An Intermetamorphic Larval Stage of a Mantis Shrimp and Its Contribution to the 'Missing-Element Problem' of Stomatopod Raptorial Appendages
Annual Research & Review in Biology,
In eumalacostracan crustaceans the thoracic appendages usually have seven elements along the main axis of the appendage. Mantis shrimps are an exception: their raptorial sub-chelate maxillipeds (anterior thoracopods) have only six such elements. Hence it has been speculated which two of the original seven elements have become conjoined in mantis shrimps. So far this question remains unanswered due to the lack of a proper reference point of identifying individual original elements. One candidate for such a reference point would be the exopod, which is unfortunately absent in adult stomatopods. Antizoea larvae possess exopods on the maxillipeds, but lack subdivision along the main axis of the appendage. We describe here a specimen that is right in the transition between the antizoea larval phase and the next larval phase (erichthus). It still possesses an exopod, but also additionally a subdivision into discrete elements on the maxillipeds. With this it provides an important reference scheme for solving the elemental identity in mantis shrimp maxillipeds. Our study aims at contributing new data to the identification of individual original elements of stomatopod maxillipeds. Our findings clearly falsify the suggestion that the basipod has become conjoined either with the coxa or with the endopod element 1 (ischium). In conclusion, our findings in combination with data from the fossil record suggest that stomatopods possess a carpo-propodus.
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