Foods & feeding
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Preferred foods & feeding ecology: sacoglossans


This section continues with information on diets and feeding ecology of west-coast nudibranch & relatives genera. Information on diets and feeding ecology of west-coast genera is provided below for sacoglossans and in 6 other sections:

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Research study 1

drawing of sacoglossan Aplysiopsis enteromorphaedrawing of head region of a sacoglossan Aplysiopsis enteromorphae
In the San Juan Islands, Washington the sacoglossan Aplysiopsis (Hermaeina smithi) enteromorphae inhabits mud flats in amongst green algal mats, Ulva and Enteromorpha.  Egg masses of Aplysiopsis are often found entangled in the Enteromorpha.  Amongst the Enteromorpha, as well, are several species of filamentous red algae, including a favoured food Rhizoclonium.  The sacoglossan crawls onto a filament of Rhizoclonium, grips it with foot and oral lobes, slices open an algal cell with a single radular tooth, and sucks out the contents.  Gonor 1961 Veliger 4: 85.

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Research study 2
  photograph of a sacoglossan Olea hansineensis Photo courtesy Ron Long, SFUAlso in San Juan Islands, Washington, the sacoglossan Olea hansineensis (6-8mm in length) uses its single-toothed radula to suck out the contents of eggs of other opisthobranchs.  If the eggs are in an early stage of development, Olea simply swallows them whole.  The eggs are consumed row by row at a rate of 15-20 min-1. Olea will push its head right into an egg mass and, as it consumes the eggs, it moves through the jelly by ciliary propulsion and by pushing with its head.  Olea lays its egg masses on or near the eggs of other opisthobranchs.  The author notes that O. hansineensis is one of the few carnivorous sacoglossans known.  Crane 1971 Veliger 14: 57. Photo courtesy Ron Long, SFU.
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Research study 3

photograph of a sacoglossan Placida dendritica courtesy Erling Svensen, Norwaygraph showing total number of cusps in the radulae of the sacoglossan Placida dendritic eating different types of algaeThe sacoglossan Placida dendritica is cosmopolitan, being found on the Pacific, Atlantic, Asian, and Medterranean coasts.  On the west coast it generally feeds on green algae, either Codium (a perennial) or Bryopsis (a seasonal). Interestingly, its radula form and dentition varies depending on the type of alga being eaten.  On a diet of Codium, the radular ribbon forms a tight coil and the cusps are many and small (see graph on Right).  On a diet of Bryopsis, the radular ribbon is straighter, and the cusps are fewer and larger. The differences appear to be a morphometric response of radula structure to the structure of the alga being eaten - the cusps respond to the degree of difficulty in penetrating and sucking out the algal cells.  Codium is an alga with a tough texture, while Bryopsis has a soft texture.  If a veliger of Placida settles onto Codium, it grows more slowly, attains a small size, and produces few eggs.  If it settles onto Bryopsis, it grows quickly, attains large size, and produces more eggs.  This information may explain the size differences noted by the author in an earlier study.  The author comments that these observations add a new definition to “preferred” food.  In the case of P. dendritica, the “preferred” food may, in fact, depend on which type of alga is first encountered as a larva, to which the individuals adapt morphologically and physiologically.  Bleakney 1990 Veliger 33: 111. Photo of Placida courtesy Erling Svensen, Norway and seaslugforum.

photograph of the alga Codium fragile NOTE  Placida dendritica also eats the green alga Derbesia spp. About one-third of the "Bryopsis/Derbesia" points on the graph are from Placida eating Derbesia marina in a Japanese study

NOTE  in an earlier paper, which uses Atlantic-coast P. dendritica, the author concludes that the size differences noted in specimens eating different algae may have been a geographic effect.  Bleakney 1989 Veliger 32: 171.

