Janet R. Voight

Echinoderm Phylogeny Including Xyloplax, a Progenetic Asteroid

Reconstruction of the phylogeny of the five extant classes of the phylum Echinodermata has proven difficult. Results concerning higher-level taxonomic relationships among echinoderms are sensitive to the choice of analytical parameters and methods. Moreover, the proposal of a putative sixth class based on a small enigmatic disc-shaped echinoderm, Xyloplax, from the deep seas of the Bahamas and New Zealand in the 1980s further complicated the problem. Although clearly an echinoderm, Xyloplax did not have clear affinity among known groups. Using molecular sequence and developmental data from recently collected Xyloplax adults and embryos, we show that rather than representing an ancient distinct lineage as implied by its status as a class, Xyloplax is simply a starfish that is closely related to the asteroid family Pterasteridae. Many members of the Pterasteridae and all Xyloplax inhabit deep or polar seas and brood young. Brooding pterasterids and Xyloplax hold their young in specialized adult chambers until the young reach an advanced juvenile stage after which they are released as free-living individuals. We hypothesize that the unique morphology of Xyloplax evolved via progenesis--the truncation of somatic growth at a juvenile body plan but with gonadal growth to maturity. Although the overall phylogeny of extant echinoderms remains sensitive to the choice of analytical methods, the placement of Xyloplax as sister to pterasterid asteroids is unequivocal. Based on this, we argue that the proposed class and infraclass status of Xyloplax should be suppressed.

Egg brooding by deep-sea octopuses in the North Pacific Ocean

Videotapes made from the submersible Alvin on Baby Bare, a 2600-m-deep North Pacific basalt outcrop, and at two other deep-sea localities document that octopuses of the genera Graneledone and Benthoctopus attach their eggs to hard substrate and apparently brood them through development. The behavior of brooding females was generally similar to that of shallow-water octopuses, but the genera showed apparent differences. In addition to the high density of brooding females observed at Baby Bare, which may relate to the increased availability of exposed hard substrates for egg attachment and of prey, females are suggested to increasingly associate with hard substrates as they mature. The biology of Baby Bare may seem unduly unique because the outcrop is isolated on a sedimented plain and is among the few exposures of hard substrate other than hydrothermal vents that have been explored by submersible. On the sediment-covered ocean floor, the availability of hard substrate may strongly affect the distribution of brooding octopuses. The size and shape of boreholes in 19 of over 400 thyasirid clam shells collected from Baby Bare support the hypothesis that octopuses had preyed upon the clams.

Paleoecologic and taphonomic potential of deepsea gastropods

Bathyal gastropods, characteristic of island arc settings, are commonly represented in Cenozoic deposits. They provide an excellent basis for paleoecological and paleoenvironmental studies of the deepsea. We predict the following based on a review of the Cenozoic literature and our examination of deepsea gastropods (>200 m): 1) Taphonomic processes affecting gastropod shells may be comparable between bathyal zones and shallow water environments, 2) death assemblages from bathyal habitats may be subject to the same time-averaging processes (e.g., census and within-habitat time-averaging) as in shallower water, and 3) complex ecological interactions, such as predation and epi- and endobiosis can commonly affect large, deepsea gastropods (e.g., the archaeogastropods, Bathybembix and Gaza)

Geographic variation of shell bionts in the deep-sea snail Gaza

Recent studies of bathyal environments indicate that microhabitats contribute significantly to biodiversity. We suggest that microhabitats may also be useful in direct comparisons of biotic diversity patterns across large areas. Using a technique developed by palaeontologists, we document the diversity and abundance of taxa occurring on the hard substrate provided by gastropod shells of the bathyal (200 m) trochid gastropod Gaza. Because the shell surfaces are comparable, shell-associated biota and their diversity may be contrasted across basins with relatively few compounding variables. Animals found attached to the outside of the shells, epibionts, included folliculinids, encrusting bryozoans and foraminifera; within the shell, endobiontic spionid polychaetes had occupied the columella of the shell, apparently destroying both the protoconch and part of the shells base. Both types of bionts were common on shells from continental margins of North and South America but were wholly absent on shells from the Lesser Antilles. Hypotheses that shell depth distributions (372–910 m) and shell age, biont dispersal potential and biotic interactions contribute to the large-scale differences in biont distribution are considered but rejected in favour of the hypothesis that terrigenous sediments and their associated nutrients are primary contributors to the observed geographic pattern.

