Innervation and structure of the adipose fin of a lanternfish.

Adipose fins of teleost fishes have been shown to function as mechanosensory organs that respond to minute bending forces created by turbulence in fast-flowing streams. Nonetheless, adipose fins also exist in some fishes that occupy still waters, including lanternfish (Myctophidae) in the deep sea. The authors examined adipose fin structure in northern lampfish, Stenobrachius leucopsarus, from coastal British Columbia. After fixation, embedding and sectioning of the adipose and supporting tissue, it was evident that lanternfish adipose fins are stiffened by compound actinotrichia, acting like fin rays, that would create a higher aspect ratio. The actinotrichia converge at the base of the fin in a hinge point complex that anteriorly interacts with a cartilaginous endoskeletal rod, controlled by skeletal muscles. Afferent nerves enter the fin at this point and form fine branches as they track deeper alongside actinotrichia. The authors propose that the vertical nightly migration to surface waters, as well as predator evasion within large schools, results in microturbulence. In these circumstances, the adipose fin acts as a mechanosensor providing feedback to the caudal fin, as it occurs in salmonids and catfish.

Fish Parasite Dinoflagellates Haidadinium ichthyophilum and Piscinoodinium Share a Recent Common Ancestor.

The dinoflagellate Haidadinium ichthyophilum Buckland-Nicks, Reimchen and Garbary 1997 is an ectoparasite of the spine-deficient, three-spine stickleback Gasterosteus aculeatus L. Reimchen 1984, a fish endemic to Rouge Lake, Haida Gwaii. Haidadinium ichthyophilum proved difficult to assign taxonomically because its morphology and complex life cycle exhibited defining characteristics of both autotrophic and heterotrophic dinoflagellates, and was tentatively assigned to the Phytodiniales. Here, we characterized a 492 bp fragment of the small subunit ribosomal RNA (SSU rRNA) from preserved H. ichthyophilum cysts. In SSU phylogeny, H. ichthyophilum branches with the fish parasites, Piscinoodinium sp., strongly supporting the inclusion of H. ichthyophilum within the Suessiales.

On fertilization in Chaetopleura apiculata and selected Chitonida.

Early events of fertilization are described in Chaetopleura apiculata and other selected Chitonida. C. apiculata egg hulls are elaborated into multi-branched spines with interlocking polygonal bases. Around the perimeter of each base are a series of open pores, ranging in size from 0.1-0.5 microm, which permit sperm direct access to the vitelline layer. In Callochitonidae (Chitonida) even larger pores occur in egg jelly coats, but this is considered to be the plesiomorphic condition, found also in Lepidopleurida such as Deshayesiella curvata. Other Chitonina, such as Rhyssoplax tulipa and Acanthopleura granulata, have a continuous outer dense layer that lacks pores and must be digested by penetrating sperm. Fertilization in Chitonida is unique and involves injection of chromatin into the egg via a narrow tubular nuclear extension that appears to exclude other sperm organelles, including mitochondria, centrioles, and flagellum. New evidence from studies of fertilization in Mopalia muscosa (Chitonida: Acanthochitonina) supports this hypothesis. This type of fertilization implies maternal inheritance of both mitochondria and centrioles, which is highly unusual, because in most animals one sperm centriole assists movements of pronuclei and regulates organization of the mitotic spindle. This mechanism of fertilization is defined by a series of apomorphic characters that unify the order Chitonida.

Paraspermatogenesis of Cerithioidean snails: retention of an acrosome and nuclear remnant.

