Search

What are the greatest sizes that the largest marine megafauna obtain? This is a simple question with a difficult and complex answer. Many of the largest-sized species occur in the world's oceans. For many of these, rarity, remoteness, and quite simply the logistics of measuring these giants has made obtaining accurate size measurements difficult. Inaccurate reports of maximum sizes run rampant through the scientific literature and popular media. Moreover, how intraspecific variation in the body sizes of these animals relates to sex, population structure, the environment, and interactions with humans remains underappreciated. Here, we review and analyze body size for 25 ocean giants ranging across the animal kingdom. For each taxon we document body size for the largest known marine species of several clades. We also analyze intraspecific variation and identify the largest known individuals for each species. Where data allows, we analyze spatial and temporal intraspecific size variation. We also provide allometric scaling equations between different size measurements as resources to other researchers. In some cases, the lack of data prevents us from fully examining these topics and instead we specifically highlight these deficiencies and the barriers that exist for data collection. Overall, we found considerable variability in intraspecific size distributions from strongly left- to strongly right-skewed. We provide several allometric equations that allow for estimation of total lengths and weights from more easily obtained measurements. In several cases, we also quantify considerable geographic variation and decreases in size likely attributed to humans.

Background: The marine environment is comprised of numerous divergent organisms living under similar selective pressures, often resulting in the evolution of convergent structures such as the fusiform body shape of pelagic squids, fishes, and some marine mammals. However, little is known about the frequency of, and circumstances leading to, convergent evolution in the open ocean. Here, we present a comparative study of the molluscan class Cephalopoda, a marine group known to occupy habitats from the intertidal to the deep sea. Several lineages bear features that may coincide with a benthic or pelagic existence, making this a valuable group for testing hypotheses of correlated evolution. To test for convergence and correlation, we generate the most taxonomically comprehensive multi-gene phylogeny of cephalopods to date. We then create a character matrix of habitat type and morphological characters, which we use to infer ancestral character states and test for correlation between habitat and morphology., Results: Our study utilizes a taxonomically well-sampled phylogeny to show convergent evolution in all six morphological characters we analyzed. Three of these characters also correlate with habitat. The presence of an autogenic photophore (those relying upon autonomous enzymatic light reactions) is correlated with a pelagic habitat, while the cornea and accessory nidamental gland correlate with a benthic lifestyle. Here, we present the first statistical tests for correlation between convergent traits and habitat in cephalopods to better understand the evolutionary history of characters that are adaptive in benthic or pelagic environments, respectively., Discussion: Our study supports the hypothesis that habitat has influenced convergent evolution in the marine environment: benthic organisms tend to exhibit similar characteristics that confer protection from invasion by other benthic taxa, while pelagic organisms possess features that facilitate crypsis and communication in an environment lacking physical refuges. Features that have originated multiple times in distantly related lineages are likely adaptive for the organisms inhabiting a particular environment: studying the frequency and evolutionary history of such convergent characters can increase understanding of the underlying forces driving ecological and evolutionary transitions in the marine environment.

A new species of dicyemid mesozoan is described from Octopus hubbsorum Berry, 1953, collected in the south of Bahia de La Paz, Baja California Sur, México. Dicyema guaycurense n. sp. is a medium-size species that reaches about 1,600 µm in length. It occurs in folds of the renal appendages. The vermiform stages are characterized as having 22 peripheral cells, a conical calotte, and an axial cell that extends to the base of the propolar cells. Infusoriform embryos consist of 39 cells; 1 nucleus is present in each urn cell and the refringent bodies are solid. This is the first of a dicyemid species from a host collected in the Gulf of California.

Aggregata bathytherma sp. nov. is described from the digestive tract of Vulcanoctopus hydrothermalis, a deep-sea octopus recently discovered associated with hydrothermal vents in the northeast Pacific Ocean. Oocysts typically are spherical in shape, sometimes irregular, 163 to 356 microm in length, and 219 to 313 microm in width. Each oocyst contains from 50 to over 200 sporocysts. Sporocysts measure 27 to 32 microm in longest diameter. The cyst wall is smooth and 1 microm thick. Each sporocyst typically contains 14 to 17 sporozoites, 49 microm in length. Histological lesions associated with the presence of A. bathytherma include rupture of the basal membrane and detachment of the epithelial cells. In heavily infected areas, most of the tissue of the host digestive tract is replaced by parasites. A. bathytherma is the first Aggregata species described from a host that lives in association with hydrothermal vents, and the third species of Aggregata from eastern North Pacific waters.

