The Class Gastropoda is the largest and most diverse group within the Phylum Mollusca, including the snails familiar to all. Few people would associate gelatinous zooplankton with any kind of snail, however. In fact, an astounding variety of planktonic gelatinous snails visit the waters of California. Some retain the remnants of the gastropod shell and resemble typical benthic marine snails. Others lack shells completely and would at first glance appear to have abandoned any apparent relationship with their benthic brethren. Most have modified the gastropod foot into either a single fin or paired structures that in some species enable surprisingly fast locomotion. While not true “jellies”, these molluscs nevertheless are endowed with delicate gelatinous tissue. Gelatinous molluscs are more complex creatures than ctenophores and cnidarian jellies, possessing true organ systems for circulation, reproduction, excretion, and digestion.
The heteropods (a small group within the Subclass Prosobranchia, Order Mesogastropoda) are an amazing group of gelatinous predatory gastropods. Using a single ventral fin for propulsion, these active visual predators capture other gelatinous prey such as salps and ctenophores using a highly mobile proboscis that holds a radula with chitinous teeth. Heteropods possess well developed eyes for locating their victims. Some species have an obvious coiled shell (Family Atlantidae); others have lost it completely (Family Pterotracheidae). Heteropods rarely make a significant contribution to the total gelatinous biomass in an area. Most prefer tropical and warm-temperate waters.
The Subclass Opisthobranchia is home to most of the gelatinous gastropods. Within this diverse group there is a tendency for the shell to be reduced or totally absent. The mantle cavity, if present, may have a single gill or lack it completely. The colorful “sea slugs” known as nudibranchs (Order Nudibranchia) are the ophisthobranchs most familiar to marine naturalists. A few species have adopted planktonic lifestyles, with some barely different from benthic forms, while others are highly modified for life in the water column. Other opisthobranchs include the pteropods, which utilize paired modified extensions of the foot (the “wings”) to swim by flapping. Although many gelatinous groups have bioluminescent (link from UC Santa Barbara) representatives, pteropods and heteropods are not known to have any species with this ability.
Thecosome pteropods (Order Thecosomata) include about 50 planktonic species and generally possess relatively large wings for swimming, hence the common name “sea butterfly” for certain species. Most possess a mantle cavity, usually with a single small gill. They produce large mucus feeding webs for ensnaring phytoplankton prey such as dinoflagellates and diatoms, and small zooplankton, such as tintinnids, foraminiferans and radiolarians. Cilia on posterior and lateral footlobes assist in bringing the food-laden mucus to the mouth. The mouth typically has lateral jaws and a radula to aid in gathering the feeding web.
Within the Suborder Euthecosomata, the Family Limacinidae includes species possessing a relatively well developed coiled calcareous shell, the ability to withdraw into the shell, and a shell operculum. Most thecosomes are members of the Family Cavoliniidae, which generally have a symmetrical cone-shaped or globular shell that may be slightly curved. Although quite abundant and species-rich in many areas, cavoliniid pteropods favor tropical and warm-temperate seas and are not very common in California waters and areas to the north. Two species that you may occasionally encounter in West Coast waters are Clio pyramidata and Cavolinia tridentata. The Suborder Pseudothecosomata includes the Family Cymbuliidae. Instead of an external calcareous shell, they possess a relatively tough gelatinous internal structure known as a pseudoconch. The lateral and posterior foot lobes are joined as a ciliated proboscis that leads to the mouth, and the wings are united ventrally to form a single plate.
A more energetic lifestyle is employed by the 50 or so species of gymnosome, or “naked” pteropods (Order Gymnosomata), which lack any kind of shell or mantle cavity. Despite their designation as pteropods, the gymnosomes and thecosomes are not closely allied. Most gymnosome species inhabit warm seas, but one species, Clione limacina, can be incredibly abundant in cold northern areas. Gymnosomes swim actively in search of thecosome prey using a pair of swimming fins located near the head. The well-defined head has two pairs of tentacles. The fins are generally smaller than thecosome wings, but are more powerful and adapted for rapid swimming. The body of gymnosomes is covered by a somewhat tough elastic skin. They use a terminal mouth that may be equipped with specialized mouthparts, including a radula, hooks, adhesive tentacles (some with suckers), and a jaw to grasp, manipulate and devour their captives. Gymnosome species vary in their possession of these mouth structures.
Gelatinous molluscs typically use copulatory organs for internal fertilization, rather than casting eggs and sperm to a likely demise in the open water. Male heteropods have a conspicuous penis to transfer spermatophores into females. Fertilized eggs are released in long strings that drift until hatching. Pteropods are typically hermaphrodites, a characteristic that is common among opisthobranchs. Thecosome pteropods start off as males, and later convert over to the female side (known as protandrous hermaphrodites). This may sound pretty bizarre but is actually not an uncommon practice in the marine realm, including many species of fish (some start as females, other species start as males!). Fertilized eggs from a mating episode are released as floating masses. Gymnosome pteropods are simultaneous hermaphrodites with internal fertilization. All individuals possess both a female genital pore (right ventral side) and a male copulatory organ (right side of the foot lobes).
All images in the JelliesZone © David Wrobel and may not be copied or used in any form without permission.