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Among starfishes, fascinating life cycles and complex morphological patterns have evolved within a familiar but unusual basic body plan. In spite of these rich complexities, available phylogenetic interpretations conflict, and the history of this important group remains uncertain. The symposium brought together current perspectives on phylogeny, the implications of certain poorly known aspects of asteroid morphology, and consideration of significant events that preceded the diversification of extant asteroid groups in the Mesozoic.
It has become axiomatic in modern organismal biology that strongly supported phylogenetic reconstructions are crucial to the understanding of biological pattern and progress. Although asteroids exhibit complex morphologies, life cycles, and behaviors that indicate their status as ideal model organisms in the study of marine invertebrates, their evolutionary history remains obscure. This is in part due to a lack of treatment by researchers but also in part due to inherent limitations of the available data.
In the following pages, the state of inquiry into the study of relationships among starfishes is arranged in three sections: the nature of the fossil record (an introduction to modern starfishes), taxa and morphology, and phylogenetic interpretations.
Because of limited research, a generally accepted hypothesis has not emerged for the phylogeny of the Asteroidea. The fossil record is a potential source of needed data, although fossil asteroids are rare, and they tend to be poorly preserved.
Emphasis in the taxonomy of both recent and fossil asteroids has been on characters visible from the exterior, and paleontologists have sought to fit even the most ancient (i.e., Ordovician) specimens into taxonomic ordinal schemes devised for recent asteroids. Animal form and arrangement of body wall ossicles of Paleozoic asteroids can be similar to those of younger species, thereby suggesting close affinities, yet ambulacral arrangements indicate clear separation of Paleozoic stem groups from the crown group.
Traits taken from the ambulacral column that mark crown-group asteroids include presence of dorsal podial pores (which allowed transfer of the ampullae to the arm interior), an offset arrangement of ambulacrals on the adambulacrals, and increased complexity of the articulation structures between ambulacrals and adambulacrals. Transfer of ampullae to the arm interior provided protection and more space for ampullae within the arm, as well as space within the furrow and between the ambulacral and adambulacral ossicles for elaboration of the soft tissues that enhance arm motion.
The Extraxial-Axial Theory (EAT) is applied to the body wall homologies of asteroids. Attempts to characterize major plate systems of asteroids as axial or extraxial, particularly those that are highly organized into series, can be problematic. However, the Optical Plate Rule (OPR) is instrumental in establishing that ambulacrals and terminals are axial. It is equally clear that the region aboral to the marginal frame is a part of the perforate extraxial body wall (with the possible exception of the centrodorsal, which is likely imperforate extraxial). Previously established EAT criteria, particularly those strongly rooted in the embryologically expressed boundary between axial and extraxial body wall in larvae, suggest that marginals, and perhaps adambulacrals, are extraxial in origin. We also explore the extraxial nature and phylogenetic significance of the odontophore. Our data from both juveniles and adults show that plate and tube foot addition sequences occur according to the OPR, and shed light on poorly known homologies of the asteroid mouth frame. These data indicate that the mouth angle ossicle must at least contain the first ambulacral, although we cannot rule out the possibility that the first adambulacral also contributes to the construction of this ossicle. The interpretations provided by the EAT for all ossicles suggest a synapomorphy scheme for somasteroids, ophiuroids, and asteroids.
Multiradiate starfish evolved independently in fourteen living families. Twenty living families are strictly 5-rayed. The FIVE-PLUS hypothesis is that supernumerary rays develop separately from the five primary rays. The ontogeny of the primary rays is proposed to be highly integrated (“en bloc” hypothesis), closely timed (synchronic hypothesis) and a developmental constraint (“tamper-proof” hypothesis). The “en bloc” hypothesis postulates that the five primary rays develop as a unit. The deep structure of this unit is believed to be a 2-1-2, BA-A-BA, organization. The synchronic hypothesis postulates that there is only a brief time at metamorphosis during which the “en bloc” pathway operates. There is a pause before the development of supernumerary rays. The “tamper-proof” hypothesis postulates that the “en bloc” pathway has no heritable variation and cannot be co-opted for the production of supernumerary rays. There is diversity of timing and pattern in the development of supernumerary rays. Postgeneration of rays in the rudiment and intercalary regeneration of rays in the imago are independent ray-producing pathways that may have been co-opted variously and recurrently in the multiple origins of multiradiate starfish.
