Invertebrate Biology, Volume 121, No. 2

Contents and Abstracts
© 2002 American Microscopical Society, Inc.

HEADER ILLUSTRATION: The aeolid nudibranch, Aeolidia papillosa, preying on sea anemones (Anthopleura elegantissima). Seavy & Muller-Parker tested the influence of the symbiotic algae (zooxanthellae and zoochlorellae) in the anemome on the nudibranch's selection of its prey. They speculate that the predator plays a role in maintaining the symbiosis. (Photo by Dan Welsh-Bon ©)


Abstracts for all articles in this issue are presented below.
Individual abstracts can also be accessed directly by clicking on the highlighted titles here:

Attachment to the substrate by soft coral fragments: desmocyte development, structure, and function
Orit Barneah, Zvi Malik, and Yehuda Benayahu. pp 81-90

Sperm structure of Rhopalura littoralis (Orthonectida)
George S. Slyusarev and Marco Ferraguti. pp 91-94

New model for the formation and function of sagittocysts: Symsagittifera corsicae n. sp. (Acoela)
Robert Gschwentner, Sanja Baric, and Reinhard Rieger. pp 95-103

Reduction and variability of trunk spines in the acanthocephalan Corynosoma cetaceum: the role of physical constraints on attachment
F. Javier Aznar, Albert O. Bush, and Juan A. Raga. pp 104-114

Chemosensory and feeding responses of the nudibranch Aeolidia papillosa to the symbiotic sea anemone Anthopleura elegantissima
Barbara Emerson Seavy and Gisèle Muller-Parker. pp 115-125

Ultrastructure of the mushroom body: digestion during metamorphosis of Utterbackia imbecillis (Bivalvia: Unionidae)
Ginger R. Fishera and Ronald V. Dimock, Jr.. pp 126-135

Ecdysis in scorpions: supine behavior and exuvial ultrastructure
R. David Gaban and Roger D. Farley. pp 136-147

Morphology of stomatopod chemosensory sensilla facilitates fluid sampling
Kristina S. Mead and Tina M. Weatherby. pp 148-157

Are the two physiological races of Pollicipes polymerus (Cirripedia) genetically divided along the California coast?
Benjamin G. Miner. pp 158-162

Fusion between incompatible colonies of a viviparous ascidian, Botrylloides lentus
Makiko Okuyama, Yasunori Saito, and Euichi Hirose. pp 163-169


Invertebrate Biology 121(2):81-90
© 2002 American Microscopical Society, Inc.

Attachment to the substrate by soft coral fragments: desmocyte development, structure, and function Abstract. Pieces cut from colonies of the soft coral Dendronephthya hemprichi exhibited rapid and effective attachment to hard surfaces. Attachment involved development of root-like processes (RLPs), which appeared at the basal part of the fragment 4 days after its removal from the colony. The fine structural changes and cascade of cellular events occurring in the RLP before and after attachment were studied using SEM, TEM, and LM. The epidermis of the RLPs is actively involved in the attachment process and several distinct phases are documented: appearance of numerous oval vesicles, extrusion of these vesicles resulting in the formation of an outer layer composed of extracellular organic matrix, and organellar debris which functions as an adhesive device leading to initial attachment. The latter phase was followed by the formation of desmocytes, which develop in the RLP epidermis and function as anchoring devices, mediating the firm attachment of the fragment to the substrate. This is the first evidence among anthozoans that desmocytes play an active role in anchoring tissue to substrate and thus extends the range of functions exhibited by desmocytes among anthozoans.

Additional key words: Octocorallia, Dendronephthya hemprichi, extracellular organic matrix


Invertebrate Biology 121(2):91-94
© 2002 American Microscopical Society, Inc.

Sperm structure of Rhopalura littoralis (Orthonectida) Abstract. We present here a description of the spermatozoon of Rhopalura littoralis, the first fine-structural study of an orthonectid sperm. The spermatozoon contains a slightly elongate nucleus and two centrioles orientated along the longitudinal axis of the sperm. The proximal centriole bears one rootlet. A single mitochondrion is present in the mid-piece region. An acrosome is absent. The sperm tail is a simple flagellum with 9+2 structure. We consider the orthonectan spermatozoon to be closer in structure to those of Porifera, Cnidaria, and Annelida, than to Aschelminthes and Platyhelminthes, to which orthonectids have previously been allied.

Additional key words: ciliary rootlet, Mesozoa, phylogeny


Invertebrate Biology 121(2):95-103
© 2002 American Microscopical Society, Inc.

New model for the formation and function of sagittocysts: Symsagittifera corsicae n. sp. (Acoela) Abstract. This study is focused on the formation and function of sagittocysts, which are secretions typical of members of the acoel family Sagittiferidae. The needle-shaped sagittocysts are produced in specialized gland cells (sagittocytes) whose distal necks are often surrounded by muscle mantles. Contraction of the muscle mantle ejects the sagittocyst. We establish a model for the development of sagittocytes and muscle mantles out of the stem cell pool of the new acoel species Symsagittifera corsicae. We used various techniques, especially interference and phase-contrast microscopy of living specimens as well as labeling of the body-wall musculature, for species characterization. In addition to the morphological features, we provide the third complete sequence of the 18S rDNA gene in the family Sagittiferidae.

