hydroids

Pycnogonid Morphology
Intertidal sea spider from Australia

Pycnogonids have a very thin body, four pairs of walking legs (five or six pairs in some genera), a pair of chelicerae, a pair of pedipalps, and a pair of ovigerous legs or ovigers. Ovigerous legs, which are located between the pedipalps and the first pair of walking legs, are unique to pycnogonids. These appendages have a variety of functions including: grooming by both sexes (Cole, 1909; Hedgpeth, 1947; King, 1973; Brusca and Brusca, 1990; Bain, 1992; Ruppert and Barnes, 1994); food handling (Hoek, 1881; Cole, 1901b; B. Bain, personal observation); courtship, mating, and egg transfer from the female to the male (Cole, 1901a; Cole, 1901b; Cole, 1906; Hooper, 1980; Nakamura and Sekiguchi, 1980); and transport of eggs and larvae by the males (Hedgpeth, 1947; King, 1973; Brusca and Brusca, 1990; Bain, 1991; Bain, 1992; Ruppert and Barnes, 1994). In the more primitive pycnogonids, the last four segments of the ovigerous leg have one or more rows of compound spines. The shape and arrangement of these spines are species specific (Bain, 1992) and are very helpful in separating closely related species. A handful of pycnogonid species have either one (decapodous) or two (dodecapodous) extra pairs of walking legs. Hedgpeth (1947) attributes these extra legs to a polymorphism, but a more likely explanation is that they are just duplicated segments since many of the decapodous and dodecapodous species are otherwise very similar to octopodous forms found in the same area.

Integumentary System

The cuticle of pycnogonids adheres to the general structural design of the integument in other primitive arthropods. However, some striking differences become immediately evident which have not been previously observed. Since museum specimens provided good cuticle samples that could be rehydrated, fixed for electron microscopy and then processed, I solicited the cooperation of a number of investigators in the field, to wit, Drs. J. W. Hedgpeth, C. V. A. Child, R. Olson, and J. Staude, to provide me with preserved samples of all families of pycnogonids. Cuticles from 20 species in 9 families were examined by scanning and transmission electron microscopy. This material and methodology has been described previously (Fahrenbach, 1994), an article that can be consulted for further detail.

Cuticle thickness ranges from 0.5 µm in the hindgut, 1-3 µm in the foregut, 30-50 µm in moderate-sized animals to a maximum of about 250 µm in Colossendeis robusta. It is composed of a thin epicuticle, several microns of exocuticle, a conspicuously layered and cross-banded endocuticle and the underlying complex-appearing epidermis.

In suitably and fortuitously stained specimens, the epicuticle consists of several thin strata, decorated with tubercles or ridges and sometimes covered with an evidently secreted layer that appears to protect the pycnogonid against epiphytic organisms (Figs. 1, 2).

The thin exocuticle, largely featureless in most preparations, displays typical swirling filaments in originally alcohol preserved specimens that obviously underwent some extraction (Fig. 2).

The substantive endocuticle is composed of several dozen laminae, each with the typical lamellar arrangement of filaments, which rotate in orientation progressively from layer to layer and display a conspicuous crossbanded pattern (Figs. 3, 4). The primary 21 nm periodicity can be resolved into a 12 nm light band and a 9 nm dark band (Fig. 5). This periodicity is displayed identically in the pure chitin pen of the squid Loligo vulgaris (Hunt and El Sherief, 1990), in all pycnogonids examined as well as untanned cuticles of many arthropods, especially lower crustaceans (Rieder, 1972a,b; Rieder et al., 1984). Where it has been found in higher crustaceans, chelicerates and insects, it is associated with thin, permeable cuticles found in respiratory or excretory surfaces. The universal lack of tanning among the pycnogonids allows them to increase substantially in size between molts. Flexibility of the cuticle at the site of arthrodial membranes is indicated by disappearance of the quasi-crystalline chitin structure, which imparts rigidity thereby.

The epidermis is basically a tall, columnar epithelium, but the cells are elaborately interfolded with each other and thereby suggest a many-layered structure (Fig. 14). Apically the cells abut the cuticle with stubby microvilli and a cleft of varying width, presumably indicative of the initial stages of ecdysis.

