Defining the Doratopsis: Investigating the Endpoint of the Paralarval Stage in Chiroteuthis Calyx
Master's Thesis (Open Access)
Master of Science (M.S.)
Moss Landing Marine Laboratories
The life histories of open ocean squid are often poorly understood, due to the difficulty of collecting and observing them in the wild. In order to understand life histories fully, species must be identifiable at all life stages. In cephalopods there is uncertainty surrounding the endpoint of the paralarval stage, and this boundary must consequently be determined on a species by species basis. Some common morphological events used as markers of this life stage change are the loss of paralarval characters, and rapid changes in the relative size and growth of certain key structures. Alternatively, the end of the paralarval stage is sometimes defined ecologically, by changes in predatory behavior, diet, and distribution.
In this study I take a comprehensive survey of all possible markers of the end of the paralarval stage in Chiroteuthis calyx, the Swordtail Squid. This squid is abundant in the mesopelagic community of Monterey Bay, California, and is significant in the diet of local predators. Its paralarvae, like those of other chiroteuthids, are known as “doratopses,” readily identified by their long and ornate tails. Live specimens were collected via Remotely Operated Vehicle and Tucker Trawl from the Monterey Bay Submarine Canyon. Preserved specimens were obtained from the Monterey Bay Aquarium Research Institute’s (MBARI) collection. Additionally, animals captured vii within the Video and Annotation Referencing System (VARS) at MBARI were analyzed and equations were derived to estimate their mass and tail length.
This species has three paralarval characters, the brachial pillar, tail, and paralarval club, all of which are lost at some point during the transition from doratopsis to sub-adult. The presence and absence of these three characters were plotted in a logistic regression, and a non-metric Multidimensional Scaling analysis. The loss of the three characters were found to be simultaneous, meaning that the historical morphological marker of the doratopsis, the tail, is an appropriate signifier of the end of the paralarval stage. The loss of the paralarval club and subsequent development of the adult tentacle club has a significant effect on predatory behavior, as the adult clubs are equipped with photophore lures for catching fish. The length of the tail is observed to be variable amongst paralarvae, and length was examined with respect to mass to look for trends in size. It was found that tail length does not correlate with mass, and so tail length is not an appropriate tool with which to estimate size or age.
Relative growth of key structures was examined using an Analysis of Covariance (ANCOVA) between life stages (as determined by presence of the paralarval club), with Dorsal Mantle Length (mm) as the independent variable. These structures included those associated with body shape, the feeding apparatus, and locomotion. The results indicated consistent changes in the size and relative growth of the feeding apparatus and body shape during the metamorphic transition between paralarvae and sub-adult. The fin length and shape did not exhibit significant changes during metamorphosis, but the fin width did enlarge relative to the body size in sub-adults. These results indicate a potential change in habitat, activity level, and predator-prey interactions following a shift viii from a long and cylindrical morphology (with a tail mimicking a common siphonophore) to a wider and more spherical body shape. The reduction of hydrodynamic efficiency and changes in tentacle morphology indicates a shift to a more sedentary lifestyle and a new mode of feeding.
The adults have been observed eating myctophid fish, but the diet of the paralarvae is unknown. To examine dietary change, pigmentation of the digestive gland was used for a qualitative comparison. Again life stage was designated according to presence of the paralarval club. The majority of paralarvae had red digestive glands, while sub-adults had a majority of orange or yellow digestive glands, suggesting a change in diet from pigment-rich zooplankton to fish which was associated with morphological changes consistent with shifts in prey capture techniques.
Trends in depth distribution were examined with both a gradual ontogenetic descent, and diel vertical migration (DVM) in mind. A VARS query for all Chiroteuthis calyx observed from 1998-2013 revealed a significant difference in mean depth between life stages. To examine differences in variability that would indicate vertical mobility in the water column, depth vs mass for specimens in this study were examined and found to not only have a difference in mean depth, but much higher variability in the paralarvae indicated that the behavior of DVM was present in the paralarvae but not the sub-adults.
The results of this study reveal a coordinated process of metamorphosis involving morphological and ecological factors. The difference between paralarvae and adults is therefore drastic both in their appearance and their behavior, suggesting that perhaps the typical description of squid development as gradual is not always true.
Bitondo, Alicia, "Defining the Doratopsis: Investigating the Endpoint of the Paralarval Stage in Chiroteuthis Calyx" (2016). Capstone Projects and Master's Theses. 75.