Paraspermatozoa of Cerithioidea are oligopyrenic with a central nuclear remnant surrounded by glycoprotein bodies and an anterior acrosome complex. Posteriorly, the parasperm have one long and several small flagella. Biosynthesis of proteins begins in the rough endoplasmic reticulum (RER), early in paraspermatogenesis, prior to the degradation of the nucleus. The breakdown of the nucleus has features characteristic of apoptosis but a nuclear remnant remains that is composed of intact DNA. The acrosome complex of parasperm is Golgi-derived, forming posteriorly and migrating anteriorly along the plasma membrane to the apex of the nuclear remnant as the paraspermatid matures. This mechanism of acrosome formation is similar to that in euspermiogenesis in neomenioid aplacophorans and neritid snails and is plesiomorphic to mollusks. However, eusperm acrosomes of Cerithioidea form differently. In the paraspermatid, small, dense granules secreted by the Golgi body fuse to form larger granular vesicles, many of which are released by exocytosis into the seminal fluid. These granular vesicles stain red with acridine orange at pH 7 under 489-nm light, indicating that they are lysosomes. The retention of a nuclear remnant and development of an acrosome complex in the parasperm of cerithioideans, as well as the secretion of lysosomes into the seminal fluid, suggests novel functions for these unusual germ cells.

SEM analysis of marine invertebrate gametes.

Scanning electron microscopy (SEM) enables a close-up investigation of topographical features at the cellular level down to a resolution of about 3 nm. In terms of gametes this allows for examination of minute surface details and changes that occur during fertilization. Sometimes these features are hidden from view and must be carefully exposed during preparation in order to be made visible in the SEM. This chapter describes methods that can reveal surface details of gametes for observation, as well as fix them permanently, while keeping fixation artifacts to a minimum.

Paraspermatogenesis in Ceratostoma foliatum (Neogastropoda): confirmation of programmed nuclear death.

Sperm dimorphism, where ejaculates contain both fertile "eusperm" and sterile "parasperm", has been implicated in sperm competition in animals. Most parasperm lack an acrosome, eliminate the nucleus and develop a complex cytoplasm, swollen with secretory vesicles. This study of parasperm in the foliate whelk Ceratostoma foliatum, focuses on characterizing the process of nuclear breakdown and the nature of cytoplasmic secretions. Testis squash preparations were treated with a Promega apoptosis detection [terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling-TUNEL] assay (Cedar Lane) to detect nuclear apoptosis; acridine orange was used to localize lysosomes. Parasperm were isolated on Percoll gradients and their proteins characterized by running electrophoresis of lysed, purified fractions on polyacrylamide gels. Results show that the process of nuclear breakdown follows the hallmarks of apoptosis in ultrastructure, which is confirmed by the presence of 3'-OH overhangs of DNA fragments. This research implicates snail parasperm as models for elucidating the nuclear events of apoptosis.

Ultrastructure of the male reproductive system and of spermatogenesis in the viviparous brittle-star, Amphipholis squamata.

The fine structure of the male reproductive system of the hermaphroditic brittle-star, Amphipholis squamata, has been studied in specimens from both the Pacific coast (Washington) and the Atlantic coast (New Hampshire). Each testis is a small (100-µm) sphere and is attached to the internal wall of the bursa by peritoneal suspensor cells. Occasional flagellated cells are found on the external surface of the testis. The testicular wall of A. squamata is a multilayered structure, similar to that of other ophiuroids, but the hemal sinus is PAS negative in this species. Germinal cells are surrounded throughout their development by the filopodia of interstitial cells. Adjacent interstitial cells are interconnected, and thus form a structural network within the testis. Positionally and functionally, the interstitial cells resemble Sertoli cells; however, their origin, behavior and ultrastructure are different in many ways. Spermatogenesis includes a series of cyclical changes (aspermatogenic phase, proliferative phase, differentiative phase, and evacuative phase). Within a single testis, the resulting production of sperm is in short pulses, but if all 10 testes are taken together, sperm are produced continuously throughout the year. The events of spermiogenesis closely follow those that have been described in other echinoderms. However, we have provided new information on the release of excess cell membranes and the fusion process of mitochondria. The mature spermatozoa of A. squamata are flagellated and motile, and have "primitive" structural features, in spite of the fact that they fertilize the eggs inside the genital bursae. The spermatozoa do not, as was previously thought, enter the bursa by rupture of the adjacent walls. Instead, they are ejaculated through a gonoduct into the rapid incurrent flow of water entering the bursa. The locomotion of the spermatozoa is in eccentric spirals, due to the unusually large angle at which the flagellum is inserted into the base of the sperm.