Additional key words: Turbellaria, Platyhelminthes, Acoelomorpha, Symsagittifera poenicea


Invertebrate Biology 121(2):104-114
© 2002 American Microscopical Society, Inc.

Reduction and variability of trunk spines in the acanthocephalan Corynosoma cetaceum: the role of physical constraints on attachment Abstract. In this study, we investigated a functional trade-off between trunk attachment and trunk-spine development in the acanthocephalan Corynosoma cetaceum. The worms live attached to the stomach and upper intestine of their cetacean definitive hosts, using the proboscis and spiny foretrunk as the main holdfast; the spiny hindtrunk can also attach by bending ventrally. When the hindtrunk bends, ventral compression generates an anterior fold (AF) and a posterior fold (PF). A morphological analysis based on 7,823 individuals collected from 10 franciscana dolphins, Pontoporia blainvillei, revealed that spines were smaller and more variable in size and occurrence in the folds than on neighboring areas; the growth of fold spines seemed to be inhibited to various degrees. Spines were more reduced in the AF than in the PF, and spines of both folds were more reduced in females than in males. Patterns of reduction appeared to be directly related to the intensity of fold compression associated with hindtrunk bending. Fold compression could induce plastic inhibition of spine growth, and/or could make fold spines maladaptive, spines being reduced by natural selection. Apparently, fold spines neither contact the substrate, nor are they exposed to the environment when the hindtrunk attaches. Therefore, fold spines could have reduced, or lost, their primary function, at least in the definitive host. The reduction and variability of spines in C. cetaceum seem to be unique among Corynosoma species.

Additional key words: phenotypic plasticity, vestigiality


Invertebrate Biology 121(2):115-125
© 2002 American Microscopical Society, Inc.

Chemosensory and feeding responses of the nudibranch Aeolidia papillosa to the symbiotic sea anemone Anthopleura elegantissima Abstract. The aeolid nudibranch Aeolidia papillosa is an important predator on the sea anemone Anthopleura elegantissima, a host to two kinds of endosymbiotic algae: zooxanthellae and zoochlorellae. The possible influence of the algae on the nudibranch's predatory response to this anemone was examined in a laboratory study. In chemosensory experiments, the nudibranch detected and chose anemone scent over a seawater control, but in both chemosensory and feeding experiments showed no preference for zooxanthellate or zoochlorellate anemones. Ingestive conditioning on zooxanthellate or zoochlorellate anemones had no effect on choice of these two anemone types in chemosensory experiments. Comparisons of the productivity and photosynthetic pigments of algae obtained from nudibranch feces and from anemones show that both algae survive passage through the nudibranch gut. The productivity of fecal zooxanthellae was 1.6× greater than that of zooxanthellae freshly isolated from anemones, although the chlorophyll a content of fecal zooxanthellae was reduced. The productivity and amount of pigments were the same for zoochlorellae in nudibranch feces and freshly isolated from anemones. Comparing fecal and isolated algae, there was no significant difference in the percentage of zooxanthellae in the process of cell division. However, the percentage of dividing cells was 2.6× higher in fecal than in freshly isolated zoochlorellae (18% and 6.9% respectively). Although the endosymbiotic algae do not make their host more or less attractive to the nudibranch, this predator may play an important role in maintaining the symbiotic relationship of Anthopleura elegantissima with zooxanthellae and zoochlorellae by providing viable algae in its feces as a source for the anemone host.

Additional key words: algal symbiosis, zooxanthellae, zoochlorellae


Invertebrate Biology 121(2):126-135
© 2002 American Microscopical Society, Inc.

Ultrastructure of the mushroom body: digestion during metamorphosis of Utterbackia imbecillis (Bivalvia: Unionidae) Abstract. Larvae of the freshwater mussel Utterbackia imbecillis metamorphose to juveniles either during their attachment to a host fish, or in vitro in a culture medium. This transformation includes degeneration of larval structures and development of the juvenile morphology. Early in metamorphosis the cells comprising the larval mantle enlarge and project into the mantle cavity, forming a structure referred to as the mushroom body. Its cells, which are ultrastructurally very similar to digestive cells of adult bivalves, are involved in pinocytosis or phagocytosis of the larval adductor muscle and of tissue from the host fish that is enclosed between the larval shells. Ingested material is passed from pinosomes to heterophagosomes which in turn fuse with heterolysosomes, where final degradation of ingested material occurs. Acid phosphatase activity was detected in heterophagosomes and heterolysosomes of all animals examined. In larvae that metamorphosed in vitro, the apical cytoplasm of the cells of the mushroom body, and the extracellular spaces among them, also exhibited acid phosphatase activity. Larvae reared on a host fish accumulated substantial deposits of lipids and glycogen within larval mantle cells during metamorphosis, whereas larvae reared in vitro did not. The larval mantle cells which constitute the mushroom body appear to be the primary sites of intracellular digestion of the larval adductor muscle and host tissue during metamorphosis.

