Crustacean larvae are usually recognised as small organisms, below one millimeter body size. However, in different crustacean groups such as Stomatopoda, Polychelida, or Achelata, also very large larvae occur with sizes of 20 mm and beyond. Also from few meiuran species (“short-tailed” crustaceans, including crabs, hermit crabs, or squat lobsters), rather large larvae are known, though still considerably smaller than 20 mm. We present here two specimens of anomalan meiuran larvae, each with a total length of 24 mm, which by far exceed the previously known/reported maximum sizes of meiuran larvae. Yet, both specimens exhibit characters that indicate their identity as zoea larvae (first larval phase with several stages), most likely shortly before the metamorphosis to the megalopa (second larval phase with one stage). Due to this early developmental state, it is difficult to provide a narrower systematic identification of the larvae. In addition to the description of the developmental status of all appendages, we also investigated the gizzard and especially the compound eyes. The latter possess a mixture of hexagonal, intermediate, and square-shaped facets in an unusual arrangement. We documented the exact arrangement of the facets in both specimens and discuss the possible re-structuring during metamorphosis. The arrangement of the different types of facets indicates that transformation to an adult eye structure takes place over several moults and that the facets are being rearranged in this process. The findings demonstrate that also meiuran larvae contribute to the fraction of the macro-plankton.

A central function of sensory systems is the gathering of information about dynamic interactions with the environment during self-motion. To determine whether modulation of a sensory cue was externally caused or a result of self-motion is fundamental to perceptual invariance and requires the continuous update of sensory processing about recent movements. This process is highly context-dependent and crucial for perceptual performances such as decision-making and sensory object formation. Yet despite its fundamental ecological role, voluntary self-motion is rarely incorporated in perceptual or neurophysiological investigations of sensory processing in animals. Here, we present the Sensory Island Task (SIT), a new freely moving search paradigm to study sensory processing and perception. In SIT, animals explore an open-field arena to find a sensory target relying solely on changes in the presented stimulus, which is controlled by closed-loop position tracking in real-time. Within a few sessions, animals are trained via positive reinforcement to search for a particular area in the arena ("target island"), which triggers the presentation of the target stimulus. The location of the target island is randomized across trials, making the modulated stimulus feature the only informative cue for task completion. Animals report detection of the target stimulus by remaining within the island for a defined time ("sit-time"). Multiple "non-target" islands can be incorporated to test psychometric discrimination and identification performance. We exemplify the suitability of SIT for rodents (Mongolian gerbil, Meriones unguiculatus) and small primates (mouse lemur, Microcebus murinus) and for studying various sensory perceptual performances (auditory frequency discrimination, sound source localization, visual orientation discrimination). Furthermore, we show that pairing SIT with chronic electrophysiological recordings allows revealing neuronal signatures of sensory processing under ecologically relevant conditions during goal-oriented behavior. In conclusion, SIT represents a flexible and easily implementable behavioral paradigm for mammals that combines self-motion and natural exploratory behavior to study sensory sensitivity and decision-making and their underlying neuronal processing.

This work was carried out in collaboration between all authors. Author JTH initiated and coordinated the study, produced the 3D models and drafted the manuscript with input from all authors. Author CH discovered the specimens and made them available for this study. Authors PTG and LLF documented the specimens and processed the images. Author NRR produced the drawings. Authors PW, NRR, PTG and LLF interpreted and described the morphological structures. All authors read and approved the final manuscript. ABSTRACT 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.