 |
 |
| Highlights |
| |
|
| Highlights |
Arden
Ashley
Research Training Program, 2004
|
Introduction Sand dollars are invertebrate animals closely related to sea stars (starfish). In 1974, sand dollars which appear to be hybrids between two different genera were found near Fort Pierce, FL. Hybridization between genera is rare and not well-documented. Previously, fertilization experiements conducted by Pawson showed that Mellita eggs and Encope sperm are capable of fertilization -- but the reverse is not possible. This study involves morphological analysis of characteristics of the test (skeleton), spines and internal anatomy of the proposed hybrids and their presumed parent stocks, Encope michelini and Mellita isometra to determine whether they are hybrids or a new genus and species.
Spine Curvature: Spine lengths were measured first using a straight line from tip to tip (see Mellita with orange line), then by using a flexible ruler to measure the full length along the center of the spine -- including the curve (see Mellita with purple line). The difference between the two measurements can be used to quantify the degree of curvature of the spines. Mellita has more curved spines than Encope and the hybrids are intermediate in terms of curvature.
Position of the Anus:
The placement of the anus on the bottom of the test varies in Encope,
the hybrid, and Mellita. Measurements were taken of the positions
of the mouth (maroon arrow in photograph), anus (purple arrow in photograph)
and the top of the posterior interambulacral lunule (blue-green arrow
in photograph). Both the anus and lunule of Mellita are closer
to the mouth than those of Encope. The anus of the hybrid assumes
a position intermediate between the extremes of the parent taxa.
Shape of the test: The thickest point of the test is much higher on Mellita than Encope. The thickest point of the hybrid test falls in between the ranges of Mellita and Encope. Mellita is much wider than it is long, whereas Encope is longer than it is wide. The hybrids are intermediate between these two shapes.
Aristotle's Lantern: Study of the Aristotle's lantern (the feeding apparatus) using x-ray photographs revealed that the average ratio of the length of the lantern (measured as pictured) to the total length of the test was slightly higher in the proposed hybrid (5.28) than in either Mellita (4.59) or Encope (4.95), however the ranges of all three overlapped.
Genital Pores: Encope has five genital pores, Mellita has four. The proposed hybrids vary in this characteristic. Of the specimens with visible genital pores, seven of the hybrids had four and nine had five. Variability in a reproductive characteristic is not indicative of a distinctive species. Therefore, this is strong evidence for the hypothesis of hybridization between Mellita and Encope.
The evidence presented indicates that the animals are indeed hybrids. The morphological evidence provided here is supported by the fact that Mellita eggs are successfully fertilized by Encope sperm. The hybrids have not been found since 1974. It is unknown whether they are still living in the ocean. Their sudden appearance and disappearance is puzzling and requires further study. Was the proposed breeding between the two triggered by an environmental occurance? Is breeding between Mellita and Encope common, but the offspring rarely survive? Or were the hybrids merely the products of an isolated incident of late 1960s free love? These questions are not easily answered. Further study of these organisms will help elucidate the mysteries surrounding hybridization.
This research was supported by a grant to the NMNH Research Training Program from the National Science Foundation Research Experiences for Undergraduates program, Award Number DBI-0243512 as well as a generous donation by Battelle to support studies in the Marine Sciences. It would not have been possible without the work of Doris Vance, Scott Whittaker, Sandra Raredon and Mary Sangrey. Want to ask a question? Visit the Message Board Virtual
Poster Session Message Board Research
Training Program
|
||
|
Research
from
Biology |
||
|
INVERTEBRATE
ZOOLOGY "Intergeneric Hybridization between the sand dollars Encope michelini and Mellita isometra (Echinodermata: Echinoidea: Mellitidae)" |
||
|
VERTEBRATE
ZOOLOGY "Morphological
adaptations for terrestrial habits of the Scimita-Billed Woddcreeper (Drymornis
bridgesii)." |
||
|
VERTEBRATE
ZOOLOGY "Identity
of an atypical bear from Alaska: The first documented wild hybrid bear
(Ursus americanus X Ursus arctos)?" |
||
|
Research
from
Paleobiology |
||
|
Research
from
Mineral Sciences |
||
|
Research
from
Anthropology |
||
|
More RTP Class of '04 Links |
Information | Highlights | Alumni Pages
| NMNH Home | What's New ? | Calendar of Events | Information Desk | Search |
Petals:
Petal lengths were measured from the ocular pore to the last inner pore
of the petal (see pink line on picture of petal). Petal widths were
measured at the widest point between the inner pores (see purple line
on picture of petal). Mellita petals are wider in relation to
their widths. Hybrids tend to be longer for their width, like Encope.
The number of pores in each petal is greater in Encope than in
Mellita and the hybrids fall in between. The number of pores
per milimeter of petal length is also slightly higher in Mellita
than in Encope, with the hybrids falling in between these two
ranges.