Tube-foot morphology has been included among a variety of taxonomic criteria for the Asteroidea over the past twenty-five years. Other than a few families belonging to the order Paxillosida, which are thought to have pointed, non-suckered tube feet that are used for digging and burial in soft sediments, the presumption has been that asteroids have flat-tipped, suckered tube feet. This has become an accepted model despite the fact that the comparative morphology of asteroid tube feet has not been considered. In the present study we examine tube-foot morphology of 45 species of Asteroidea representing 19 families. Our analysis confirms that members of the Luidiidae and Astropectinidae (order Paxillosida) lack suckers on the tips of their pointed tube feet. We demonstrate that there is considerable variation in tube-foot morphology among members of the Asteroidea including an entirely new type of flat-tipped, non-suckered tube foot in species belonging to the order Valvatida. The external morphology of tube feet in species belonging to the order Velatida could not be distinguished from “typical” flat-tipped, suckered tube feet; nonetheless, histological sections revealed a distinctive internal morphology. Finally, we report the first observations of the tube-foot morphology of representatives of deep-sea asteroids belonging to the orders Notomyotida and Brisingida, a group that also lacks the typical flat-tipped, suckered tube-foot morphology. The results of our study demonstrate that the current tube-foot morphology model needs to be reconsidered, as there is considerably greater variation than was previously believed to be the case. Moreover, we conclude that while tube-foot morphologies show consistent similarities within orders, tube-foot morphology is less appropriate as a taxonomic character below this level.
Among diverse marine invertebrate taxa, the evolution of a non-planktotrophic larva is often associated with increased rates of cladogenesis, compared to related taxa that retain the ancestral planktotrophic larval form. Molecular phylogenetic analyses of non-planktotrophic (and, presumably species-rich) clades of marine invertebrates are rare. Here we analyze 1542 base pairs of mitochondrial DNA sequence comprising two gene regions, the cytochrome oxidase I gene and the putative control region and flanking sequences, for 23 molecular lineages in the obligately brood-protecting asteroid genus Leptasterias. Using maximum likelihood, minimum evolution, and maximum parsimony methods, five major clades were identified that corresponded to five taxa (species or species complexes) in the subgenus Hexasterias, section camtschatica (following the taxonomy of Walter K. Fisher). Two clades (L. aequalis> complex and L. aleutica/L. camtschatica complex) were composed of numerous molecular lineages (7–8 lineages/clade), and several clades had multiple shallow nodes, suggestive of recent radiations. Two of the clades (L. aleutica/L. camtschatica complex and L. hexactis complex), with geographic ranges restricted to latitudes higher than 48°N, were lacking deep phylogenetic nodes. This pattern is consistent with the hypothesis that high-latitude taxa have high rates of extinction due to repeated climatic crises. A log-likelihood ratio test performed on the camtschatica section, including a member (Leptasterias polaris) of the polaris section and using a representative (L. mülleri) of Leptasterias subgenus Leptasterias as an outgroup, demonstrated that the camtschatica section is monophyletic.
The superorder Forcipulatacea (Asteroidea, Echinodermata) includes two orders, the Brisingida and the Forcipulatida. The Forcipulatida is diverse, including the Asteriidae, Coscinasteriinae, Pedicellasteridae, Labidiasteridae, Neomorphasteridae, Pycnopodiinae (Asteriidae), Heliasteridae, and the Zoroasteridae, whereas the Brisingida is limited to the Brisingasteridae, Brisingidae, Freyellidae, Hymenodiscidae, and Odinellidae. A phylogenetic analysis of forcipulataceans using morphological characters resulted in 12 most parsimonious trees at a tree length of 68 steps.
The Brisingida, recognized as basal in one early analysis and derived in another, is here considered to be derived. Two genera of pedicellasterids emerged as the sister group to the remaining Forcipulatida.
Bremer and bootstrap measures show strong support for the brisingidan and zoroasterid plus neomorphasterid clades. Certain other traditional taxonomic groupings, including the Pedicellasteridae, Labidiasteridae, Asteriidae, and Pycnopodiinae, are not supported as monophyletic. Support for the pedicellasterids as a sister group to the remainder of the Forcipulatida is not robust.