First Report of the Enigmatic Echinoderm Xyloplax from the North Pacific

More than 100 specimens of the rare and enigmatic echinoderm Xyloplax, a deep-sea wood-fall specialist described as the new class Concentricycloidea on the basis of nine specimens from New Zealand (1), were recently collected from a depth of 2675 m in the North Pacific. These first submersible-collected specimens of Xyloplax from the Pacific contrast with the submersible-collected specimens of the Atlantic’s X. turnerae in developmental mode. Embryos of the Pacific specimens—including those from New Zealand—undergo extended development in the ovary; members of the Atlantic species apparently release small embryos or eggs. In what may be an associated difference, females appear to dominate the North Pacific population in numbers and exceed males in individual size. In addition, the marginal spines of North Pacific specimens show a nonlinear relationship to body diameter on an ln-ln plot; relatively rapid growth of marginal spines in small individuals may enhance dispersal on near-bottom currents. In the Atlantic species, the sexes are equally represented (although sexual size dimorphism exists), and marginal spines grow at a constant rate. The increased biomass of females among North Pacific specimens may reflect their increased investment in each offspring. Animals specialized to live on deep-sea wood falls remain little known, both taxonomically and biologically. Nine specimens from five New Zealand wood falls collected at depths from 1057 to 1208 m were assigned to the genus Xyloplax, which has been interpreted as composing a new class of echinoderms, the Concentricycloidea (1) and, alternatively, as a highly derived genus of the Asteroidea (2 and references therein). Detailed anatomical study of the genus (3), and a brief report of population parameters (4) were based on more than 200 specimens collected from an experimental wood deployment at 2066 m off the Bahamas. The recent collection of more than 100 specimens of the genus from an experimental wood deployment in the North Pacific provides a contrast between this viviparous species and the Atlantic species with its shorter embryonic development. Despite distinct reproductive strategies, both species exploit the rare and unpredictable habitat provided by deep-sea wood falls. This study reports the population composition and growth patterns of specimens of the undescribed North Pacific species. The specimens probably represent the original population accurately because the deployments were recovered inside a closed box on the Deep Submergence Vehicle Alvin. C. Mah, University of Illinois at Urbana-Champaign, is describing the new species represented by these specimens.

Erectile tissue in an invertebrate animal: the Octopus copulatory organ

The most familiar examples of erectile tissue are the genitalia of mammals, notably the penis and clitoris. Among the soft-bodied invertebrates, erectile tissue is virtually unknown, even in sex organs. Here we report that the ligula, the copulatory organ that is a modification of an arm tip of male octopuses, is erectile in Octopus bimaculoides. The normally minute ligula was observed during an unsuccessful mating attempt to be engorged. Histological sections of the ligula reveal striking structural convergences with mammalian erectile tissue: large, well-vascularized internal cavities subdivided by networks of collagen fibres and enclosed by an array of collagen fibres. This internal structure differs markedly from the dense, three-dimensional array of muscles and connective tissues seen in the other octopodid ligulae examined. Erectile tissue may represent an evolutionary compromise between opposing selective forces. Small ligulae may be advantageous because O. bimaculoides hunts in daylight and the white-faced ligula may be conspicuous to predators. Large ligulae, however, may be advantageous if the intense sexual selection thought to occur among octopodids selects for large ligulae, which can transfer larger spermatophores that carry more sperm. An erectile ligula may minimize the impact of these opposing selective forces.