Additional key words: glochidium, mussel


Invertebrate Biology 121(2):136-147
© 2002 American Microscopical Society, Inc.

Ecdysis in scorpions: supine behavior and exuvial ultrastructure Abstract. Ecdysis in scorpions has some common features in all species in which it has been examined. Immature scorpions about to molt become less active, and the cuticle changes in appearance. When humidity and other conditions are suitable, the animal begins ecdysis with cheliceral and pedipalpal movements and internal processes that tear the pleural membrane just ventral to the anterior and lateral edges of the carapace. The carapace is pushed upward from within, and the animal starts to emerge through the opening made by the elevated carapace. This is usually done in the prone position. As the anterior body emerges, the pedipalps and walking legs are stretched posteriorly with their distal ends temporarily confined within the old exoskeleton. The buthid scorpions Androctonus australis and Parabuthus transvaalicus become supine before they begin to emerge. Ecdysis is completed lying supine, and the animal rights itself shortly thereafter. A turnover from upright to supine is not seen at any other time in scorpions. After prone or supine extraction, the exuvium includes the cuticle of booklungs, bristles, and sensilla and is relatively intact except for the wedge-shaped opening at the anterior end.

Additional key words: molting, exuvium, fluorescence


Invertebrate Biology 121(2):148-157
© 2002 American Microscopical Society, Inc.

Morphology of stomatopod chemosensory sensilla facilitates fluid sampling Abstract. Stomatopods, like many marine crustaceans, rely on their sense of smell to detect prey and to find mates (Ache 1982; Zimmer-Faust 1989; Atema & Voigt 1995). In lobsters, crabs, crayfishes, prawns, leptostracans, anaspidans, mysids, amphipods, tanaids, isopods, ostracodes, phyllopods, and cumaceans (Heimann 1984; Hallberg et al. 1992), this detection of odors from distant sources involves specialized chemosensory setae called aesthetascs located on the antennules. The external structure of stomatopod sensilla appears to follow the typical crustacean aesthetasc pattern, but their internal structure has not been previously examined. In this study, we use serial reconstruction from transmission electron microscopy to show that the stomatopod sensilla are aesthetascs. For chemoreception to occur, chemical-containing fluid must be very close to the surface of the aesthetascs, such that odor molecules can diffuse to chemoreceptors on the olfactory receptor neurons inside the aesthetasc. Flicking of stomatopod antennules maximizes fluid penetration near the parts of the sensilla where the cuticle is thinnest, and where the outer dendritic segments are most fully branched with the greatest surface area. Thus, the external and the internal structure of the stomatopod aesthetasc are "matched" to maximize the efficiency of odor arrival at the surface of the olfactory receptor neurons.

Additional key words: aesthetasc, flow


Invertebrate Biology 121(2):158-162
© 2002 American Microscopical Society, Inc.

Are the two physiological races of Pollicipes polymerus (Cirripedia) genetically divided along the California coast? Abstract. Genetic work on numerous species with planktonically dispersing larvae has demonstrated gene flow throughout populations along the California coast, despite steep environmental gradients around Point Conception. Previous ecological work on a planktonically dispersing intertidal barnacle, Pollicipes polymerus, demonstrated that this species is divided into two physiological races near Point Conception, at Point Latigo. The northern race broods when water temperature either warms or cools to 14°C and the southern race broods as water warms to 20°C. I examined 8 polymorphic allozyme loci from five populations between Santa Cruz and San Diego to test if the two physiological races are genetically divided. No genetic differences were found over this geographic range, supporting a previous molecular study. These results suggest that reproductive differences between the two races of this stalked barnacle may result from plasticity in brooding activity.

Additional key words: allozymes, brooding, gooseneck barnacle, population structure


Invertebrate Biology 121(2):163-169
© 2002 American Microscopical Society, Inc.

Fusion between incompatible colonies of a viviparous ascidian, Botrylloides lentus Abstract. The mode of allo-recognition among colonies was investigated in a viviparous ascidian, Botrylloides lentus. Each embryo is enveloped by a pouch of epithelial cells (brood pouch), and is brooded in the vascular lumen of the parental colony. That is, the parental colony tolerates the presence of semi-allogeneic conspecifics (embryos) in the vascular system. A rejection reaction occurs when incompatible colonies are brought into contact at their growing edges. The inflammatory rejection reaction is limited to a small area where the tunic of two colonies has partially fused. On the contrary, incompatible colonies fuse and their blood vessels become interconnected with one another, when they are brought into contact at artificially cut surfaces. This mode is the same as those of other viviparous species of Botrylloides, B. fuscus and B. violaceus. A relationship between viviparity and the loss of allo-recognition in the vascular system is suggested.

Additional key words: self or non-self recognition, surgical fusion, subcuticular rejection