Morphological data suggest widespread homoplasy and therefore comprehensive generic or even species-level analyses are required to further evaluate questions of derivation and relationships among these taxa.
Phylogenetic approaches have sparked controversy in asteroid systematics since 1987. Despite recent attempts at resolving these differences and evidence of some consensus, our understanding of relationships among asteroid taxa remains unsatisfactory. This paper presents results of an investigation into asteroid evolutionary history using DNA sequence data from mitochondrial transfer RNA and the cytochrome oxidase c subunit I genes analyzed with and without previously published ribosomal gene sequences. Analysis of these genes provides an assessment of familial relationships but does little to elucidate ordinal relationships. A basal position for the Paxillosida is not supported. However, close relationships of some velatid and valvatid taxa are upheld. The resulting phylogenies are not a definitive answer to controversies in asteroid systematics. However, with new insights to some asteroid relationships, they highlight the need for a redirection of future systematic studies so a consensus can be made.
Often, as environmental endocrinologists, we observe animals in nature with the goal of describing their normal endocrinology. However, the contamination of virtually all natural habitats by chemicals of anthropogenic origins (e.g., PCBs, organochlorines, phytoestrogens, alkyphenols, heavy metals) that might alter endocrine conditions suggests we need to reevaluate many of our field studies with respect to points of reference or controls. The impaired response of the stress axis of feral brown trout, Salmo trutta, correlated with chronic exposure to heavy metals is examined as a case in point although the problems extend to other hypothalamic axes as well. Our studies emphasize that measurement of one static endocrine parameter to assess the health of any hypothalamus-pituitary axis (e.g., plasma cortisol levels to indicate stress) should not be used as a biomarker for field studies.
The developing organism is particularly sensitive to exposure to estrogenic chemicals during a critical period in the induction of longterm changes in female reproductive organs, and persistent molecular alterations induced by the perinatal estrogenic agents. The perinatal mouse model can be utilized as an indicator of possible longterm consequences of exposure to exogenous estrogenic compounds including environmental endocrine disruptors. Attention should be paid to abnormalities in female genital organs exposed to estrogenic endocrine disruptors during fetal and early postnatal development in mammals including humans.
In the present paper, we are reviewing experimental evidence demonstrating that phagocytic cells, such as macrophages, may be used as a biomarker of immunotoxicity in wildlife studies. We will first present data obtained after exposure in vitro with selected chemicals showing the comparative sensitivity of phagocytic cells from different species. These results demonstrate that, at least for metals, each species produce a similar shaped dose-response curve, although considerate interspecies sensitivity is evident. These results also demonstrate the sensitivity of the phagocytic activity, suggesting indeed that this function could be used to monitor exposure to chemicals. The similar shaped dose-response curves imply that mechanisms of action may also be similar. Furthermore, based on the relative speices sensitivity, sentinel species could be selected for field monitoring. Such an approach may also be useful to establish correction factors required to extrapolate results between species. This sensitivity of the phagocytic activity of macrophages will be further under controlled conditions in laboratory animal models. Finally, the reliability of this approach will be demonstrated using case studies with wildlife species.
