THE VELIGER LARVA HOME PAGE
(under construction)

 THIS HOME PAGE PROVIDES INFORMATION ON THE CHARACTERISTIC MOLLUSCAN LARVA, THE VELIGER, AND EXCERPTS FROM CURRENT VELIGER LARVAL INVESTIGATIONS.

FOR THE PRESENT, THIS PAGE IS CONCERNED WITH MY OWN RESEARCH INTERESTS, OPISTHOBRANCH VELIGER LARVAE.

Peptidergic and monoaminergic components of the veliger nervous system

 A variety of neurotransmitters/modulators may be found in the veliger larval nervous system. Recent studies suggest that interaction between neuronal systems utilizing these neuroactive substances and with associated sensory structures, adds a higher degr ee of complexity (and perhaps capability) to the larval nervous system than would be suggested by the relatively small number of neurons present and cursory examination of its apparently simple morphology. My lab has been examining the morphology of speci fic neuronal systems in opisthobranch veliger larvae that are characterized by the presence of various monoamines or peptides. One interesting neuronal system is that characterized by the presence of 5-hydroxytryptamine (serotonin). While this neuronal s ystem includes neurons present in the cerebral and pedal ganglia and axonal tracts that extend through most of the larval CNS and into the larval foot, velum and viscera, one particularly interesting component is a group of 5 neurons present in the larva' s apical ganglion (= apical organ). The following images describe parts of the apical serotonergic complex that are apparently involved in both sensory and somatic functions.

The movie and figures below present various aspects of this sensory structure. The following movies and single images were prepared from serial optical sections of anti-serotonin labeled whole larvae. Serotonin antigenicity was visualized with FITC conjug ated secondary antibodies. Optical sections were collected with a Bio-Rad MRC-1000 confocal, laser-scanning microscope and processed with Vital Images', VoxelView, Volume Microscopy Workbench. Final image preparations, including labels and GIF conversion, were performed with Silicon Graphics Showcase, Snapshot, and Movie Convert software.

I. Serotonergic components of the opisthobranch veliger larva's nervous system

A. The apical serotonergic sensory complex in opisthobranch veliger larvae:

The apical serotonergic sensory complex consists of a group of 5 serotonergic neurons and associated dendrites and axons that reside in the apical ganglion (cephalic sensory organ - Bonar, 1978) of every opisthobranch larva thus far examined. In larvae ex amined at the ultrastructural level the two pairs of neurons on the left and right sides reside in the lateral lobes of this ganglion, with a centrally located neuron being present in the medial lobe. The central neuron and the anterior neuron of each lat eral pair project dendrites through the larva's pre-trochal epidermis. Each dendrite ends in a terminal varicosity that supports 2 cilia (presumably sensory). In addition, each of the 5 neurons of this complex extend an axon into an axonal pexus that is l ocated just posterior to the central neuron, between the two lateral pairs of neurons. This plexus consists of many axons from other neurons within the apical ganglion in addition to those from the 5 serotonergic neurons. This plexus might be considered a neuropil; however, rather than being centrally located within a layer of cortical neurons, it buldges into the larval hemacoel along the posterior side of the apical ganglion. Two serotonergic nerve trunks exit the apical axonal plexus ventrally. These n erve trunks extend laterally, one to the left and one to the right, and branch soon after leaving the plexus. One branch of each nerve trunk continus into the ipsolateral velar lobe where it branches a number of times. The other branch extends apporximate ly perpendicular to the ipsolateral branch, anterior toward the pre-trochal epidermis; however, befor reaching the epidermis it makes a 90o turn toward the contralateral side and runs beneath the dendrites of the serotnergic sensory neurons and into the c ontralateral velar lobe. The general layout of the apical serotonergic sensory complex suggests a compensitory sensory system that adjusts some aspet of the velar lobes (perhaps muscle tension or ciliary activity) in response to an unknown external stimul us.

Descriptive pictures (click on small image to view in greater detail)

IMAGE Oblique dorsal view of the apical serotonergic sensory complex in a typical dendronotid nudibranch veliger (Melibe leonina)

The serotonergic neuronal complex in the larval apical organ of Melibe leonina. This complex consists of 5 neurons (NC, NL1, NL2, NR1, NR1) and an apical axonal plexus. In addition, the 3 anterior neurons (NC, NL1, NR1) project dendrites to the sur face through the pre-trochal epidermis. Only a small portion of two of the dendrites (D) is visible in this re-construction. Each of these dendrties ends in a swollen varicosity that supports 2 cilia (presumably sensory). A detail of one of these neurons may be seen in the "Sensory neuron" image below. Adjacent serotonergic neurons (NCL, NCR) are present in the left and right cerebral ganglia.

IMAGE Posterior view of the apical serotonergic sensory complex in a typical dendronotid nudibranch (Melibe leonina)

View of the apical serotonergic neuronal complex from the posterior aspect. Four of the 5 neurons present in the complex are visible (NL1, NL2, NR1, NR1) as are the apical axonal plexus and neurons present in the left and right cerebral ganglia (NCL, NCR) . Two nerve trunks (NTL, NTR) project ventrally from the apical axonal plexus. Each of these nerve trunks branches soon after leaving the plexus (not visible in this re-construction). On both the left and right sides, one branch continues into the ipsolat eral velar lobe, while the other branch extends anterior and approximately perpendicular to the ipsolateral branch, makes a 90° turn, and then projects beneath the dentrites of the 3 anterior neurons of the apical serotonergic complex and into the con tralateral velar lobes.