The cuticle is superficially decorated with spines and stout setae (Fig. 8), sensory setae with blunt termini, sensilla with one to six fine setae to each unit (Figs. 9,10), to complex, branched spiny projections (in Nymphopsis spinosissima).

Pycnogonids are unusual in that they have a diversity of cuticular glands. Copious pore canals are not readily visualized, but appear to receive their secretions from normal epidermal cells and deposit it on the surface as the epicuticle or one of its components (Fig. 6). Other glands are simple acinar glands well below the epidermis, connected to the surface by simple cuticular ducts, as in the salivary glands (to be described in comjunction with the proboscis). Other cuticular glands discharge through presumptive cuticle-derived shafts that each carry a number of ducts with complex cross-sections (Fig. 7). These ducts open preferentially through arthrodial membranes.

Finally, there is an enormous number of special mucus glands with a valvular opening, up to 1,400+ per mm²(Fig. 11,12,) Depending on their size, they can be contained within the cuticle or spread out below it. The component cells resemble pancreatic acinar cells, i.e. they are filled with endoplasmic reticulum and large Golgi cisternae, indicative of a proteinaceous secretion rather than mucus composed of polysaccharides (Fig. 13). The cells open into a small lumen which communicates with the surface through a bivalved cuticular opening. The glands are commonly supplied with autonomic innervation, fibers containing large dense granules making occasional synaptoid contacts upon the gland cells (Fig. 13).

References

[N.B. A complete list of references to pycnogonid morphology, behavior and ecology, is contained separately in this website.]

Fahrenbach, W.H. (1994) Microscopic anatomy of Pycnogonida: I. Cuticle, epidermis, and muscle. J. Morphol. 222: 33-48.

Hunt, S., and A. El Sherief (1990) A periodic structure in the pen chitin of the squid, Loligo vulgaris. Tiss. Cell 22a: 191-197.

Rieder, N. (1972a) Ultrastruktur der Carapaxcuticula von Triops cancriformis Bosc. (Notostraca, Crustacea). Z. Naturforsch. 27b: 578-579.

Rieder, N. (1972b) Ultrastruktur und Polysaccharidanteile der Cuticula von Triops cancriformis Bosc. (Notostraca, Crustacea). Z. Morphol. Tiere 7: 361-380.

Rieder, N., P. Abaffy, A. Hauf, M. Lindel, and H. Weishäupl (1984) Funktionsmorphologische Untersuchungen an den Conchostraceen Lepestheria dahalacensis and Limnadia lenticularis (Crustacea, Phyllopoda, Conchostraca) Zool. Beitr. N. F. 28: 417-444.

The material found on this webpage is copyrighted © by W. Henner Fahrenbach.
Use of this material for private, academic or professional purposes is granted as long as the author is acknowledged and this web page is cited.

Excretory System

The pycnogonid literature, consisting of about 1,000 articles in 14 languages over the past century, contains only minimal references to an excretory system or states that these animals lack one altogether. However, Meisenheimer (1902), Dogiel (1911), and Loman (1917) found a suggestive gland in the first larval appendage of Ammothea echinata and Nymphon strömii, to which they did not attribute any function other than a secretory one. Thompson (1909) also observed what he described as a nephrocyte in the same location in larval pycnogonids.

In the process of sectioning the anterior end of an adult specimen of Nymphopsis spinosissima (Ammotheidae), I found a conventional, though primitive excretory organ. It occupies the length of the scape of the chelifore and consists of an end sac, a straight proximal tubule, a short distal tubule, and a raised nephropore. The existence of this organ has been briefly mentioned in a prior publication (Fahrenbach, 1994).

The end sac, as seen in near-cross section of the scape (Fig. 1), is a thin-walled and polygonal chamber, about 150 µm in cross section, suspended in the hemocoel of the appendage, its edges radially guyed to the cuticle at half a dozen or more locations. This wall consists of an unadorned, in parts fibrous basement membrane, 1-4 µm thick, facing the hemocoel, and internally of a continuous carpet of podocytes and their pedicels (Figs. 2, 3). The podocytes, measuring maximally 10 x 15 µm, have a rich and varied cytoplasmic content: a few large mitochondria; large, dense and presumably proteinaceous inclusions that are obvious with the light microscope; a substantial Golgi system; and a labyrinthine system of connected intracellular channels that open into a central vacuole that often rivals the nucleus in size (Fig. 4).

