The fine structure of spermiogenesis in the Amblypygi and the Uropygi (Arachnida)

The male reproductive system of the sea spider Phoxichilidium femoratum (Rathke, 1799)

Sea spiders (Pycnogonida) are marine chelicerates. As a sister clade to Euchelicerata, Pycnogonida are an interesting group for comparative anatomy, however data on pycnogonid anatomy and biology remain scarce. This research provides a detailed account of the complete male reproductive system, gametogenesis, and sperm structure of a sea spider at the ultrastructural level. The male reproductive system of P. femoratum includes the testis, femoral, and ovigeral glands. The testis is typical of Pycnogonida: U-shaped with pedal outgrowths, opening with gonopores on legs 2-4. The testis lays within the horizontal septum, separated from it by ECM. The reproductive sinus is reduced. The ventral wall of the testis is germinative, spermatogenesis proceeds in cysts, all stages are evenly distributed throughout the whole testis. Sperm of P. femoratum is a typical sperm of animals with fertilization in mucus but without an acrosome. It lacks apomorphic euchelicerate features such as an acrosomal filament and implantation fossa. Femoral and ovigeral glands are sex-specific and likely related to reproduction. Ovigeral glands possibly secrete a fungicide substance, while the function of femoral glands remains obscure.

Evolutionary morphology of the male reproductive system, spermatozoa and seminal fluid of spiders (Araneae, Arachnida)--current knowledge and future directions

The male reproductive system and spermatozoa of spiders are known for their high structural diversity. Spider spermatozoa are flagellate and males transfer them to females in a coiled and encapsulated state using their modified pedipalps. Here, we provide a detailed overview of the present state of knowledge of the primary male reproductive system, sperm morphology and the structural diversity of seminal fluids with a focus on functional and evolutionary implications. Secondly, we conceptualized characters for the male genital system, spermiogenesis and spermatozoa for the first time based on published and new data. In total, we scored 40 characters for 129 species from 56 families representing all main spider clades. We obtained synapomorphies for several taxa including Opisthothelae, Araneomorphae, Dysderoidea, Scytodoidea, Telemidae, Linyphioidea, Mimetidae, Synotaxidae and the Divided Cribellum Clade. Furthermore, we recovered synspermia as a synapomorphy for ecribellate Haplogynae and thus propose Synspermiata as new name for this clade. We hope that these data will not only contribute to future phylogenetic studies but will also stimulate much needed evolutionary studies of reproductive systems in spiders.

Spermatozoa of an Old World Ricinulei (Ricinoides karschii, Ricinoidae) with notes about the relationships of Ricinulei within the Arachnida

The ultrastructure of spermatozoa is a valuable tool for phylogenetic and systematic studies. Ricinulei are enigmatic and poorly studied arachnids. So far, spermatozoa are only known from New World ricinuleids. The goals were to study, by means of light and transmission electron microcopy, the spermatozoa of an Old World species with regard to their phylogenetic implications, e.g., does the sperm structure contribute to the debated sister-group relationship of Acari and Ricinulei. The spermatozoa are coiled-flagellate and characterized by a cap-like acrosomal vacuole covered by electron-dense material, an elongated nucleus covered by a manchette of microtubules during spermiogenesis, an axoneme with a 9+2 microtubular pattern, a nuclear tube and axonemal basis which both originate underneath the acrosomal vacuole and cleistospermia as transfer form equipped with three intracellular plates. The data of the present study did not support a close relationship of Ricinulei and Acari which have aflagellate sperm with various synapomorphies as e.g., lacking nuclear envelopes/membranes in Actinotrichida (very similar to Solifugae) or vacuolated spermatozoa in Anactinotrichida. Affinities of Ricinulei are discussed in the light of the ultrastructure of arachnid spermatozoa.

Spermatozoa and spermiogenesis of Liphistius cf. phuketensis (Mesothelae, Araneae, Arachnida) with notes on phylogenetic implications

The present study deals with the spermatozoa and spermiogenesis of Liphistius cf. phuketensis, a representative of the most primitive and enigmatic spider group Mesothelae. The general organization of the spermatozoa is very similar to the condition known from Amblypygi supporting a sister-group relationship between Araneae and Amblypygi. Besides plesiomorphic characters such as, e.g., an elongated and corkscrew shaped nucleus, the sperm cells are characterized by several apomorphic characters, e.g., the giant body and conspicuous membranous areas which are formed at the end of spermiogenesis. As the transfer form, coenospermia are formed at the end of spermiogenesis, which strongly supports the idea that this type of sperm aggregation is the primitive transfer form within spiders. A very remarkable character of the spermatozoa of some groups of arachnids is the coiling of the main cell organelles at the end of spermiogenesis. Previously, the Mesothelae were believed to be the only spider group which does not show a complete coiling of the main cell organelles. With the present study the first evidence of a complete coiling of spermatozoa within this primitive spider group could be documented, indicating that this character is part of the ground pattern of spider spermatozoa. Consequently, the incomplete coiling seems to be a synapomorphy of certain species of Mesothelae, which sheds new light on the discussion of the phylogenetic relationships of this group.

