Giant larvae in crustaceans
Among numerous crustacean groups giant larvae have been reported, especially among decapods. Decapods usually have (at least) two larval phases: The pelagic zoea larvae swim with the outer locomotion branches (exopods) of their thoracopods. This phase may include up to ten stages. The zoea is followed by the megalopa, which mediates the transition between the pelagic larva and the benthic juvenile. Most megalopae have lost their exopods on the thoracopods and swim with their pleopods. In many groups there is only a single megalopa stage. Sometimes larvae show a kind of mixed morphologies somewhere “between” zoea and megalopa. The latest zoea as well as the megalopa usually measure only few millimeters in total length. Yet, there are quite some exceptions:
Zoea larvae of prawns (Dendrobranchiata) are usually small with shield lengths rarely reaching 1 mm. Yet, within Aristidae zoea larvae formerly addressed as “Cerataspis monstrosa” reach shield lengths of almost 12 mm (Bracken-Grissom et al., 2012).
Polychelidan lobsters (Polychelida) only have a short zoea phase (Torres et al., 2014), but have several megalopa stages that reach astonishing sizes. These eryoneicus larvae reach sizes of more than 100 mm in length (Martin, 2014b; Eiler et al., 2016). Fossil forms that show some similarities to modern forms and also an increased size have been reported from the Jurassic Solnhofen limestones (Eiler and Haug, 2016), and from the Cretaceous limestones of Lebanon (Haug et al., 2015a).
Achelatan lobsters (Achelata) develop through a characteristic type of zoea larva, the phyllosoma (Palero et al., 2014). Phyllosoma larvae have been recognized as giant larvae frequently in the literature. They can reach up to 80 mm in body length, with their thin legs extending even longer (Guérin, 1822; Richters, 1873; Johnson, 1951; Sims, 1964; Sims and Brown, 1968). Phyllosoma larvae most likely represent the largest decapod larvae (Palero et al., 2014). As a consequence, also the megalopa larvae of achelatans (nisto and puerulus larvae) are significantly larger than other types of megalopa larvae. Giant phyllosoma larvae have been reported from the fossil record with body length up to 100 mm. Besides “typical phyllosoma larvae (Polz, 1972; 1973; 1984; 1987; 1995; 1996; Haug et al., 2009; 2011a), large nisto larvae (Audo et al., 2014; Haug and Rudolf, 2015), but also transitory forms with a “mixed” morphology of phyllosoma and post-phyllosoma stages have been reported (Haug et al., 2013b; Haug and Haug, 2013, 2016).
Larvae of false sand crabs (Hippidae) usually reach a total length of 2 mm. A single specimen has been reported to have reached 15 mm in total (Martin and Ormsby, 1991). Yet recently more material turned up demonstrating that reaching such a size may be quite more common among false sand crabs than expected (Rudolf et al., 2016).
While mantis shrimps (Stomatopoda) are not decapods, they show certain similarities to them including various aspects of their larval development. Their later larva can be roughly seen as the functional equivalent to the megalopa larva in decapods. Depending on the specific ingroup these larvae are of the alima-type or of the erichtus-type. Both reach sizes of several centimeters. Alima-type larvae have been known to reach up to 5 mm (Ahyong et al., 2014). Just recently new very large erichthus-type larvae have been described (Haug et al., 2016). Erichthus-type larvae have also been described from the Jurassic lithographic limestones from Germany with up to 18 mm (Haug et al., 2008; 2015b). Notably, giant fossil larvae from the Triassic Hallstatt limestone from Austria with shield lengths of 13 mm show certain characters of the mantis shrimp larvae, and also similarities to the false sand crab larvae (Hyžný et al., 2016).