Current risk assessment techniques for almost all chemicals involve establishing a threshold dose, or the dose below which no adverse effects are seen. But threshold doses may not apply in situations where the chemical mimics the actions of an endogenous compound—such as a steroid hormone—that is important to development. In such cases, any exposure may exceed the threshold. Recent studies with the red-eared slider turtle have shown that exogenous estradiol—even when applied to eggshell in doses as low 0.4 ng—will affect sex development during embryogenesis. Considering that only 0.2% of the estradiol applied to the eggshell ends up in the embryo, it becomes apparent that even very low dosages of steroid hormones or their mimics can have profound biological effects. We tested this idea using eight compounds identified in the yolk of alligator eggs from Lake Apopka, Fla. Five of the compounds—the PCB mixture Aroclor 1242,trans-Nonachlor,cis-Nonachlor, p,p′-DDE, and chlordane—altered sex ratio outcomes when applied to eggshells during development. Aroclor 1242 produced the most powerful effects, shifting the ratio of females almost twofold, while chlordane had the greatest effect when combined with estradiol. Administration of all eight compounds together also increased the ratio of females to males. However, comparing the single-compound exposures at the same dosages indicate that these compounds behave differently in combination than they do singly, emphasizing the need for further studies using chemical mixtures reflecting proportions found in nature. The effect of chlordane and Aroclor 1242 on aromatase activity levels during embryogenesis in the brain and adrenal-kidney-gonad (AKG) complex was also examined. Chlordane, a suspected anti-androgen in this species, did not affect aromatase activity in either the brain or the AKG. However, Aroclor 1242 significantly altered aromatase activity levels in the red-eared slider turtle brain—but not in the AKG—during a crucial developmental period. After this crucial period, Aroclor 1242 caused an increase in aromatase activity in the AKG of embryos just prior to hatch. Additionally, hatchling males treated during embryogenesis with Aroclor 1242 and chlordane exhibited significantly lower testosterone levels than controls in response to follicle-stimulating hormone administration, while chlordane-treated females had significantly lower progesterone, testosterone, and 5α-dihydrotestosterone levels relative to controls. These results are similar to those found in juvenile alligators from Lake Apopka. Males treated with Aroclor 1242 and trans-Nonachlor displayed an elevated estradiol response to FSH administration vs. control males. Taken together, these results suggest that EDCs exert effects during embryonic development that extend beyond birth. They also suggest that the alterations in sex steroid hormone levels observed in animals from contaminated areas may result from EDC-induced alterations in the neuroendocrine axis controlling gonadal sex steroid hormone production.
Individual variability in endogenous hormones can confound the interpretation of effects of developmental exposure to endocrine disrupting chemicals. In single-birth species, such as humans, there are many sources of variability in fetal sex hormone levels, such as birth order or race. In litter-bearing species a source of fetal variability in serum levels of estradiol and testosterone is the sex of adjacent fetuses due to fetus-to-fetus steroid transport (called the intrauterine position phenomenon or IUP). Distinct phenotypes of reproductive physiology and behavior are due to IUP in house mice and other litter-bearing animals. We review here the effects of background levels of sex steroids in fetuses due to IUP in an experiment in which pregnant mice were exposed to an environmentally relevant low dose of the estrogen-mimicking chemical, bisphenol A. Bisphenol A is the monomer used to make polycarbonate plastic products (such as baby bottles), the resin lining of food and beverage cans, dental sealants, and a host of other products. Fetal exposure via the mother to bisphenol A increased the rate of postnatal growth in males and females and also advanced the timing of puberty in females. However, the greatest response to bisphenol A occurred in males and females with the highest background levels of endogenous estradiol during fetal life due to their IUP, while fetuses with the lowest endogenous levels of estradiol showed no response to maternal bisphenol A treatment. This finding suggests that estrogen-mimicking chemicals interact with endogenous estrogen in altering the course of development.
Many xenobiotic compounds introduced into the environment by human activity have been shown to adversely affect wildlife. Reproductive disorders in wildlife include altered fertility, reduced viability of offspring, impaired hormone secretion or activity and modified reproductive anatomy. It has been hypothesized that many of these alterations in reproductive function are due to the endocrine disruptive effects of various environmental contaminants. The endocrine system exhibits an organizational effect on the developing embryo. Thus, a disruption of the normal hormonal signals can permanently modify the organization and future function of the reproductive system. We have examined the reproductive and developmental endocrinology of several populations of American alligator (Alligator mississippiensis) living in contaminated and reference lakes and used this species as a sentinel species in field studies. We have observed that neonatal and juvenile alligators living in pesticide-contaminated lakes have altered plasma hormone concentrations, reproductive tract anatomy and hepatic functioning. Experimental studies exposing developing embryos to various persistent and nonpersistent pesticides, have produced alterations in gonadal steroidogenesis, secondary sex characteristics and gonadal anatomy. These experimental studies have begun to provide the causal relationships between embryonic pesticide exposure and reproductive abnormalities that have been lacking in pure field studies of wild populations. An understanding of the developmental consequences of endocrine disruption in wildlife can lead to new indicators of exposure and a better understanding of the most sensitive life stages and the consequences of exposure during these periods.