Double spermatogenesis in Chelicerata

Sperm dimorphism is a rare phenomenon in Chelicerata. Until now, it was known only from three species of the opilionid genus Siro (Sironidae, Cyphophthalmi). Fertilizing (eusperm) and nonfertilizing spermatozoa (parasperm) develop in the same cyst and are thus sister cells. The fine structure of the spermatozoa of two species has been examined and is compared here. In contrast to Siro rubens, S. duricorius spermatozoa lack an acrosomal complex. Both sperm types produce a transitional process, a more or less modified flagellum, which is later retracted. Hence, the spermatozoa are aflagellate. Eusperm and parasperm of all three species form highly ordered sperm balls that are stored in the deferent duct. Reviewing and adding new results about the sperm dimorphism in this arachnid taxon provides the basis for some considerations of another enigmatic morphological character found in Uropygi and Amblypygi, i.e., the tubular accessory genital glands that show holocrine extrusion. These glands are suggested to represent modified, infertile derivatives of the testis anlage. Their secretion is produced in a way reminiscent of a strongly degenerated spermatogenesis. Consequently, these products may be regarded as strongly degenerated germ cells representing a line of germ cell development, which has been separated very early in spermatogenesis from the usual line leading to fertilizing sperm cells. This further, although less evident, case of probable dichotomous germ cell development is discussed with respect to the controversial phylogenetic-systematic relationships between Uropygi (Thelyphonida and Schizomida), Amblypygi, and Araneae.

The ultrastructure of the peculiar synspermia of some Dysderidae (Araneae, Arachnida)

The present study reports on the ultrastructure features of spermatozoa and spermatogenesis of several species of Dysderidae (Dysdera crocata, Dysdera erythrina, Dysdera ninnii, Harpactea arguta, Harpactea piligera, Dasumia taeniifera). Dysderid spiders are known to possess a peculiar sperm transfer form known as synspermia, characterized by fused spermatozoa surrounded by a secreted sheath. Until now the exact mode of formation of the synspermia is unknown. The present study demonstrates that the spermatids are connected via narrow cell bridges during the entire spermiogenesis as is usual, although in Dysderidae they do not separate at end of the spermiogenesis. Instead, they fuse completely within the testes shortly after the spermatid has coiled to get a spherical shape. The number of fusing sperm cells is different in the different observed species. The species of the genus Harpactea thus have synspermia consisting of two fused spermatozoa; whereas in the species of the genus Dysdera four sperm cells are fused and in D. taeniifera at least three spermatozoa are fused. In contrast with other known families with this peculiar form transfer of sperm, the synspermia in Dysderidae are mainly characterized by a conspicuous vesicular area which extends through the entire synspermium surrounding the cell organelles. Thus, all main cell components (e.g., nucleus, acrosomal vacuole, and axoneme) are covered by the vesicular membrane. The vesicular area seems to be functional and probably it is important during sperm activation in female genital system. Simultaneously to the extension of the vesicular area, the synspermium accumulates large amounts of glycogen. The glycogen is mainly located around the centriolar adjunct and along the axoneme accompanying the postcentriolar elongation of the nucleus. A further peculiar feature is the extremely elongated acrosomal vacuole, which seems to be synapomorphic trait for sperm cells of dysderids. Interestingly, spermatogenesis, including the fusion, exclusively occurs within the testes (in contrast to the formation of coenospermia). In the vas deferens only synspermia were found. The secreted sheath surrounding the spermatozoa is finally synthesized in the parts of the vasa deferentia, which are close to the genital opening where numerous vacuoles and microvilli are seen in the epithelial cells.

On the occurrence of coenospermia in mesothelid spiders (Araneae: Heptathelidae)

Two species of the early derivative spider family Heptathelidae (Heptathela kimurai yanbaruensis and Ryuthela nishihirai nishihirai) have been investigated with respect to spermiogenesis, focussing on late events during which peculiar transfer forms are developed. It is shown, for the first time in detail, that these spiders produce coenospermia. The coenospermia of these species are large aggregates containing more than 20 individual encapsulated spermatozoa. The coenospermia possess a likely flexible envelope formed by a thick multilayered secretion, which protects the spermatozoa during transfer to the female genital system and storage in the receptacula. In addition, a short description of the main cell components of the individual spermatozoa is given as a complement to previous studies. With the observation presented here, the suggestion that coenospermia are an ancestral character in the Araneae is further confirmed, and plesiomorphic features of spider sperms are consolidated.