Where they touch, podocytes are connected by adherent junctions. Substantial portions of the podocytes touch the basal lamina (Fig. 2), but they generally stand off from this surface and cover it with an uninterrupted layer of pedicels. These crowd together, often overlapping each other and forming a covering more than one pedicel deep. The gap between adjacent pedicels is crossed by an ultrafiltration slit membrane (Fig. 5).

The end sac fuses with the surface of the proximal tubule over a circle of about 100 µm in diameter (Figs. 6). In the center of this contact zone (Fig. 7), the two constituent epithelia lose their specializations and form a valve between them. The cells of the proximal tubule, a simple 150 µm long sac (Fig. 8), have not been characterized with the electron microscope, but they have a deep and irregular brush border and a thick epithelium of complex ap pearance.

The proximal tubule opens through a constriction, but no apparent valve, into the short (35 µm), cuticle lined distal tubule, actually a small chamber (Fig. 9). The opening of the nephropore has a raised cuticular rim projecting above the surrounding cuticle (Fig. 10). Musculature in the scape is largely restricted to the two ends and does not impinge on the excretory system. Only a few minor muscle cells are located within the region illustrated here.

Comments.

The end sac is bowed inward under the substantial blood pressure that the pycnogonids are capable of developing by virtue of their vigorous heart beat (90-180 beats/min; Tjønneland et al., 1985). Although the proximal tubule was not imaged with the electron microscope, its structure suggests close similarities with the same organ in, for example, lower crustaceans.

This excretory system has astounding similarity to that of primitive crustaceans, specifically to the maxillary gland of Hutchinsoniella macracantha (Hessler and Elofsson, 1991). Without delving into appendage homologies (Hedgpeth, 1954), it is tempting to compare the pycnogonid excretory system to the yet more primitive antennary gland of immature crustaceans, especially in view of its location in the first, preoral appendage. Conversely, the coxal glands of merostomates (Fahrenbach, 1999), scorpions (Farley, 1999), spiders (Felgenhauer, 1999), ticks (Coons and Alberti, 1999), and mites (Alberti and Coons, 1999) have substantial similarities with this gland, both with regard to position near the base of the first leg as well as ultrastructure. Thus, this gland, as seen in pycnogonids, can be considered representative of an ancestral arthropod excretory organ.

It remains to be seen where this gland is to be found in the numerous achelate species of pycnogonids.

References

Alberti, G., and L.B. Coons (1999) Acari: Mites. In: Microscopic Anatomy of Invertebrates. 8C: 515-1215. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Coons, L.B., and G. Alberti (1999) Acari: Ticks. In: Microscopic Anatomy of Invertebrates. 8B: 267-514. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Dogiel, V.A. (1911) Studien über die Entwicklungsgeschichte der Pantopoden. Nervensystem und Drüsen der Pantopodenlarven. Zeitschrift f. wissenschaftliche Zoologie, 98: 110-146.

Fahrenbach, W.H. (1994) Microscopic anatomy of Pycnogonida: I. Cuticle, epidermis, and muscle. J. Morphol. 222: 33-48.

Fahrenbach, W.H. (1999) Merostomata. In: Microscopic Anatomy of Invertebrates. 8A: 21-115. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Farley, R.D. (1999) Scorpiones. In: Microscopic Anatomy of Invertebrates. 8A: 117-222. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Felgenhauer, B.E. (1999) Araneae. In: Microscopic Anatomy of Invertebrates. 8A : 223-266. F.W. Harrison and R.F. Foelix (eds.) New York: Wiley-Liss, Inc.

Hedgpeth, J.W. (1954) On the phylogeny of the Pycnogonida. Acta Zool. 35: 1-21.

