METAMORPHOSIS OF LIMULUS POLYPHEMUS TRILOBITE LARVAE: ROLE OF CHEMICAL AND STRUCTURAL CUES, COMPETENCY, AND THE COST OF DELAYED METAMORPHOSIS
Kronstadt, Stephanie M.
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Many marine and estuarine arthropods exhibit complex life cycles that include planktonic larval stages and benthic juvenile and adult phases. Chemical and structural cues associated with juvenile habitats often induce settlement and metamorphosis, thereby shortening the duration of the larval phase. These cues can trigger metamorphosis only after larvae reach competency, or developmental maturity. The point at which larvae reach this competency period and the ability to retain competency is highly species specific. Once competency is attained, a decrease in the time to metamorphosis (TTM) can decrease dispersal potential but may increase the chance of settling in a suitable habitat. Alternatively, an increase in TTM (delayed metamorphosis) may enhance dispersal and the possibility of finding a more suitable habitat. However, delaying metamorphosis may reduce energy stores, affecting growth and survival in later life stages. The American horseshoe crab (Limulus polyphemus) inhabits estuarine and coastal habitats from the Yucatan Peninsula to northern Maine. It possesses a single lecithotrophic larval stage (trilobite) that molts, or metamorphoses, to a benthic juvenile stage. Metamorphosis is accelerated in the presence of chemical cues from several habitat-associated cues, including conspecifics and the seagrass Halodule wrightii (Boleman 2011). This thesis research further examined the effect of these two habitat-associated sources (i.e., conspecifics and H. wrightii) on metamorphosis of L. polyphemus. The first set of experiments tested the hypotheses that (1) the effect of both cues on time to metamorphosis (TTM) is dose-dependent, (2) the molecule(s) responsible for inducing metamorphosis is (are) a thermally-stable, low molecular weight compound(s), similar to those found to induce metamorphosis in other marine invertebrate species, and (3) L. polyphemus larvae also respond to structural cues by reducing the TTM. When larvae were exposed to conspecific- and H. wrightii-exudate water at concentrations between 0.3-30 g L-1, TTM declined in all treatments, even at the lowest concentration tested (0.3 g L-1). The observed dose-dependent effect on TTM suggests that the chance of metamorphosis increases as larvae approach a chemical source (i.e., juvenile population or seagrass bed). Heating and cooling (-70 °C or 100 °C) exudate water did not alter or reduce the potency of the molecules responsible for inducing metamorphosis, suggesting that the molecule(s) are thermally stable. Trilobite larvae exposed to exudate water dialyzed through membranes of different pore sizes (0.5-1.0 kDa, 8.0-10 kDa, and 100 kDa) responded similarly to all size fractions, suggesting the effective molecules in both source waters were relatively small (< 0.5 kDa) compounds. Finally, artificial H. wrightii structure decreased TTM in L. polyphemus trilobite larvae, indicating that larvae respond to both chemical and structural cues. There was no additive or synergistic effect when H. wrightii structural and chemical cues were combined, suggesting that there is a hierarchy of cues in which the chemical cue takes precedence (at a concentration of 30 g L-1). In order to determine the effect of timing of exposure to chemical cues on the metamorphosis of L. polyphemus, a second series of experiments tested the hypotheses that (1) trilobite larvae become competent within a few days in the plankton, and (2) delaying exposure to cues (i.e., delaying metamorphosis) negatively impacts post-metamorphic size, shape, and survival of L. polyphemus juveniles. The beginning of the competency period was determined by measuring time required for 25% of larvae to metamorphose (TTM25) after exposure to a known inducer of metamorphosis (conspecific exudate). To determine the effect of delayed metamorphosis on competency, larvae were exposed to conspecific cues either immediately following hatching (control) or at delay intervals of 7, 14, 21, and 28 days post-hatching. Larvae in the control and 7-day delay treatment had similar patterns of metamorphosis. In both treatments, TTM25 was 16 days, suggesting that larvae become competent about 16 days post-hatch. The effect of delayed metamorphosis on post-metamorphic size and survival was examined by measuring survivorship (%), the molt-stage duration (MSD), prosoma length (PL), prosoma width (PW), and the shape (PL: PW) for the first three juvenile instars (J1-J3) of crabs in the control, 21-day, and 28-day delay treatments. Delaying metamorphosis had no significant effect on survivorship, MSD, and PL for any of the treatments. However, third juvenile instars (J3) that were in the 28-day delay treatment were significantly narrower than those in the control. This difference resulted in individuals that were slightly more circular in shape (close to a 1:1 PL: PW ratio) when compared with juveniles in the control (close to a 1:2 PL: PW ratio). This study demonstrated that delaying metamorphosis of trilobite larvae had no lethal effects, and minimal sublethal effects, on later life stages. Therefore, extending the larval phase in order to find a suitable habitat may be an adaptive advantage for L. polyphemus.