IMAGE A putative sensory neuron in the apical serotonergic complex of a typical aeolid nudibranch veliger (Berghia verrucicornis).

Detail of a portion of the apical serotonergic complex showing the left sensory neuron, NL1, imaged in a ventral view. The two sensory cilia at the swollen tip of the dendrite are indicated, as is the axon (ANL1) that extends from NL1 into the apical axon al plexus. The transverse axon tract (AT) that extends beneath the dendrites of the serotonergic sensory neurons of the apical organ is also visible. This axon tract contains branches of the two major nerve trunks that ventrally exit the apical axonal ple xus. These branches extend to the contra-lateral velar lobe relative to each of the nerve trunks. NLC - serotonergic neuron in left cerebral ganglion.

IMAGE Location of the apical serotonergic sensory complex within the head of the veliger larva

Perspective reconstruction showing the position of the apical serotonergic sensory complex in the head of a veliger larva of the aeolid nudibranch Berghia verrucicornis. As can be seen, this sensory complex lies between the larva's eyespots, above the larval esophagus. Both lateral pairs of serotonergic neurons (NR1 & NR2, NL1 & NL2), the medial neuron (NC), and the axonal plexus are present. A few other neurons to the left and right of the complex are in the edges of the cerebral ganglia. The dend rites with swollen terminals extending from the two anterior neurons (NR1, NL1) of the lateral pairs are evident. The dendrite from the central neuron is not apparent. The axons at the lower left, below the larval eyespot, are extending into the right vel ar lobe. The "haze" surrounding all these structures is from background fluorescence in unlabeled tissues of the larval "head".

Movies

Quicktime movies of a complete rotation of the apical serotonergic sensory complex of a veliger larva of Melibe leonina. Two movies are provided, one in small and the other in large format. The small format movie will download approximately twice as fast as the large format movie; however, the large format movie will show more detail at higher resolution.
B. Other components of the opisthobranch veliger larval serotonergic neuronal system:

1. Serotonergic innervation of the larval foot in a competent veliger of the aeolid nudibranch, Berghia verrucicornis

IMAGE Serotonergic innervation of the larval foot

A perspective, 3-D reconstrruction of serotonergic innervation in the right pedal ganglion and foot of a competent larva of Berghia verrucicornis. The right pedal ganglion (PGR), associated neurons and neuropil, ia shown as a sagittal view from the larva's left side. Three nerve trunks (NT1P, NT2P, NT3P) can be seen to arise from the pedal ganglion and extend into the right half of the larval foot. The axons in these nerve trunks branch repeatedly (ABP) with many varicosities along the length and a t the ends of the branches. Possible functions for this innervation include modilation of muscle contractility or ciliary activity.

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FAQ:

1. What is a larva?

A larva is a developmental stage characteristic of many, but not all, types of aquatic and amphibious animals. For example, many invertebrate animals such as starfish and vertebrate animals such as frogs and fish have a larval stage in their development. To really answer this question, we have to go back to the beginning, back to when an animal first starts to develop. For most, this would be at the point of conception when the sperm fertilizes the egg. Fertilization leads to a developmental stage we cal l the embryo. An embryo develops in some sort of protected environment. In the case of a starfish, frog, or regular fish, there is a protective layer that initially surrounds the egg, and after fertilization surrounds the embryo, that is called the vitell ine membrane. This layer is not composed of cells, rather, it is acellular and composed of a substance called glycoprotein. The vitelline membrane protects the embryo from vectors such as bacteria that might interfere with its development and also from th e effects of water turbulence that might break apart the fragile cells that the embryo is composed of. At some point an embryo reaches a developmental stage that is able to survive in the external environment. When this happens, the embryo breaks out of t he vitelline membrane and is, thus, released into the external melieu. Once outside the protected embryonic environment, the organism is no longer referred to as an embryo. If the organism resembles a miniature adult and feeds on the sorts of things that the adult does, it is called a juvenile (i.e. it essentially looks like an adult, but must grow to become one and, unlike an adult, is not yet able to be sexually active). However, for many animals the organism released bears either no, or only slight, re semblance to the adult stage, often feeds on something entirely different than the adult types of food, and is not capable of sexual activity. In addition, this newly released developmental stage will, at some point, have to undergo a major morphological transition (called metamorphosis) in order to take on adult-like form and become a juvenile. Thus, in animals that have a larval stage in their life history, the larva exists during a period in their development that lies between the embryonic and juvenil e stages.

Just to set things in proper perspective, definitions for the gametes and various developmental stages might be as follows:

2. What is a veliger larva?

The veliger is the typically descibed larva of the inverterbate phylum known as the Mollusca, and is characteristic of the molluscan classes, Gastropoda and Bivalvia. This larval form may be easily identified by the presence of a bi-lobed swimming organ k nown as the velum. The velum looks rather like a pair of Mickey Mouse ears with a fringe of fine long hairs. The "hairs" are really cilia that beat in a synchronous whip-like motion. The beating of the cilia accomplishes four primary functions,
  1. it is responsible for larval swimming,
  2. for the collection of food particles from the water,
  3. directing the food particles to the mouth, and
  4. rejecting particles that are non-palatable because of their size or composition.
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