Hessler, R.R., and R. Elofsson (1991) The excretory system of Hutchinsoniella macracantha (Crustacea, Cephalocarida). J. Crust. Biol. 11: 356-367.

Loman, J.C.C. (1917) Beiträge zur Anatomie und Biologie der pantopoden. Tijdschrift der Nederlandsche Dierkundige Vereeniging 16: 53-102.

Meisenheimer, J. (1902) Beiträge zur Entwicklungsgeschichte der Pantopoden. I. Die Entwicklung von Ammothea echinata Hodge bis zur Ausbildung der Larvenform. Zeitschrift f. wissenschaftliche Zoologie, 72: 191-248.

Thompson, W. D’Arcy (1909) Pycnogonida. In: The Cambridge Natural History, 4: 501-542. S.F. Harmen and B.E. Shipley (eds.). London: Macmillan &Co.

Tjønneland, A., H. Kryvi, J.P. Ostnes, and S. Økland (1985) The heart ultrastructure in two species of pycnogonids and its phylogenetic significance. Zoologica Scripta 14: 215-219.

Muscular System

Pycnogonid body musculature has been described by Dencker (1974) and that of the proboscis by Fry (1965); hence, this brief chapter will be restricted to the submicroscopic anatomy of the muscles. The fine structure of somatic and cardiac muscles has been treated by Tjønneland et al (1985), Totland and Kryvi (1986) and Fahrenbach (1999).

The fine structure of the muscles is that of striated arthropod design throughout the body, but three aspects stamp it as primitive compared to even that of microcrustaceans:

A) Each muscle consists of a single cell and there are no muscle bundles, surrounded by connective tissue. In the appendicular musculature, such single fibers may converge on a common tendon, on which they commonly insert in bipinnate fashion, but solitary muscle cells (myofibers) are the rule. They traverse the hemocoel directly, separated from it only by a thin external lamina.
B) A myofiber is not subdivided into myofibrils, whereas myofibrillar design of various levels of complexity has been described innumerable times in all other arthropods (e.g., Harrison et al, ). At most, peripheral incursions of the surface membrane in the fashion of a T-system suggest incipient myofibrillar subdivisions.
C) In contrast to all other arthropods, the entire somatic musculature consists of slow fibers. Aside from the obvious, slow locomotor performance of pycnogonids, these slow fibers have long sarcomere lengths (5-10 µm, A-band length up to 7 µm)) and a myofilament distribution of one thick myofilament (myosin) surrounded by approximately 12 thin filaments (actin). This design has been shown by both physiological and ultrastructural studies in arthropods to be diagnostic of slow contractile function. Nonetheless, some pycnogonids are capable of catching copepods with their chelifores.
D) In contrast to all other arthropods, in whom the visceral musculature is of the slow type, the heart muscle of pycnogonids is of the fast type. This is evidenced not only by the visibly fast heart rate (between 90 and 180 beats / minute), but also by a substantially shorter sarcomere length of about 2.8 µm, a condition that also applies to the intestinal musculature.

The basic design of the somatic myofiber has been detailed in the preceding paragraphs and is illustrated in Figure 1. Diameter of the cells ranges from 4-60 µm. Visceral musculature, like that of the pharynx, the digestive and reproductive systems adheres to the same ultrastructure. Muscles that cover the surface of the midgut are disposed as an irregular lattice of narrow strap-like fibers, about 1-3 µm wide and 0.5 µm thick.

The sarcoplasmic reticulum consists of standard dyads and triads facing the surface and the sarcolemmal infoldings, respectively (Fig. 2). Innervation consists of synaptic boutons depressed into the surface of somatic and visceral muscles without substantial synaptic modifications.

As a fiber approaches its insertion in the cuticle, it subdivides into numerous conical projections that interlock with corresponding recesses in the epithelial cells mediating the attachment (tendinal cell). The interlocking contacts between muscle and tendinal cell show septate desmosomal specializations. The tendinal cell is filled with robust microtubules that evidently provide the high tensile strength connection betwen muscle and cuticle. It should be pointed out here parenthetically that these “structural” microtubules differ from the standard “dynamic” ones, in that they fix well with osmium tetroxide and were seen long before tubulin microtubules were discovered after the advent of aldehyde fixation.

