Contributions to Zoology, 86 (3) – 2017Christina Nagler; Jens T. Høeg; Carolin Haug; Joachim T. Haug: A possible 150 million years old cirripede crustacean nauplius and the phenomenon of giant larvae

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Introduction

The larval phase of a metazoan organism, an animal, is a discrete post-embryonic period. Different authors apply various criteria what identifies a larva in comparison to a non-larval immature. Among these are, for example, 1) a morphology that is significantly different from that of the adult (Hickmann, 1999), 2) the occupation of a different ecological niche than the adult (Giese and Pearse, 1975; Young, 1999), or 3) the possession of organs that become reduced and are absent in the adult (Strathmann, 1993; Anger, 2001).

In many organisms the larval phase is comparably short, ended by a metamorphosis that produces the juvenile/adult morphology within a short period of time (Passano, 1961). As a consequence of a short larval phase in most organisms the larvae are rather small (Cowen and Sponaugle, 2009).

In classical zoological literature, we often encounter the differentiation between so-called primary and secondary larvae (although the value of this differentiation remains questionable). The first type should represent ancestral larval types, while the second represents derived forms (Werner, 1988). Especially larval forms that are classified as primary are usually microscopic entities, more or less invisible to the naked eye. The trochophora (of annelids and molluscs) and the pluteus (of echinoids) are often given as examples (Young, 2002), although both forms are clearly derived types of larvae characterizing specific monophyletic groups (hence, could also be interpreted as secondary larvae). Yet, also many larval forms that clearly fall into the secondary larvae category are often quite small, for example, crustacean nauplii (Martin et al., 2014, fig. 2.3; Haug and Haug, 2015).

Despite the fact that larvae are usually small, in many lineages larvae of astonishing sizes have evolved, i.e., forms that can well be called ‘giant larvae’. A rather simple case example is that of flying insects. As flying insects perform a terminal molt and can no longer grow as adults, their larval phase is comparably long and the late stages are quite large, almost as large as the adults (Truman and Riddiford, 2002). Yet, giant larvae are also known in numerous further metazoan groups.

The phenomenon of giant larvae can also be observed in the fossil record. There are cases of exceptional types of fossil preservation that seem only to preserve rather small forms. Most notably, fossils in an Orsten-type preservation include many forms of larval arthropods and larval cycloneuralians, no specimen being larger than 2 mm (Haug et al., 2014a). Yet, for many other types of preservation especially the large forms appear to have a higher chance to be preserved. For malacostracan crustaceans, we have fossils of especially super-sized larvae such as those of achelatan lobsters (Polz, 1971; 1972; 1973; 1995; Haug et al., 2011a; Haug and Haug, 2016), polychelidan lobsters (Haug et al., 2015a; Eiler and Haug, 2016) or mantis shrimps (Haug et al., 2008, 2015b), some of them in thousands of specimens (Polz, 1987; 1996), while smaller-sized larvae like those of crabs are very rare (Haug et al., 2015c). It seems therefore common that giant larvae occur as fossils.

With this contribution we aim at briefly reviewing the known occurrences of giant larvae. Due to the still very incomplete knowledge of Mesozoic plankton (Haug and Haug 2017), the description of a possible further case of a 150-million-year old crustacean larva that was found in the famous lithographic limestones of southern Germany add important details. Additionally, this fossil larva is of unusually large size.