Müller's larva

Müller's larva or Mulleria is a larva of some Polycladida.^[1] It has eightfold symmetry and resembles a trochophore; homology between the two types of larvae from related groups of the Spiralia is possible but is not well studied.^[2] Müller's larva is ciliated and has several paired and unpaired lobes. The cilia on the lobes are longer than cilia on the rest of the body. At the anterior and posterior ends of the larva are tufts of longer cilia (apical and caudal). The apical tuft originates from the apical organ, a sensory structure associated with the central nervous system.^[2]

It is named after Johannes Peter Müller (1801–1858), a German physiologist who invented the plankton net, and first described larval forms of many phyla.^[3]

Müller's larva is one of several larval forms described for polyclad flatworms, a group in which many species develop directly but others develop indirectly through a planktonic stage with transient larval structures before settling and transforming into benthic juveniles.^[4][5]

Polycladida are mostly marine, free-living flatworms, and species vary in developmental mode. In indirect developers, larvae are part of the plankton and later settle to the seafloor, where they undergo a transition to a juvenile form resembling the adult body plan.^[4][5] Comparative phylogenomic analyses suggest that developmental modes (including indirect development) have changed repeatedly over polyclad evolutionary history, with gains and/or losses inferred in multiple lineages.^[5]

The defining external features of polyclad larvae include protruding lobes and a prominent ciliary band or marginal ciliary specializations associated with the lobes.^[4] While the article lead describes an eight-lobed form, larvae with other lobe counts (including 4–8 and 10) have also been reported, and many species hatch with eight lobes.^[4] In some cotylean polyclads, a ventral sucker develops in late larval stages; this has been proposed as a settlement-related structure.^[4]

In addition to the apical and caudal ciliary tufts noted above, Müller's larvae can bear larval eyes, and the structure and development of larval and adult eyes have been studied in polyclads using ultrastructural methods.^[6]

Descriptions of embryonic development and post-embryonic change in polyclads indicate that the planktonic phase can include substantial growth and reorganization before settlement.^[2][4] In the polyclad Maritigrella crozieri, larvae can survive for weeks under laboratory conditions, enabling experimental study of the planktonic phase and the transition toward juvenile traits.^[7] During late larval development in some groups, larval-specific structures are reduced and juvenile/adult structures become more prominent, consistent with a metamorphic transition associated with settlement on the benthos.^[4]

Larvae are active swimmers and can show phototactic behavior. Experiments on Maritigrella crozieri larvae have been used to characterize phototaxis, spectral sensitivity, and changes in phototactic responses over ontogeny.^[8][7]

Müller's larvae have long been inferred to feed in the plankton, and direct observations of prey capture have been made using high-speed videography. In one study, captures of individual microalgal prey were associated with transient, large-scale reversals of ciliary beating over portions of the main ciliary band, altering near-field flow and redirecting food-containing water toward the mouth.^[9]

Because Müller's larva shares several gross morphological features with trochophore-like larvae, polyclad larvae have been discussed in relation to broader hypotheses about the evolution of spiralian life cycles.^[2][4] Reviews of polyclad larval diversity have emphasized that similar larval features across phyla can arise through a mixture of evolutionary conservation, modification, and convergence, and that the origin(s) of specific larval characters within polyclads remain an active area of research.^[4] Phylogenomic analyses have also been used to place larval forms and developmental modes in an explicit evolutionary framework across Polycladida.^[5]

Polyclads with Müller's larvae have been used in studies of larval anatomy, sensory biology, and development, including work on larval phototaxis, ciliary locomotion and feeding, and the timing of juvenile trait appearance.^[7][2][10]

• Götte's larva
• Trochophore
• Spiralia
• Plankton

• Brusca, Richard C.; Moore, Wendy; Shuster, Stephen M. (2016). Invertebrates (3rd ed.). Sinauer Associates/Oxford University Press. ISBN 978-1-60535-375-3.
• Ruppert, Edward E.; Fox, Richard S.; Barnes, Robert D. (2004). Invertebrate Zoology: A Functional Evolutionary Approach (7th ed.). Brooks/Cole. ISBN 978-0-03-025982-1.