Cell numbers and cellular composition were examined in infusoriform larvae of 44 species of dicyemid mesozoans belonging to 6 genera; Conocyema, Dicyema, Dicyemennea, Dicyemodeca, Microcyema, and Pseudicyema. In addition, literature on infusoriform larvae of another 20 species was reviewed. Infusoriform larvae consist of a constant cell number which is species-specific. Small interspecific variations are found in total cell numbers, 35, 37, 39, 41 and 42. The most frequent cell number encountered in infusoriform larvae studied is either 37 or 39. Infusoriform larvae with 35 cells are found in three genera. Infusoriform larvae with 37 cells are found in four genera. Infusoriform larvae with 39 cells are found in four genera. Most differences in total cell numbers are due to the absence or presence of particular ventral cells. In all infusoriform larvae, the lateral, dorsal and caudal areas are cell constant, whereas in the apical and ventral areas a distinct and variable configuration of cells are present. In cellular composition, a total of 29 cells (15 cell types) were recognized in all infusoriform larvae examined. Additional cell types are characteristic of a relatively few species. Even in infusoriform larvae with the same total cell numbers, cellular composition varies by species. Thus, there are 7 variations of cellular composition in infusoriform larvae with 37 cells. Differences in larval cell numbers and types do not warrant traditional generic separation of dicyemids.

Two new species of dicyemids are described from 2 species of deep benthic cephalopods, Graneledone antarctica and G. macrotyla, collected in the Southern Ocean south of the Antarctic Convergence. Dicyemennea bathybenthum n. sp. was found in G. antarctica. It is a medium-sized dicyemid whose length does not exceed 1,000 microm. The calotte is bluntly rounded or conical in small individuals, but the shape becomes discoidal in large individuals. Nematogens and vermiform embryos have 23 peripheral cells. An anterior abortive axial cell is present in vermiform embryos. Other stages in the life cycle were not observed. Dicyemennea dorycephalum n. sp. was found in G. macrotyla. It is a medium to large dicyemid that rarely exceeds 4,000 microm. The calotte is distinctly pointed, similar in shape to a spearhead. Vermiform stages typically have either 25 or 27 peripheral cells. An anterior abortive axial cell is present in vermiform embryos. Infusoriform embryos have 37 cells, the refringent bodies are solid in composition, and 2 nuclei are present in each urn cell.

Nematogens and vermiform embryos of a new species of Dicyema are described from an octopus collected off Veracruz, Mexico. Dicyema shorti n. sp. is a small dicyemid species that rarely exceeds 500 microm in length. It is further characterized by the presence of 18 peripheral cells in the vermiform stages, a conical-shape calotte, and an axial cell that extends to the base of the propolar cells. Other stages in the life cycle of the parasite are not known. This is the first dicyemid to be described from Octopus burryi Voss. 1950, and also from both the southern Gulf of Mexico and the country of Mexico.

Dicyemennea canadensis n. sp. is described from a bathyal octopus collected off Canada in the Bay of Fundy. The dicyemid is a small species that rarely exceeds 600 microm in length. The vermiform stages are further characterized as having 17-23 peripheral cells, a conical-shaped calotte, an axial cell that extends to the base of the propoplar cells, and no abortive axial cells. Infusoriform embryos consist of 37 cells. There is 1 nucleus in each urn cell, and refringent bodies are absent. This is the first dicyemid to be described from the cephalopod Bathypolypus arcticus (Prosch. 1847), and the first dicyemid reported from Canada. In addition, it is the first species of Dicyemennea from the northwestern Atlantic Ocean to be described.

Patterns of cell division and cell lineages of the vermiform embryos of dicyemid mesozoans were studied in four species belonging to four genera: Conocyema polymorpha, Dicyema apalachiensis, Microcyema vespa, and Pseudicyema nakaoi. During early development, the following common features were apparent: (1) the first cell division produces prospective cells that generate the anterior peripheral region of the embryo; (2) the second cell division produces prospective cells that generate the posterior peripheral region plus the internal cells of the embryo; (3) in the lineage of prospective internal cells, several divisions ultimately result in cell death of one of the daughter cells. Early developmental processes are almost identical in the vermiform embryos of all four dicyemid genera. The cell lineages appear to be invariant among embryos and are highly conserved among species. Species-specific differences appear during later stages of embryogenesis. The number of terminal divisions determines variations in peripheral cell numbers among genera and species. Thus, the numbers of peripheral cells are fixed and hence species-specific.