On the cuticular side of the myocuticular junction, the microtubules of the tendinal cells insert on membrane infoldings with opaque, thickened walls that each surround the origin of an anchoring filament. In some preparations, these can be followed into the endocuticle, where they provide the ultimate anchoring for the muscles akin to that of Sharpey’s fibers in vertebrate bone (Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7).

Despite its vigorous action, the wall of the heart is extraordinarily thin, measuring generally no more than 0.5 µm in a small pycnogonid like Ammothea hilgendorfi and attenuating in places to about 50 nm. It consists of basal laminae facing both the cardiac lumen as well as the hemocoel and highly attenuated myofibers with minimal overlap and frequent stretches devoid myofilaments. The myofibers have internal and external projections, possibly as a result of contraction after fixation and loss of distending blood pressure and are about 2.5 µm long. The need for a sarcoplasmic reticulum is obviated by the reduced diffusion distances in this structure. Adjacent cells are connected by gap junctions (Fig. 8).

References
[N.B. A complete list of references to pycnogonid morphology, behavior and ecology is contained separately in this website.]

Dencker, D. von (1974) Das Skelettmuskelsystem von Nymphon rubrum Hodge, 1862 (Pycnogonida: Nymphonidae). Zool. Jahrb. 93: 272-287.

Fahrenbach, W.H. (1994) Microscopic anatomy of Pycnogonida: I. Cuticle, epidermis, and muscle. J. Morphol. 222: 33-48.

Fry, W. G. (1965) The feeding mechanisms and the preferred foods of three species of Pycnogonida. Bull. Brit. Mus. Nat. Hist. (Zool.) 12: 195-223.

Harrison, F. W. et al. (eds.)(1992-1999) Microscopic Anatomy of Invertebrates, Vols. 8 -11. Wiley-Liss

Tjønneland, A., H. Kryvi, J. P. Ostnes, and S. Økland (1985) The heart ultrastructure in two species of pycnogonids and its phylogenetic implications. Zoologica Scripta 14: 215-219.

Totland, G. K., and H. Kryvi (1986) The fine structure of the somatic muscles and their attachment to the cuticle in two species of Pycnogonida. Zoologica Scripta 15: 69-72.

Pycnogonid Classification

Traditionally, pycnogonids were classified as the most primitive chelicerates and their relationships with other chelicerate taxa had been considered to be problematical at best. Recent research has demonstrated that pycnogonids are closely related to mites and ticks and they are now considered to be an order of Arachnids, Order Pycnogonida (Bain, in press). Within the pycnogonida, there are currently a number of different classification schemes, most with seven to nine different families. The most commonly used classification schemes are those of Stock (1994) and Hedgpeth (1947, 1982) each with eight families. Arnaud and Bamber (1987), a recent summary of pycnogonid biology, use a combination of these two classification schemes. There are currently 86 pycnogonid genera and at least a thousand species (Bain, 1992). There is no recent published species catalog for the group.

Table 1
A Comparison of Current Pycnogonid Classifications
(modified from Bain, in press)

Hedgpeth (1947, 1982); King (1973)	Stock (1994)	Arnaud & Bamber (1987)
Nymphonidae	Nymphonidae	Nymphonidae
Pallenidae¹	Callipallenidae	Callipallenidae
Phoxichilidiidae	Phoxichilidiidae²	Phoxichilidiidae
Endeidae		Endeidae
Ammotheidae	Ammotheidae³	Ammotheidae³
Tanystylidae
Colossendeidae	Colossendeidae	Colossendeidae
Pycnogonidae	Pycnogonidae	Pycnogonidae
	Austrodecidae⁴	Austrodecidae⁵
	Rhynchothoracidae	Rhynchothoracidae

¹ Pallene is a preoccupied genus name (Flynn 1929, p. 252). Most species of Pallene have been changed to Callipallene and the family name changed to Callipallenidae.
² Includes the genus Endeis
³ Includes the family Tanystylidae
⁴ Contains only the genus Austrodecus
⁵ Contains both Austrodecus and Pantopipetta

The material found on this webpage is copyrighted © by Bonnie Bain.
Use of this material for private, academic or professional purposes is granted as long as the author is acknowledged and this web page is cited.