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Research study 4

A fascinating glimpse of social interaction amongst ascoglossans Placida dendritica is provided in laboratory and field observations at the Hatfield photograph of green alga Codium sitchensis courtesy Gary McDonaldMarine Science Center, Newport, Oregon.  In the field, group foraging by members of the species occurs commonly on green algae Codium histogram showing group sizes of sacoglossans Placida dendrica on a food alga Codium sitchensissetchellii and C. fragile (see histogram upper Right). The histogram for C. setchellii is actually truncated, as groups as large as 72 may be found. For Placida, specifically, the behaviour may be beneficial in searching for and identifying abundant and nutritious food sources. 

histogram showing effect of group size on growth in the sacoglossan Placida dendritica
But what kinds of “on-site” disadvantages might there be to members of a group feeding on a common food item?  For example, will all participants obtain relatively less food, or will smaller individuals be out-competed by larger ones?  In fact, results show that for similar-sized individuals, growth in a group is faster than when single (see histogram lower Right). Further, in groups of mixed sizes, smaller individuals benefit more than larger ones; in fact, growth of the latter is actually reduced.  The way this size-related facilitation and competition works is not clear, but it seems that one slug’s feeding not only stimulates others to feed, but it in some way modifies the physical or physiological state of the food alga, leading to enhanced feeding and growth.  The first, or physical, aspect of this is idea is tested by comparing attractiveness and edibility to Placida of algae wounded both from natural feeding and from puncturing with a pin.  Neither treatment significantly enhances attractiveness of the alga to other Placida; however, physical damage of the food alga does lead to significantly more feeding and to significantly faster growth - but only if the damage occurs from feeding by other conspecifics.  If the damage is caused by a pin, no feeding/growth effects are evident.  The stimulating effects may include lubricating mucus or pheromones of some kind.  The author concludes by saying that gregarious feeding allows the herbivores to consume certain algal species not otherwise available to individuals.  Trowbridge 1991 Ecology 72: 2193; for a comparison of herbivory patterns of ascoglossans feeding on Codium spp. in 3 world locations see Trowbridge 1998 J Exp Mar Biol Ecol 230: 31. Photograph courtesy Gary McDonald, Long Marine Laboratory, UC Santa Cruz.

NOTE   clustering is more evident on C. setchellii than on C. fragile.  A third green-algal species, Bryopsis corticulans, is eaten but only by single individuals, never by aggregations.  The author suggests that the ephemeral nature of Bryopsis, in comparison with Codium spp. may not allow time enough for aggregations of Placida to form

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Research study 5

Studies on ascoglossans in high intertidal zones of the central Oregon coast show that Alderia modesta frequents estuarine algal mats (Vaucheria spp.), while Aplysiopsis enteromorphae prefers open-coast tidepools containing filamentous green algae such as Cladophora spp., Chaetomorpha spp., and Enteromorpha spp.  Growth of both species in such habitats is fast, and Aplysiopsis may become sexually mature within a few days of metamorphosis, and Alderia within about 10d of metamorphosis.  Interestingly, unlike in many other opisthobranchs, spawn mass in Aplysiopsis is unrelated to body mass. Trowbridge 1993 Veliger 36: 99; Trowbridge 1993 Veliger 36: 303. For an excellent and comprehensive review of feeding habits of many of the 25 species of west-coast sacoglossans, see Trowbridge 2002 Veliger 45: 1. Drawing of Aplysiopsis from Gonor 1961 Veliger 4: 85; photo of Alderia courtesy A. Chernyshev, Russia and seaslugforum.

NOTE  despite reaching relatively high densities in high-level tidepools along the central coast of Oregon, the grazing effects of Aplysiopsis in summer are about 2 orders of magnitude less than those of another mesoherbivore, Littorina scutulata, also present in high numbers in the tidepools.  Trowbridge 1993 J Exp Mar Biol Ecol 169: 233.

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Research study 6

photograph of sacoglossans Alderia willowi and A. modesta courtesy Patrick Krug, California State University, Los AngelesAlderia willowi, found only in southern California, and the cosmopolitan Alderia modesta are specialised herbivores that feed solely on green algae Vaucheria spp. These algae are common on mudflats in temperate estuaries throughout the northern hemisphere.  In southern California A. willowi eats Vaucheria longicaulis, while populations of A. modesta in other west-coast areas eat Vaucheria spp.  Krug 2007 Amer Malac Bull 23: 99. Photograph courtesy Patrick Krug, CSU Los Angeles, California krugLab.

NOTE  the taxonomy of the genus Vaucheria is uncertain; hence, the lack of specificity

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