Sexual Dimorphism and Niche Divergence in a Mid-Water Octopod (Cephalopoda: Bolitaenidae)

In the translucent mid-water octopod Eledonella pygmaea, the posterior salivary glands that release proteolytic enzymes into the esophageal crop grow five times faster in males than in females. I suggest that the sexes vertically partition the water column and that large glands have evolved in males as a result of their deepwater habitat. Members of the species undergo ontogenetic vertical descent and are suggested to mate at the lower end of the adult depth range where receptive females signal males with light organs. Selection for increased fitness is inferred to result in females increasing their fecundity by feeding at the upper limit of the adult range and in mature males increasing their encounters with mates by living at depths where mating occurs. To further increase their fitness, mature males--despite occurring in a prey-limited habitat--must expend energy to visually detect potential mates, to travel over wide areas, and to attempt to copulate. To increase the energy available to them, males at depth may exploit bioluminescent prey. The large glands protect the translucent males from increased predation by physically blocking light emitted by bioluminescent prey in their crops, and by speeding digestion.

Morphometric Analysis of Male Reproductive Features of Octopodids (Mollusca: Cephalopoda)

Taxonomic accounts of octopodids frequently describe the spermatophore, the penis that releases the spermatophore from the internal organs, and the ligula and calamus that transfer it to a female. To explore relationships among these male features and body size, this study applies principal components analysis to data from 43 species of the family Octopodidae, or benthic octopuses. Covariation in penis and mantle length opposed by covariation in ligula and calamus lengths forms primary shape variation. Secondary shape variation is due to opposing variation between ligula and calamus lengths. Primary shape variation is greatest among shallow-water species. The calami and ligulae of diurnal and crepuscular shallow-water species are short compared to those of nocturnal shallow-water species. Because these structures contain heterogeneous collagen arrays and lack camouflaging chromatophore organs, they are white. Diurnal and crepuscular octopus species may minimize their lengths due to selection imposed by visual predators. Secondary shape variation is greater in deep-sea and high-latitude octopuses. Members of Voss’s Eledoninae (except Eledone) and Graneledoninae and two species of Benthoctopus have exceptionally long calami and comparatively short ligulae; these lengths vary among members of the Bathypolypodinae. Variation in spermatophore length is independent of the structures considered.

Meristic variation in males of the hydrothermal vent octopus, Muusoctopus hydrothermalis (Cephalopoda: Octopodidae)

Intraspecific variation in characters such as arm sucker and gill lamellae counts in octopodids is yet to be thoroughly investigated, potentially hampering our ability to recognize species. In this study, data from 13 specimens of Muusoctopus hydrothermalis collected at four hydrothermal vents on the East Pacific Rise between 8°38′N and 12°48′N are considered. Although the northern and southern octopuses differ minimally in size, mean sucker count by arm in the northern group is 11.7 to 22.8% higher than it is in the southern group; in addition these octopuses typically have an additional gill lamella and bulkier funnel organs. The arms of each individual carry a different number of suckers. The difference is significant on nonadjacent arms, a pattern that merits examination in a broader taxonomic context. Why these differences exist among conspecifics remains unknown, the incidence of parasitic copepods is not different between the groups and the between-group variation in arm suckers seen here compares well with a previous report of variation among 18 specimens from the type locality. Increases in meristic characters (counts) in fish are attributed to lower temperatures during embryonic development following Jordans rule. Northern and southern vents offer the octopuses a wide temperature range, but vent fluid chemistry differs. Northern vent fluids may be more toxic; if so, developing octopus embryos may survive only minimal vent fluid exposure and therefore develop at low temperatures. At the less toxic southern vents, eggs may survive greater exposure to vent fluids and thus develop at higher temperatures.

Cholidya polypi (Copepoda: Harpacticoida: Tisbidae), a parasite of deep-sea octopuses in the North Atlantic and northeastern Pacific

Abstract The female of the harpacticoid Cholidya polypi Farran, 1914, is redescribed and the male is described for the first time. This copepod parasite of octopuses, reported previously only from Benthoctopus (= Polypus) ergasticus off Ireland, is here documented from the North Atlantic east of Florida on Tetracheledone spinicirrus and off North Carolina and New Jersey on Bathypolypus anticus, and from the northeastern Pacific west of Oregon and Washington on Graneledone pacifica and off central California on Graneledone spp.