Pycnogonid Collecting

Pycnogonids can be found in all marine habitats from the intertidal zone to the abyssal depths. A number of species are also found in estuaries since many pycnogonids are very tolerant of changes in salinity (B. Bain, unpublished data). In the intertidal zone, the best places to look for pycnogonids are under rocks and in or on hydroids, bryozoans, algae, and corals. In many cases, they are very hard to find since the color of the pycnogonid often very closely matches its habitat (Bain, 1991). Pycnogonids have also been collected from the tube feet of sea stars (Stock, 1981), from brittle stars (Stock, 1979) and have been found clinging to nudibranchs (Merton, 1906; Ohshima, 1933b), holothurians (Ohshima, 1927b), polychaetes (Stock, 1959; Salazar-Vallejo and Stock, 1987) and medusae (Lebour, 1916; Okuda, 1940. Mauchline, 1984; Child and Harbison, 1986). They have also been found living in (and feeding on) several different species of clams (Ohshima, 1927a, b; Ohshima, 1933a; Ohshima, 1935; Benson and Chivers,.1960; Kikuchi, 1976; Ogawa and Matsuzaki, 1985).
Rocky shore near Phillip Island, Australia

Pycnogonid Courtship, Mating Behavior, and Parental Care

Pycnogonids are one of the few groups of animals in which the males exclusively care for the developing eggs and young and the ovigerous legs play a major role in these activities. Male pycnogonids appear to use their ovigerous legs during courtship to induce egg laying by the female (Nakamura and Sekiguchi, 1980). Then, following courtship, the female begins to lay the eggs and the male fertilizes them while the female holds them on her ovigerous legs (Cole, 1901a; Nakamura and Sekiguchi, 1980). After fertilization, depending on ovigerous leg type (Bain, in press), the male either gathers the eggs one by one onto his ovigerous legs (Nakamura and Sekiguchi, 1980) or hooks his ovigerous legs into the egg mass (held by the female) and, in one motion, gathers most of the eggs into a single mass on his ovigerous legs (Cole, 1901a). The eggs then remain on the ovigerous legs for variable amounts of time depending on species and geographic location (Cole, 1901b; Thompson, 1909; Hedgpeth, 1948; Nakamura, 1981). While most pycnogonids carry the eggs only until they hatch, some Arctic species of Nymphon and Boreonymphon continue to carry the juveniles after they hatch until the juveniles are one third the size of the adult. (Thompson, 1909).

Pycnogonid Preferred Foods and Feeding Behavior

We currently know very little about pycnogonid feeding habits. Some pycnogonids, such as Anoplodactylus californicus and Anoplodactylus carvalhoi are generalized predators which feed on hydroids, polychaetes, nudibranchs and other small invertebrates (Bain, 1991; Piel, 1991) while others, like Achelia pribilofensis, are scavengers (B. Bain, personal observation). Others, like Pycnogonum litorale and Pycnogonum rickettsi feed by inserting the proboscis into a sea anemone and sucking out its internal fluids. The pycnogonid then drops off and wanders away when satiated. This type of feeding behavior rarely results in the death of the anemone unless the pycnogonid is as large or larger than the anemone (B. Bain, personal observation).

Pycnogonid Post-Embryonic Development

The most common larval type to hatch out of the egg is the protonymphon. This larva is a free-swimming feeding stage and shortly after hatching, it leaves the male’s ovigerous legs and swims away. Depending on species, this larva can follow one of three different developmental pathways: it can remain free-living and undergo a series of molts during which it adds appendages in a sequential fashion until the adult number of appendages has been reached (Morgan, 1891; Okuda, 1940; Bain, in press); or it can become encysted in a hydroid or a stylasteroid coral, undergo one or more molts (exact number unknown) and emerge as a juvenile with three pairs of walking legs (Hilton, 1916; Bain, in press); or it can undergo a different series of molts in which all of the appendages appear at once and increase in size and segment number after each molt (Ohshima, 1927a; Salazar-Vallejo and Stock, 1987; Bain, in press). The above three types have been named the Protonymphon or Typical Protonymphon (free-living), Encysted Larva (encysted in hydroids or corals), and the Atypical Protonymphon (all the appendages appear at once rather than sequentially) (Bain, in press). A fourth type of pycnogonid postembryonic development begins with a different kind of larva, the Attaching Larva, hatching out of the egg (Nakamura, 1981; Bain, in press). This larva is a non-feeding yolk-filled larva which attaches itself to the male’s ovigerous legs and remains attached for the next several molts. Once it molts into either a fourth instar (Propallene longiceps) or a fifth instar (Austropallene cornigera), it leaves the male’s ovigerous legs and takes up a free-living existence (Nakamura, 1981; Bain, in press).

REFERENCES

Arnaud, F. and R. N. Bamber, 1987. The Biology of the Pycnogonida. In: J. H. S. Blaxter and A. J. Southward (eds). Advances in Marine Biology, vol. 24. Academic Press, New York, pp. 1-96.
Bain, B. A., 1991. Some observations on biology and feeding behavior in two southern California pycnogonids, Bijdragen tot de Dierkunde, 61(1): 63-64.
Bain, B. A., 1992. Pycnogonid higher classification and a revision of the genus Austropallene (Family Callipallenidae). Ph.D. diss., City University of New York.
Bain, B. A., in press. Systematics and Biogeography of the Pycnogonida (Subphylum Chelicerata, Class Pycnogonida). Cladistics.
Bain, B. A., in press. Larval Types and a Summary of Postembryonic Development in Pycnogonids. Invertebrate Reproduction and Development.
Benson, P. H. and D. C. Chivers. 1960. A pycnogonid infestation of Mytilus californianus, Veliger 3: 16-18.
Brusca, R. C. and Brusca, G. J., 1990. Invertebrates, Sinauer Associates, Sunderland, Mass.
Calman, W. T., 1937. The type-specimens of Pallene australiensis Hoek (Pycnogonida). Annals and Magazine of Natural History, (10)20: 530-534.
Calman, W. T., and I. Gordon, 1933. A dodecapodous Pycnogonid. Proceedings of the Royal Society of London (B), 113:107-115.
Carpenter, G. H., 1892. Pycnogonida. Reports on the zoological collections made in Torres Straights by Prof. A. C. Haddon, 1888-89. Scientific Proceedings of the Royal Dublin Society, 7 (5) (40): 552-558.
Child, C. A., 1975. Pycnogonida of Western Australia, Smithsonian Contributions to Zoology, 190: 1-29.
Child, C. A., 1982. Pycnogonida of the Western Pacific Islands, I: The Marshall Islands. Proceedings of the Biological Society of Washington, 95 (2): 270-281.
Child, C. A., 1983. Pycnogonida of the Western Pacific Islands, II: Guam and the Palau Islands. Proceedings of the Biological Society of Washington, 96 (4):698-714.
Child, C. A., 1988a. Pycnogonida of the Western Pacific Islands, III: Recent Smithsonian-Philippine Expeditions. Smithsonian Contributions to Zoology, 468:1-32.
Child, C. A. and G. R. Harbison, 1986. A parasitic association between a pycnogonid and a scyphomedusa in midwater, Journal of the Marine Biological Association of the United Kingdom, 66:113-117.
Clark, W. C., 1963. Australian Pycnogonida, Records of the Australian Museum, Sydney, 26 (1): 1-81.
Clark, W. C., 1972a. Pycnogonida of the Snares Islands, New Zealand Journal of Marine and Freshwater Research, 5 (2): 329-341.
Clark, W. C., 1972b. Pycnogonida of the Antipodes Islands, New Zealand Journal of Marine and Freshwater Research, 5 (3 & 4): 427-452.
Clark, W. C., 1977a. Pycnogonida from the Snares Islands (Note), New Zealand Journal of Marine and Freshwater Research, 11 (1): 173-177.
Clark, W. C., 1977b. The genus Ammothea Leach (Pycnogonida) in New Zealand waters: new species and a review, Journal of the Royal Society of New Zealand, 7(2): 171-187.
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